Individual Residential Wastewater Treatment Systems - Design Handbook

Table of Contents

Foreword

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This handbook has been produced to provide guidance in uniformly implementing the NYS Department of Health's Administrative Rules and Regulations design standard (10NYCRR Appendix 75-A), entitled Wastewater Treatment Standards – Individual Household Systems. The handbook was prepared to address effective design, construction and maintenance of individual household sewage treatment systems for use by homeowners, design professionals, builders, contractors, local community officials and health department officials. Part 75 of the NYS Department of Health's Administrative Rules and Regulations (10NYCRR 75) requires that all new individual sewage treatment systems shall be designed and constructed in accordance with 10NYCRR Appendix 75-A as the generally accepted standard for individual sewage treatment systems.

New construction should routinely meet all standards. Section 75.3(d) of 10NYCRR Part 75 provides for issuance of a specific waiver when a hardship or other circumstance makes it impractical to comply with a standard. Specific waivers are also required for construction of certain new systems (listed in Sections 75-A.9(a)(2) and 75-A.10(c)) and deviations from Appendix 75-A standards unless a general or local waiver has been issued to a county health department or local government to address these matters. Although specific waivers are not required for correction or replacement of existing failing individual sewage treatment systems, local health departments may elect to issue specific waivers for such systems. All correction or replacement systems should comply with Appendix 75-A standards if possible. Wastewater treatment system expansion to meet an actual or potential occupancy increase (i.e. adding rooms to a residence that will or can be used as bedrooms) shall be in accord with Appendix 75-A requirements.

Many of the suggested designs and construction techniques included in this handbook represent recent improvements in sewage treatment and are the product of intensive nationwide research in this field. Information presented in this handbook reflects the practices and experience of the Department and local health departments, and recommendations of Federal and State Agencies.

Sections 347 and 308 of the New York State Public Health Law give county, part-county and local boards of health authority to enact ordinances and regulations for protection of public health. Many communities have consequently enacted additional ordinances and regulations which must be satisfied before household sewage treatment systems are installed. Persons contemplating construction of these facilities should consult with local authorities to ensure compliance with all existing additional requirements. Watershed rules and regulations must also be met where applicable. Local authorities to be contacted include local municipal Code Enforcement Officers, watershed inspectors and County Health Department staff or State Health Department District Office staff. Local Health Department regulations more stringent than Appendix 75-A requirements (e.g., a larger vertical separation distance between the bottom of absorption facilities and high ground water, bedrock, or impermeable soil) provide enhanced wastewater treatment. If a site meets Appendix 75-A criteria for individual subsurface wastewater treatment system construction and must be modified to meet more stringent Local Health Department regulations, site modification shall be implemented in accordance with applicable Local Health Department regulations (e.g., placement and stabilization of supplemental fill, ground water dewatering/monitoring representative tests for proposed curtain drains.)

Several statewide general waivers have been issued and are addressed in this handbook. General waivers regarding various requirements also have been issued to county health departments and the New York City Department of Environmental Protection to address unique needs and conditions. General waivers are summarized in Table 12 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and must be addressed in the localities affected.

Wherever practical, public sewerage works are recommended for the collection and treatment of household sewage. Approval of individual sewage treatment systems shall only be granted where it has been demonstrated that public facilities are not feasible and where other conditions including soils, topography and geology are suitable.

The Department recognizes that some devices and systems are not addressed in this handbook. Use of new devices and techniques on a limited and monitored demonstration basis may be allowed as a means to accumulate operational experience data. Agreement for such installations shall be obtained from the local health unit having jurisdiction prior to construction. The compilation of operational experience data can assist in the progressive evolvement of systems deemed suitable for use in New York State.

Although many of the principles discussed in this handbook are applicable to larger type sewage treatment systems, the standards of the Department of Environmental Conservation for systems with a wastewater flow of ≥ 1,000 gallons per day (gpd) shall be used.

The terms "shall" and "must" are intended to indicate a requirement. The terms "should," "recommended" and "preferred" are intended to indicate a recommendation, not a requirement.

Individual Residential Wastewater Treatment Systems Design Handbook was prepared by the Department's Bureau of Community Sanitation and Food Protection, Division of Environmental Protection, in conjunction with the NYS Conference of Directors of Environmental Health Services and the cooperation of many active and retired State and local health department environmental health professionals whose contributions are gratefully acknowledged. Special acknowledgement is due to several contributors: J. Cunnan, J. Decker, R. Denz, S. Lukowski, G. Sauda, K. Scheuer, P.J. Smith, R. Stewart, J. Strepelis, R. Svenson, and J. Yavonditte.

Introduction

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Despite the trend toward public sewerage systems, individual household sewage treatment systems still comprise the sole method of sewage treatment available to many New York State residents. It is currently estimated that 1.3 million State residents are served by individual household systems. Additional millions, who frequent summer camps, recreation and tourist facilities in New York State, rely on individual on-site systems for sewage treatment. Many New York State residents and visitors will continue to use individual sewage treatment systems in suburban and rural areas where public sewers are not available or feasible.

Adequate household sewage treatment system standard provide for a safe, sanitary means of treating household wastewater. Many gastrointestinal illnesses can be transmitted by water, food, insects, pets, and toys contaminated by human waste. Properly designed, constructed, and maintained sewage treatment systems minimize the possibility of disease transmission and potential for contamination of ground and surface waters.

The absence of septic odor, sewage overflow, water pollution and other environmental insults caused by malfunctioning treatment systems is best assured by treatment of all sewage in a sanitary manner. Sewage must be treated to assure:

  1. Drinking water supplies will not be contaminated.
  2. A health hazard will not be created as a result of sewage exposed on the ground surface accessible to people or pets.
  3. Waters of any shellfish breeding ground, bathing beach or other recreational area will not be polluted.
  4. A breeding place will not be created for insects, rodents or other possible disease carriers, which may come into contact with food or drinking water.
  5. State laws and local regulations governing water pollution or sewage discharge will not be violated.
  6. A nuisance resulting in obnoxious odors or unsightliness will be avoided.

These criteria can best be met by discharges of household sewage into an adequate public sewer system. Where public sewers are unavailable or unfeasible, the discharge shall be into a properly designed, constructed and maintained individual sewage treatment system.

This handbook and 10NYCRR Appendix 75-A apply to systems receiving domestic-type sewage flows of less than 1,000 gpd. Domestic-type sewage is produced by residential year-round and seasonal dwellings. New York State Department of Environmental Conservation Standards for Wastewater Treatment Works are applicable to domestic-type sewage flows of 1,000 gpd or more. In accord with the State Education Law, plans for individual wastewater treatment systems must be prepared by a design professional licensed to practice in New York State by the State Education Department.

For certain designs, local health department staff should be contacted to obtain additional information, assistance, and available literature regarding recent research and/or wastewater treatment methods. Specialized or critical cases may occur when soil is unsuitable, high ground water/rock/clay are too close to the ground surface, or concern exists over possible well/spring contamination or lake eutrophication. In such cases, professional consultation and special designs may be required.

Construction Safety

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It is recommended that the Public Utilities' Underground Facilities Protection Organization be contacted prior to any excavation to determine the location of any underground utilities in the area and thereby avoid potential hazards and disruption of utility service. UFPO telephone numbers for various project locations are listed below.

Upstate New York1-800-962-7962
Onondaga County1-315-437-7333
Long Island1-516-661-6000
New York City1-800-272-4480

Excavations, such as for seepage pits and septic tanks, may create safety hazards. Experience warns us that depths as shallow as five feet below ground level have caused injury and loss of life. It is the contractor's responsibility to assure that working conditions on the work site are not hazardous to workers or the public. Federal OSHA Construction Standards are applicable to excavations and trenches.

Homeowners constructing/repairing their own systems should be especially careful when working in or near excavations. Excavations should not be left open and unattended. Excavations should be covered, lighted and barricaded/fenced to prevent injury to the public.

Sewage Flows

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The quantity of daily sewage, which a system may be required to accept and treat, is a major concern in the design of household sewage treatment systems. Toilet, bathroom, kitchen, and laundry wastes routinely contribute to this flow. Roof, footing, garage, cellar, and surface water drainage must be excluded from the system. Brine backwash waste from household water softening equipment and backwash waste from iron and/or manganese removal equipment may be discharged to the septic tank of an individual sewage treatment system.

Factors which influence the quantity of daily sewage flow include:

  1. Number of occupants
  2. Number of bedrooms
  3. Garbage grinders
  4. Water pressure
  5. Dishwashers
  6. Automatic clothes washers
  7. Spas, hot tubs, and whirlpool baths
  8. Flow volume of plumbing fixtures
  9. Individual user habits
  10. Leaking faucets and water closets
  11. Prevailing temperature

Expansion attics, basements, sleeping porches, dens, and recreation rooms, which may be converted to additional permanent bedrooms in the future, should be considered in calculating design flow. Considerable variability in sewage flow rate occurs from household to household.

Prior to 1980, toilets routinely used approximately five (5) gallons per flush (gpf) and the sewage treatment system was based upon a design flow of 150 gpd per bedroom or 75 gpd per person. Section 15-0314 of the New York State Environmental Conservation Law requires that all installations of sink faucets, lavatory faucets, showerheads, urinals, and toilets manufactured after January 1, 1980 must meet specific water-saving performance standards. Toilets manufactured from 1980 to 1991 must use no more than 3.5 gpf. Individual sewage treatment systems for dwellings constructed with 1980-1991 reduced flow plumbing fixtures may be based upon a design flow of 130 gpd per bedroom as noted in Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Post 1991 toilets use 1.6 gpf. Prior to 1980, faucets and showerheads frequently used 5 gpm. Post 1979 units meeting the DEC standards use 3 gpm. Time of use and total flow for faucets and showerheads is not controlled by the flow rate standards but some reduction in total flow from these fixtures is expected. Individual sewage treatment systems for dwellings constructed with post 1991 reduced flow plumbing fixtures may be based upon a design flow of 110 gpd per bedroom as noted in Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Although the wastewater flow to the treatment system has been reduced with post 1979 fixtures, the actual biological load to the system remains the same. Therefore, certain design parameters are not reduced when water saving fixtures are used. An example is septic tank size. Water saving fixtures are important for water conservation and should be used where low yield wells and/or limited areas of soil absorption exist (i.e., helpful in addressing existing deficient water sources and/or wastewater treatment.)

Soil and Site Appraisal

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Purpose

A comprehensive soil and site investigation identifies the preferred location for a sewage treatment system and facilitates design of an economical and appropriate system.

The cost of a household sewage treatment system is heavily influenced by prevailing soil and site conditions. If a home must be located on marginal soils, considerable expense will be incurred to construct a treatment system. Sites exhibiting rock outcroppings, high ground water, poor drainage, or steep slopes will require elaborate and expensive subsurface systems if approvable. Slow soil percolation rates require large subsurface absorption areas. Poorly drained sites may require special surface and/or subsurface drainage to prevent periodic failures caused by rising ground water levels or ponding of surface drainage. The solution and control of such problems require consideration of the total drainage area. The State or local highway agency may be helpful in providing an overview of drainage provisions.

Soils with very fast soil percolation rates (i.e. less than one minute per inch) are not suitable for conventional absorption systems unless the site is modified. Fast percolation rate soils do not provide adequate treatment of wastewater because the effluent moves too quickly through the soil and may reach ground water before being fully treated. The installation of a two foot layer of less permeable soil beneath and surrounding the absorption area can provide a soil treatment layer and reduce the rate at which the effluent flows through fast soils.

Site Investigation

Selection of the sewage treatment system location must be an integral part of the initial home site layout and not a simple accommodation. Construction of the home or drilled well should not begin until the sewage treatment system has been properly located and designed. Therefore, a field evaluation of the site and soil should be conducted before purchase of development of the property. Low areas likely to be flooded every ten years or more frequently shall be avoided. Most proposed absorption facilities shall not be located where the final slope of stabilized soil exceeds 15 percent but absorption trench systems with stringent minimum horizontal and vertical separation distances (i.e. 10', 9', 8', or 7' between parallel trenches and 2', 3', 4', or 5' between trench bottom and high ground water, bedrock, or impermeable soil, respectively) may be constructed on sites with in situ soil having a slope of > 15 to ≤ 20 percent and a soil percolation rate of 1 to 60 minutes per inch.

Rock outcroppings serve as a warning that shallow soils are present and may suggest the probable direction of ground water flow. The investigation should indicate the depth of usable permeable soil at the site above rock, unsuitable soil, and high seasonal ground water.

Separation distances between subsurface treatment systems and property boundaries, structures, and facilities are required to maintain system performance, permit repairs, and reduce undesirable effects of underground sewage flow and dispersion. These include property lines, wells, wetlands, water courses, buildings, utilities, and components of subsurface sewage treatment systems. Required separation distances appear in Figures 1 and 2 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Tables 2 and 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

The required infiltrative area is determined from: (a) properly conducted soil percolation and deep hole tests that are fully consistent with the site and soil evaluation; and, (b) projected sewage flow. An additional 50 percent of the required infiltrative area shall also be identified and remain available for future expansion and replacement purposes. It is recommended that the reserved area equal 100 percent of the required infiltrative area to facilitate absorption system replacement when necessary. A properly designed, constructed, and maintained individual sewage treatment system has an average expected useful life of 20 to 25 years. In general, the least complex treatment consistent with soil and site conditions should be selected.

In some areas of New York State, individual sewage treatment systems are restricted by local regulations and watershed rules and regulations. Many county health departments require that plans be submitted and approved before construction starts, installed facilities be inspected prior to backfilling/covering, and completed systems be certified prior to household occupancy. Local sanitary codes and zoning ordinances may control the design and installation of systems, and include regulations pertaining to easements and rights-of-way. Table 12 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) summarizes general waivers applicable in various municipalities. Because of the many regulations which may be in effect, the local health unit should be consulted prior to design and construction of household sewage treatment systems. Appendix 75-A represents the minimum statewide standards for new individual sewage systems.

When a local health unit does not implement an individual sewage treatment program, the local Code Enforcement Officer (CEO) for the Uniform Fire Prevention and Building Code should utilize the Uniform Code's generally accepted standards to ensure compliance with Appendix 75-A standards. Where CEOs have program jurisdiction, plans for any of the conventional systems listed in Section 75-A.8 may be approved. CEOs may approve plans for alternative systems only after receiving an appropriate Local Waiver from the Health Department for one or more alternative systems. Plans for alternative systems shall routinely be approved by the local health department having jurisdiction unless the local health department has issued a local waiver to a local municipality for said system(s).

Separation Requirements

The effluent from a sewage treatment system contains substantial quantities of dissolved nutrients which may eventually reach the ground water. In addition, some chemical contaminants, pathogenic bacteria, and viruses are capable of traveling great distances if they reach the ground water aquifer, especially in creviced and channeled rock. To minimize the possible health hazard and pollution potential of treatment system effluents, subsurface systems should be located above the seasonal high ground water level and as far as possible from drinking water supplies, and surface and subsurface waters. The minimum required horizontal separation distances appear in Figures 1 and 2 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The minimum required vertical separation distances appear in Table 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and site requirements and figures for the various systems addressed in this handbook. The combined horizontal and vertical separation distances for each sewage treatment system are called boundary conditions.

The type and number of sewage treatment systems or other sources of pollution in the vicinity of a well indicate the potential for nearby contamination of the ground water supplying the well. Ground slope and rock outcrops may indicate the probable direction of sewage and ground water flow and the preferred location for a well to avoid potential sewage pollution. Well construction, depth to the aquifer, soil type above the aquifer, volume/rate of water pumped, and well drawdown are also extremely important since they affect the distance and travel time of polluted waters. Usually, pollution of wells is minimized by increased distance and travel time.

When pumping from a well, ground water flow will tend to be toward the well. Since the pumping level of water in the well may frequently be 50 to 150 feet below the ground surface, well pumping may exert an attractive influence on ground water as far as 500 to 1,000 feet away from the well, regardless of the elevation of the top of the well. Therefore, distances to and elevations of sewage treatment systems must be considered relative to the elevation of the water level in the well while it is being pumped. A sewage treatment system located 100 feet away on level ground or down grade from a well may still be 50 feet higher than the pumping water level in the well.

Required separation distances are sometimes impossible to meet for replacement of failing facilities. The existence of an impervious soil stratum between the sewage treatment system and a ground water aquifer used as a drinking water source plus information on well depth, casing depth, well grouting, well water quality and depth to the aquifer should be carefully evaluated when considering any reduction in separation distances. Specific waivers are needed for reductions in boundary conditions for new construction.

Considerable judgment is needed to select a suitable location for a well. The limiting distances noted in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) should be used as a guide for locating wells. Experience has shown that these distances to be reasonable and effective in most instances when coupled with proper interpretation of available hydrologic and geologic data and satisfactory well location, construction and protection. The Department's Handbook, Rural Water Supply, contains additional information regarding well water supplies. Drilled wells should be developed and tested for quality and quantity prior to commencement of home construction.

Soil Investigation

Soils vary widely in character. Most soils are mixtures of stones, sand, silt, clay, and organic matter. Soils usually contain microorganisms capable of breaking down organic matter and macroorganisms capable of assisting in soil aeration. Soil survey maps and reports have been developed for most counties in New York State and are available from the U.S. Soil Conservation Service. While helpful in determining general soil and soil-moisture characteristics and suitability for various uses, the maps/reports are not substitutes for an on-site soil investigation.

The nature of the stratum immediately under the upper soil layer can affect the treatment of wastes. Dense substrata, such as clay, fragipan, shale, argillite, and cemented limestones restrict the limits of vertical movement of wastewater. Highly fractured or channeled rock substrata underlying shallow soil profiles may facilitate such rapid water movement that contamination of ground water, and nearby streams/lakes could occur. Deep test holes or borings are used to determine: (1) the presence/absence of such underlying substrata; (2) high ground water levels; (3) depth to bedrock; (4) types of soils penetrated; and, (5) other features such as root systems, land drains, etc., which may affect the design and operation of a subsurface sewage treatment system. High ground water level can be determined by observing the free water surface in an excavated hole or/plus soil mottling (soil color patterns) in a deep hole during the normal spring high ground water period (i.e., March 15 to June 30), whichever is higher. Information regarding soil mottling/discoloration should be obtained from all deep hole tests, whenever available, to supplement any observed high ground water level. Any determination outside the normal spring high ground water period should include soil mottling/discoloration readings. Assistance from knowledgeable persons experienced in interpreting soil mottling. (e.g., soil scientists, geologists, design professionals) may be helpful in determining depth to high ground water in deep holes. In some gravelly soils, high ground water can only be determined by monitoring a free water surface in an excavated hole during the spring high ground water period.

Gray soil colorations are associated with saturated and chemically reducing conditions and yellowish-brown colorations are associated with aerobic and chemically oxidizing conditions. Soils with high water tables during some part of the year generally exhibit variable coloration (i.e., mottling) at the depth of the high water mark and below. Due to inherent color properties of some soils, it can be extremely difficult to identify mottling.

Site vegetation can also offer clues regarding surface and ground water levels. Recognizing the types of plants which grow on wet soils can help verify the findings of deep hole tests.

A deep percolation test conducted approximately one foot below the bottom of proposed absorption facilities may be used to supplement deep hole observations and verify a soil will transmit and treat wastewater (i.e., is usable).

The depth of a deep test hole is determined by the type of absorption system to be installed. If a shallow subsurface system such as an absorption field or absorption bed system is proposed, at least four feet o f usable soil shall be available above impermeable strata or the high ground water level. The four foot depth determination provides a minimum separation of two feel beneath the bottom of an absorption trench/bed. Deep test holes for proposed absorption fields shall be at least six feet deep (i.e., preferably four feet deeper than the bottom of proposed absorption trenches/bed) to facilitate observation of soil mottling/discoloration on the sidewalls of the hole and other boundary conditions. At least one deep test hole shall be dug within or immediately adjacent to the proposed absorption area to assure that uniform soil and site conditions prevail. If observations of deep test holes, percolation tests holes, excavations, grading cuts, etc. reveal widely varying soil profiles, additional deep test holes shall be dug and observed to assure that a sufficient area of usable soil is present for the installation of the proposed absorption facility. At least one deep test hole may also be required to be dug within or immediately adjacent to the proposed absorption expansion area to assure that uniform soil and site conditions prevail. Treatment systems shall be designed to reflect the most severe conditions observed within the proposed absorption field.

Three feet of usable soil must exist beneath the bottom of seepage pits (i.e., above ground water, bedrock or impervious strata) since pits provide less treatment than absorption trenches/beds. Deep test holes for proposed seepage pits shall be at least five feet deeper than the bottom of proposed seepage pits to facilitate observation of soil mottling/discoloration on the sidewalls of the hole and other boundary conditions. At least one deep test hole shall be dug at the proposed location of or immediately adjacent to the proposed seepage pits to assure that uniform soil and site conditions prevail. If observations of deep test holes, percolation holes, excavations, grading cuts, etc. reveal widely varying soil profiles additional deep test holes shall be dug and observed to assure that a sufficient area of usable soil is present for the installation of the proposed seepage pits. The effective seepage pit sidewall absorption area comprises only soils with a percolation rate of one to sixty minutes per inch. No allowance for seepage pit infiltration area is made for the bottom area of a pit. Any bottom area or sidewall soil with a percolation rate faster than one inch per minute precludes use of the site unless soil blending produces at least three feet of filtration through blended soil with a percolation rate of one to 60 minutes per inch.

Where absorption systems are to be installed above drinking water aquifers, a greater separation distance to bedrock (e.g., limestone, karst, shale) may be required by the local health department without jurisdiction. Absorption systems should not be constructed directly over visible cracks, crevices, sinkholes, etc., in such formations.

Seasonal weather variations markedly affect ground water levels. Heavy spring rains combined with annual snow melt in New York State normally raise groundwater to its high level between March 15 and June 30. Cycles of drought and flooding (i.e. less than and greater than average precipitation) obviously influence the "high ground water level" reached during any particular year. Hence, information regarding soil mottling/discoloration should be obtained from all deep hole tests, whenever available, to supplement any observed high ground water levels (i.e. free water surface in an unlined hole). Periodic observations of shallow monitoring wells throughout the normal spring high ground water period produces a very accurate determination of the high water table at a given site for a given year.

Another item used in subsoil investigation is soil texture. This refers to the proportion of sand, silt and clay. Soil textural classifications noted by the U.S. Department of Agriculture, Soil Conservation Service are depicted in Table 7 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The following descriptions can be used in identifying soil texture:

  • Sand: Individual grains can be seen and felt readily. Squeezed in the hand when dry, this soil will fall apart when the pressure is released. Squeezed when moist, it will form a cast that will hold its shape when the pressure is released but will crumble when touches.
  • Sandy Loam: Consists largely of sand, but has enough silt and clay present to give it a small amount of stability. Individual sand grains can be seen and felt readily. Squeezed in the hand when dry, this soil will fall apart when the pressure is released. Squeezed when moist, it forms a cast that will not only hold its shape w hen pressure is released, but will withstand careful handling without breaking. The stability of the moist cast differentiates this soil from sand.
  • Loam: Consists of an even mixture of the different sizes of sand and of silt and clay. It is easily crumbled when dry and has a slightly gritty, yet fairly smooth feel. It is slightly plastic. Squeezed in the hand when dry, it will form a cast that will withstand careful handling. The cast formed of moist soil can be handled freely without breaking.
  • Silt Loam: Consists of a moderate amount of fine grades of sand, a small amount of clay, and a large quantity of silt particles. Lumps in a dry, undisturbed state appear quite cloddy but they can be pulverized readily; the soil then feels soft and floury. When wet, silt loam runs together and puddles. Either dry or moist casts can be handled freely without breaking. When a ball of moist soil is pressed between thumb and finger, it will not press out into a smooth, unbroken ribbon but will have a broken appearance.
  • Clay Loam: A fine textured soil which breaks into clods or lumps that are hard when dry. When a ball of moist soil is pressed between the thumb and finger, it will form a thin ribbon that will break readily, barely sustaining its own weight. The moist soil is plastic and will form a cast that will withstand considerable handling.
  • Clay: A fine-textured soil that breaks into very hard clods or lumps when dry, and is plastic and unusually sticky when wet. When a ball of moist soil is pressed between the thumb and finger, it will form a long ribbon1.
    (1 Soil Survey Manual, Handbook No. 18, USDA, Washington, D.C., August 1951)

The problem of identifying soil texture occurs when the soil is within the loam range. What is the percentage of sand, silt, clay? Training and experience are needed to make reasonable estimate.

Structure is also a characteristic used in soil investigation. A moist or dry soil mass in its natural state tends to break into pieces of a rather definite shape resembling a geometric figure or some other material. Thus a soil may have a prismatic, block, granular, crumb or platy structure. Structure is indicative of drainage characteristics and is used to determine the limits of soil horizons. Soil structure should not be confused with the structural (strength) characteristics of a soil.

Infiltration and percolation govern the absorptive capacity of soil. Infiltration is the passage of liquid across the liquid-soil boundary or interface and percolation is the passage of liquid through soil once it has crossed the interface. Soils with high clay content may not allow adequate passage of liquid and, therefore, are generally unsuitable for subsurface treatment. Usually, the coarser the soil particles, the faster the percolation. If groundwater levels are high, even soil of high permeability will not allow sufficient liquid to percolate.

The smearing or compaction of trench and seepage pit sidewalls or bottom during excavation and construction will severely restrict infiltration. Low capacity for either infiltration or percolation may cause a household sewage system to fail. A poorly operating septic tank system may cause physical, chemical and/or biological clogging at the liquid-soil interface and restrict infiltration.

Soil Percolation

Percolation tests should be conducted by persons with training/experience in conducting such tests. They include but are not limited to design professionals (i.e. engineers and architects), surveyors, sanitarians, soil scientists, technicians, and system installers.

Soil percolation test results are indicative of the ability of a soil to absorb treated sewage. If the percolation test results are inconsistent with field determined soil conditions, additional percolation tests must be conducted and the more restrictive test results must be used for the system design. Percolation tests may be conducted anytime except when the ground is frozen or precipitation interferes with the test (i.e., adds water to the test hole.)

If a conventional absorption system is planned, at least two percolation tests shall be performed within the proposed absorption area with the bottom of the test holes at 24 to 30 inches below grade. The slowest percolation test results (i.e., worst case observed) shall be used to design the absorption facilities. At least one percolation test may also be required to determine if the soil in the proposed expansion area soil is usable.

At least two percolation tests shall be performed in any proposed seepage pit area with the bottom of the percolation tests holes at the proposed pit depth and half the proposed pit depth. If different soil layers are encountered at the proposed pit sidewall area, a percolation test shall be conducted in each permeable layer and the applicable pit design percolation rate shall comprise the weighted average of each test result based upon the depth of each permeable layer. No allowance for infiltration area is made for the bottom area of a pit or the pit sidewall area of impervious strata (i.e., percolation rate slower than 60 minutes/inch.)

If a deep absorption trench system is planned, at least two percolation tests shall be performed within the proposed absorption field with the bottom of the test holes at the depth of the proposed trenches. If a shallow absorption trench system is proposed, at least two percolation tests shall be performed within the proposed absorption field with the bottom of the test holes at the depth of the proposed trenches or at six inches below grade if the bottom of the proposed trenches will be between grade and six inches below grade. The slowest percolation rate observed shall be used to design the absorption facility.

Where absorption facilities are to be constructed in fill or disturbed soils, the soil shall be permitted to stabilize by natural settlement for a period of at least six months, including a freeze-thaw cycle, before in situ percolation and deep hole tests are performed. If the site to be modified and any fill comprise only permeable granular material (e.g., sand, sand and gravel, or sandy loam similar to fill material for mound systems with a percolation rate of ≤ 30 minutes per inch), stabilization may be achieved by mechanical compaction in approximately six inch lifts. Mechanical compaction shall be achieved via track type machines (e.g., bulldozer or front end loader with downward blade/bucket pressure) or steel wheeled roller. All nongranular soils (e.g., silt loam, clay loam, silt, clay) require natural settlement to achieve stabilization. Fill material to be used in a mound system shall undergo percolation tests at the borrow pit and exhibit a percolation rate of 5 to 30 minutes per inch.

Heavy construction equipment shall not be used in and immediately downslope of raised or mound system areas to avoid compaction of the native soil (i.e. reduction in permeability). Areas to be used for an absorption system should be disturbed as little as possible. When a raised or mound system is planned, percolation and deep hole tests should be performed within the estimated basal area of the raised or mound system.

The procedure noted below should be followed in performing a soil percolation test:

  1. Make sure proper construction safety practices are followed.
  2. Dig a hole with vertical sides approximately 12 inches wide on all four sides or 12 inches in diameter. If an absorption field is being considered, the depth of the test holes should be 24 to 30 inches below final grade or at the projected bottom of trenches in shallower/deeper systems. If a seepage pit must be used, percolation tests should be conducted at one-half the depth and at the full estimated depth of the seepage pit. In order to facilitate conducting the test and preventing cave-in, a two-tiered excavation should be made approximately two feet above the bottom of the proposed seepage pit and two feet above the half-depth of the proposed seepage pit. Percolation test pits should be dug approximately two feet deep into each tier base. It is necessary to place washed aggregate in the lower two inches of each percolation test hole to reduce scouring and silting action when water is poured into the hole. The sides of percolation holes should be scraped to avoid smearing. Figure 3 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) depicts a soil percolation test.
  3. Pre-soak the test hole by periodically filling the hole with water and allowing the water to seep away. This procedure should be performed for at least four hours and should begin one day before the test, except in clean, coarse sand and gravel. After the water from the final pre-soaking has seeped away, remove any loose soil that has fallen from the sides of the hole.
  4. Pour clean water into the hole, with as little splashing as possible, to a depth of six inches above the bottom of the test hole.
  5. Observe and record the time in minutes required for the water to drop from the six inch depth to the five inch depth.
  6. Repeat the test a minimum of three times until the time for the water to drop from six inches to five inches for two successive tests is approximately equal (i.e., ≤ 1 min. for 1 – 30 min./inch, ≤ 2 min. for 31-60 min./inch). The longest time interval to drop one inch shall be taken as the stabilized rate of percolation and shall serve as t he basis of design for the absorption system.

For example, assume the following times were recorded while performing steps (d), (e), and (f) noted above:

Run NumberTime for One Inch Drop (Minutes)
124
227
330
430

The stabilized percolation rate is 30 minutes per inch. Percolation rates, projected flow rates and observed boundary conditions coupled with Tables 4, 5, and 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) are frequently used to design needed absorptive facilities.

House or Building Sewer

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General Information

The interior house plumbing extending through the foundation wall is called the house or building drain. The pipe connecting the house or building drain to the subsurface sewage treatment system is called the house or building sewer (see Figure 4 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). The house drain and house sewer are designed to convey sewage to the septic tank at sufficient velocity to prevent settling of solids in the drain/sewer. They also enable gases from the septic tank to be vented to the atmosphere through the soil stack and vent stack. Plumbing for residences served by individual wastewater treatment systems shall be installed in a manner to avoid interference with the flow of gases and air from the absorption area, distribution box, and septic tank through the soil stack and vent stack. Where local code requires the use of building traps (i.e., on the building drain), vent piping should be installed on the building drain from a point immediately downstream of the building trap to the vent stack.

House sewers shall be of sound, durable material, of water-tight construction, have a minimum diameter of four inches, and be laid on a firm foundation at a minimum grade of one-quarter inch per foot. House sewers should be installed with as straight an alignment as possible. If bends are necessary, a maximum bend of 45ºF shall be used and fitted with a clean-out of the same size as the sewer. The clean-out should be extended to the ground surface and properly capped/plugged for maintenance purposes. At least one clean-out with a properly fitted plug is required on the house drain within the building to provide access to the house sewer.

House sewer construction including materials shall comply with applicable requirements contained in Parts 903 through 907, inclusive, and Part 1250 of the State Uniform Fire Prevention and Building Code (9NYCRR).

Water Line – House Sewer Separation

A minimum horizontal separation of 10 feet should exist between the house sewer and any water line. Where lines must cross, the water service line shall be at least 12 inches above the house sewer. If a water line must pass below a house sewer, the vertical separation must be at least 18 inches and the sewer materials shall be water main pipe or equivalent and shall be pressure tested to ensure water tightness. At crossings, water and sewer pipe joints shall be installed as far as practicable from the crossing (i.e., with full lengths of both water pipe and sewer pipe.) Suction water lines shall never cross under house sewers or any other component of a sewage treatment system.

Septic Tanks

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General Information

Raw sewage from a house sewer must undergo treatment prior to discharge to an absorption treatment system. Devices such as septic tanks or aerobic units provide varying degrees of physical and biological treatment.

Physical treatment is generally restricted to processes dependent upon the density of sewage components. Settling chambers reduce turbulence and velocity of sewage and permit separation of most floating and settleable solids from the wastewater. Materials heavier than water settle to the bottom and form sludge. Materials lighter than water rise to the surface and form scum.

Two groups of bacteria, aerobic and anaerobic, provide biological treatment of sewage. Aerobic bacteria degrade organic matter in the presence of air or oxygen. Anaerobic bacteria perform a similar function in the absence of elemental oxygen but at a slower rate.

Septic tanks are large, watertight chambers which promote the growth of anaerobic bacteria for the biological decomposition of sewage. Septic tanks should be sized for a minimum detention time of 36 hours and are constructed with an inlet, multiple baffles or sanitary tees, and an outlet to assure separation of floating and settleable solids and retention of scum and sludge. Neither scum nor sludge should be scoured from the septic tank by sewage flowing through the tank.

Performance of a septic tank can be improved by outlet modifications and compartmentalization of the tank. Rising gases routinely produced by anaerobic digestion of organic matter in septic tanks interfere with particle settling and cause resuspension of previously settled solids (i.e., sludge). Outlet baffles/tees should be equipped with a gas deflection device to minimize the flow of such particles/solids out the effluent pipe. Increasing the diameter of the vertical section of outlet sanitary tees to more than four (4) inches is recommended to decrease upflow velocity and potential discharge of suspended solids to the absorption system. Use of an outlet filter also minimizes flow of particles/solids to the absorption facility. Compartmentalization via multiple chambers or tanks in series results in improved retention of floatable and settleable solids. The second chamber/tank has reduced turbulence, velocity, and instantaneous flow rates than would occur in a single compartment tank. Tank compartmentalization and outlet modifications reduce clogging of the absorption system and are recommended. See Figures 5 and 7 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Pumped-out septic tanks (i.e., periodic maintenance) frequently contain toxic gases and shall not be entered by a homeowner. Only trained persons utilizing oxygen breathing apparatus and using the buddy system should attempt to enter or repair a pumped-out septic tank. If a leak below the liquid level cannot be repaired or sealed, the tank must be replaced.

All toilet, bathroom, kitchen, and laundry wastes from a household shall be discharged into the septic tank. Brine backwash waste from household water softening equipment may be discharged into the septic tank. Household chemicals such as bleaches, disinfectants, cleansers, etc., when used in normal household applications should not disrupt septic tank or absorption system operation. Roof, footing, garage, cellar, surface and cooling water must be excluded from septic tanks. Materials not readily degraded (e.g., paper towels, newspaper, wrapping paper, rags, sanitary napkins, disposable diapers, coffee grounds. cooking fats/oils, bones, facial tissues, and cigarette butts) should not be flushed into septic tanks. These products do not degrade in the tank and can clog inlets, outlets, and the absorption system. Examples of other products, which should not be discharged into septic tanks include antifreeze, pesticides, herbicides, oil, gasoline, paint, turpentine, and concentrated acids or alkalies (e.g., suIfuric acid or sodium hydroxide).

Septic tanks are designed to handle all the normal daily flow which a household can produce. For this reason, design should be based upon the maximum capacity of a home rather than its number of inhabitants at any particular time.

Although minimum capacities for septic tanks are established in Table 3 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook), larger units have many advantages. Longer detention times, due to increased capacity, permit improved separation of floatable and settable solids and improved retention of scum and sludge, which prolong the life of the absorption system. Larger tanks require less frequent cleaning and also accommodate expansion of the home or the addition of a garbage grinder. Larger tanks provide a good cost-benefit return.

Table 3 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) specifies minimum septic tank capacities based upon the number of bedrooms. Expansion attics shall be counted as an additional bedroom. A garbage grinder or hot tub/spa shall/should be considered equivalent to an additional bedroom, respectively, for determining tank size. Gas deflection baffles are strongly recommended for tank outlets to minimize solids carry-over from the septic tank to the absorption system. A gas deflection baffle or other acceptable outlet modification, and a dual compartment tank or two tanks in series must be provided when a garbage grinder can reasonably be expected at the time of construction or in the future. A gas deflection baffle or an outlet sanitary tee shall be provided whenever any full width outlet baffle exists in a septic tank to minimize solids carryover to the absorption area.

Location

Septic tank access covers shall always be accessible. Where manholes or removable covers are more than 12 inches below final grade, an extension collar shall be provided over each opening. Extension collars shall not be brought flush with or above the ground surface unless the cover can be locked to prevent tampering especially by children. Driveways, parking lots, etc., shall not be constructed above septic tanks unless the tanks are specially designed and reinforced to safely carry the loads imposed. Objects, such as swimming pools, shall not be constructed above septic tanks since they interfere with tank operation and maintenance.

Minimum separation distances of septic tanks from wells, watercourses, building foundations, property lines, drainage ditches, etc., must be maintained as indicated in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook), Figure 1 and Figure 2 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

A sketch or plan of the as-built sewage treatment system should be retained by the homeowner for future inspection and maintenance. The sketch/plan should indicate measured distances from system components (i.e., septic tank manholes, distribution box, corners of the tile field) to relatively permanent points (i.e., corners of house foundation, property stakes, street pavement/curbing, telephone or electrical poles. etc.) The location of each access cover or manhole for the septic tank should be identified by installing a stake from grade toward the cover/manhole. Such stakes permit rapid location for inspection/maintenance with minimal landscape disturbance (See Figures 4 and 5 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)).

Design and Installation

The following general requirements apply to all septic tanks regardless of construction material:

  1. A minimum liquid depth of 30 inches. The maximum depth for determining the allowable design volume of a tank shall be 60 inches. Deeper tanks provide extra sludge storage but no credit is given toward design volume.
  2. The minimum distance between the inlet and outlet shall be six feet. All tanks shall meet the minimum surface area requirement for the specific design volume in Table 3 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The effective length of rectangular tanks should not be less than two nor greater than four times the effective width.
  3. Tanks must be watertight, constructed of durable material, and not subject to excessive corrosion, decay, frost damage, or cracking. When installed, the top of all tanks shall be able to support at least 300 pounds per square foot (psf).
  4. Tank access covers and manhole covers shall be within 12 inches of final grade to permit inspection and maintenance. Tanks shall have at least one manhole opening and visual access openings above the inlet and outlet baffles. A manhole opening may replace a visual access opening. Tanks with a liquid depth of 48 inches or more shall have atop opening with a minimum of 20 inches in the shortest dimension to allow entry into the tank. Tanks with a liquid depth less than 48 inches shall have a top opening that is at least 12 inches in the shortest dimension. When the top of a septic tank is more than 12 inches below final grade, watertight extension collars shall be used to bring access covers and manhole covers within 12 inches of final grade. Septic tank access covers located at or above grade should be lockable to prevent entry by unauthorized persons, especially children.
  5. Tanks shall have inlet and outlet baffles, sanitary tees or other devices to prevent the passage of floating solids and to minimize disturbance of settled sludge or floating scum by sewage entering and leaving the tank. Outlet designs incorporating gas bubble deflection (i.e., gas deflection baffles) are strongly recommended to minimize solids loading of the absorption system. Inlet and outlet baffles shall extend a minimum of 12 inches and 14 inches, respectively, below the liquid level in tanks with a liquid depth of less than 40 inches, and 16 and 18 inches, respectively, in tanks with a liquid depth of 40 inches or greater. The horizontal distance between the outlet baffle and the outlet shall not exceed six inches. Baffles shall be constructed of a durable material not subject to excessive corrosion, decay, or cracking. Increasing the diameter of the vertical section of outlet sanitary tees to more than four (4) inches is recommended to decrease upflow velocity and potential discharge of suspended solids to the absorption system.
  6. There shall be a minimum of one inch clearance between the underside of the roof of the tank and the top of all baffles, and/or tees to permit venting of tank gases. Multi-chamber and multi-tank systems shall also be designed to permit venting of tank gases.
  7. Tanks shall be placed on at least a three inch bed of sand or pea gravel. This will provide for proper leveling and bearing. A five inch bed of aggregate (3/4 to 1 ½ inches in diameter) may be used in-lieu-of the required three inch bed of sand or pea gravel. Any additional instructions provided by the tank manufacturer shall also be followed.
  8. There shall be a minimum drop in elevation of two inches between the inverts (bottom of inside of pipe) of the inlet and outlet pipes.
  9. Garbage grinders. An additional 250 gallons of capacity and seven square feet of surface area are required when a garbage grinder can reasonably be expected at the time of construction or in the future. A gas deflection baffle or other acceptable outlet modification (e.g., gas baffles) and a dual compartment tank or two tanks in series must also be provided.
  10. Septic tanks may be forced toward the ground surface during cleaning or dewatering operations if they have been installed within the ground water zone. This is caused by the buoyancy effect of the displaced volume of the tank. Septic tanks should not be completely dewatered if ground water levels are significantly higher than the bottom of the tank unless said tanks are properly anchored. Tanks constructed of fiberglass, plastic, or steel are more likely to float than reinforced concrete tanks because of their lighter weight per given volume.
  11. Special care must be taken in bedding the house sewer, septic tank, and outlet line to prevent uneven settlement and possible cracking or rupture where the inlet and outlet lines connect to the septic tank.

Multi-compartment Tanks or Tanks In Series

  1. Dual compartment tanks or two tanks in series are recommended for all systems and shall be required when (1) tanks have an interior length of ten feet or more, (2) a mound system is proposed, (3) a sand filter system is proposed, or (4) a garbage grinder can reasonably be expected to be used at the time of home construction or in the future.
  2. The first compartment or tank (inlet side) shall account for 60 to 75 percent of the total design volume.
  3. The baffle separating the tank compartments shall extend from the bottom of the tank to at least six inches above the invert of the outlet pipe. The baffle separating the tank compartments shall terminate at least one inch below the underside of the tank roof to permit venting of tank gases.
  4. Flow between compartments shall be through a four inch vertical slot at least 18 inches in width, a six inch elbow, or two 4-inch elbows. The invert of the slot or elbow(s) shall be located at a distance below the liquid level equal to one-third the distance between the invert of the outlet and the bottom of the tank. Four or more 4-inch diameter holes through the dual chamber baffle may be used in-lieu-of the 4 x 18 inch slot with all inverts at the same elevation as the slot invert.
  5. Each compartment shall have at least one manhole opening and a visual access opening above the inlet/outlet baffle. A manhole opening may replace a visual access opening. A manhole opening above the inlet/outlet baffle satisfies the requirement.
  6. The volume and surface area needed to meet the requirements of Table 3 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be based upon the total volume and surface areas of all the tanks and chambers.
  7. Tanks in series shall have a minimum drop in elevation of two inches between the inverts of the inlet and outlet pipes within each tank. The tanks should be connected by a single pipe with a minimum diameter of four inches and a minimum slope of 1/32 inch per foot.

Precast Reinforced Concrete Tanks

  1. Concrete shall have a minimum compressive strength of 2,500 pounds per square inch (psi) at 28 days set; 3,000 psi concrete is recommended.
  2. Wall thickness shall be a minimum of three inches unless the design has been certified by a New York State licensed professional engineer as complying with all appropriate requirements for thin-wall construction. All walls, floor, roof, and access covers shall contain reinforcing to assure support for 300 psf.
  3. All joints shall be sealed such that the tank is watertight.
  4. Tanks with a joint below the liquid level must be tested for watertightness prior to backfilling.

Cast-in-place Concrete Tanks

  1. Concrete shall have a minimum compressive strength of 2,500 psi at 28 days set. 3,000 psi concrete is recommended.
  2. The walls and floors shall be poured at the same time (monolithic pour).
  3. The walls, floors, and roof shall be at least three (3) inches thick with adequate reinforcing to assure support for 300 psf. Unreinforced walls and floor shall be a minimum thickness of six (6) inches.
  4. Access covers shall contain reinforcing to assure support for 300 psf.

Fiberglass and Polyethylene Tanks

  1. All walls, floor, roof and access covers shall assure support for 300 psf.
  2. Installation shall not occur in areas where the ground water level can rise to the level of the bottom of the septic tank.
  3. Particular care must be taken during installation, bedding, and backfilling of these tanks to prevent damage to the tanks. Manufacturer's installation instructions shall be followed. These tanks are sometimes selected for installation in hard to reach sites due to the tank's light weight.
  4. All tanks should be sold by the manufacturer completely assembled. If, because of size, a tank is delivered to a site in sections, all joints shall be sealed with watertight gaskets and shall be tested for watertightness after-installation, and prior to complete backfilling.
  5. Inlet and outlet baffles or sanitary tees should be installed by the manufacturer or supplier.

Steel Tanks

  1. All tanks must have a label indicating corrosion protection complying with Underwriters Laboratories, Inc., Standard UL-70 or equivalent.
  2. Any damage to the interior or exterior tank coating must be refinished with an equivalent coating material prior to placement/backfilling since unprotected steel surfaces deteriorate rapidly from corrosion.
  3. All walls, floor, roof and access covers shall assure support for 300 psf.

Aerobic Units

Aerobic units comprise a watertight compartment with a pump, air compressor, or other device to inject air into the sewage in the compartment. The injected air stimulates multiplication of aerobic bacteria and results in improved biological decomposition of organic matter. Aerobic units are generally classified Class I or II in accordance with the National Sanitation Foundation (NSF) Standard 40. Class II units are not acceptable since they may occasionally discharge quantities of scum or sludge, which can easily plug an absorption system during "upsets." Class I units routinely produce a better quality effluent with lower concentrations of B.O.D. and S.S. than a septic tank and generally contain an outlet modification to prevent scum or sludge from exiting the unit during upsets. The filter on the effluent of a Class I unit should not be removed until the unit is pumped to avoid carryover of solids to the absorption area. The effluent filter must be in place before returning the unit to service. Only Class I units may be used in New York State. The volume of liquid wastewater produced by aerobic units is equal to the volume produced by a properly sized septic tank. Aerobic units are generally more expensive than a properly sized septic tank and require electrical power to continuously operate the pump, air compressor, or other device. Examples of aerobic units are depicted in Figures 8 and 8A (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

An aerobic unit may be installed instead of aseptic tank under the following conditions:

  1. The unit shall have a label indicating compliance with the standards for a Class I unit as described in the NSF Standard 40 or equivalent.
  2. The rated capacity of the unit shall be equal to or greater than the design flow as determined from Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).
  3. The absorption system that follows the unit shall be sized in the exact same manner as it would for a septic tank system.
  4. Units must include as a standard feature a service contract which provides, as a minimum, semi-annual inspections and annual pumping for three years or more. In addition, a service contract shall be in effect throughout the useful life of the unit (i.e., a series of uninterrupted service contracts).
  5. The surface discharge of aerobic unit effluent is strictly prohibited. Aerobic units are sometimes selected to provide improved wastewater treatment as a mitigative measure for replacement systems (i.e., when available horizontal separation distances for absorption facilities do not meet Appendix 75-A Table 2 values.)

Operation and Maintenance

The best designed and installed septic tank system will eventually fail to function properly without periodic maintenance. When failures occur, immediate repairs are essential to eliminate a potential health hazard and aesthetic nuisance due to sewage overflow or backup in the plumbing. Repair of a failed system is usually costly and may far exceed the cost of constructing the initial system. Evaluating and correcting system failure is addressed in Tables 13 and 14 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

The result of inadequate septic tank maintenance can be clogging of the tank and sewage backup into the home and/or sewage overflow onto the ground surface. Failure to periodically clean a septic tank commonly results in clogging of soil surrounding the absorption field by overflowing solids not removed by the septic tank. When this occurs, it is usually necessary to abandon the absorption field and construct a new one at great expense and inconvenience. Other possible causes of failure include use of "septic tank additives," a change in ground water level, water line leaks, excessive water usage, a change in surface water drainage, tank baffle failure, or flushing materials not readily degraded or harmful products (i.e., as noted under General Information) into septic tanks.

Seeding new septic tanks with sludge is not necessary since adequate bacterial activity will commence promptly after sanitary wastes enter the tank.

Septic tanks should be inspected annually to determine scum and sludge accumulation. Most tanks should be pumped out every two to three years. Septic tanks must be pumped out whenever the bottom of the scum layer is within three inches of the bottom of the outlet baffle or sanitary tee or the top of the sludge is within ten inches of the bottom of the outlet baffle or sanitary tee as shown in Figure 9 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). A pole wrapped with toweling and a four inch board attached to the bottom can be employed to measure scum and sludqe clearance from the bottom of the outlet baffle or sanitary tee (see Figure 9 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). The pump-out clearances also apply to any chamber in multi-compartment tanks and to any tanks in series (i.e., pump out all tanks/chambers as soon as any tank/chamber fails the minimum clearance).

In addition to inspecting for sludge and scum accumulation, the septic tank, baffles/tees, house sewer connection, and tank outlet pipe should also be inspected. Some concrete baffles deteriorate over time. Baffles/tees which have deteriorated and no longer perform as designed must be replaced. Occasionally, winter ground frost will displace the house sewer as it enters the septic tank or the effluent line between the septic tank and distribution box causing breakage of the lines at the septic tank wall. Cracked or broken lines must be repaired or replaced.

Cleaning is usually accomplished by pumping the contents of the septic tank into a tank truck which is operated by a commercial septic tank cleaning service. Only Department of Environmental Conservation permitted septage haulers shall be engaged to pump out and dispose of septic tank contents. Septic tanks should not be washed or disinfected after being pumped out. A small quantity of sludge should be left in the tank to encourage continued bacteriological activity.

Septic tank additives are not recommended. Additives are unnecessary to the proper operation of household systems and may cause the sludge and scum in the septic tank to be discharged into the absorption field, resulting in premature failure. Some additives may actually pollute groundwater.

Waste brine from household water softener units has no adverse effect on the operation of a septic tank but may cause a slight shortening of the life of an absorption facility installed in a structured clay type soil. Hence, brine backwash waste from household water softening equipment should be discharged to the septic tank of an individual sewage treatment system. In areas with structured clay type soils, the backwash waste may be discharged to a separate absorption facility (absorption field or seepage pit) sized to handle the entire backwash volume. Separation distances required for conventional absorption facilities shall be met for the backwash waste absorption facility.

Septic tanks are capable of handling the normal production of household grease and fat without requiring a grease trap. Grease traps require frequent cleaning. When grease traps are used, they shall be installed to handle only waste from grease generating fixtures (i.e., a kitchen sink) and sized to handle one half-day flow to assure proper cooling of the waste in the trap and retention of the grease. The grease trap effluent shall be discharged to the septic tank. Unless the amount of grease and fat discharged is unusually large, such as in a restaurant, grease traps are not recommended for household wastes.

Whenever septic tanks are to be abandoned (i.e., when public sewers are installed to handle household wastes), the tanks shall be removed or pumped out and refilled with soil to prevent future cave-ins.

Spas, hot tubs and whirlpool baths are sometimes installed in residences served by on-site wastewater treatment systems. Rapid draining of these units, which may contain a few hundred gallons of water, can interfere with the proper operation of a septic tank (i.e., separation of floating and settleable solids from the wastewater). Draining should be controlled via the drain pump/valve to no more than 5 gpm to minimize undesirable impacts upon the on-site wastewater treatment system. Misuse can result in premature failure of the absorption system due to carryover of solids from the septic tank.

Distribution Devices

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General Information

Septic tank or aerobic unit effluent is usually conveyed to multiple absorption facilities (i.e., laterals, seepage pits). For the treatment system to function properly, the septic tank/aerobic unit effluent must be equally distributed to each lateral or seepage pit utilizing properly designed distribution devices. Several types of distribution devices may be used to perform this function. Distribution boxes (see Figure 10 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)) are most commonly used in conjunction with absorption fields and seepage pits. Distribution boxes may be used on sloped sites provided the inverts of the outlets are all at the same elevation and the first ten feet of outlet lines have the same slope or speed levelers are used. Drop manholes with distribution lines to absorption trenches (see Figure 12 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)) and serial distributors with elbow sections (see Figure 11 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)) may be used with serial absorption trenches on moderate to steep slopes.

Gravity Distribution

The maximum length of absorption lines used in conjunction with gravity distribution shall be 60 feet. Gravity perforated distribution lines shall be installed with a slope of 1/16 to 1/32 inch per foot. The inverts of perforated distribution lines shall not be installed deeper than 24 inches below grade.

Distribution Box

A distribution box is used to evenly distribute settled sewage to subsurface absorption laterals and seepage pits. Distribution boxes should be inspected annually to assure that: (a) all outlet inverts are at the same elevation; (b) excessive solids are not flowing out of the septic tank or aeration unit; and, (c) any required baffle is in place as designed. For accessibility, it is necessary that the distribution box be located and have a removable cover not more than 12 inches below grade. Where, due to site conditions, a distribution box must be more than 12 inches below grade, an extension collar shall be installed to have the cover within 12 inches of grade. The location of distribution box covers should be identified by installing a location stake from grade toward the cover. Such stakes permit rapid location for inspection/maintenance with minimal landscape disturbance.

To minimize frost action and reduce the possibility of movement once installed, distribution boxes must be set on a bed of sand or pea gravel at least 12 inches deep. A 12 inch bed of aggregate (3/4 to 1 ½ inches in diameter) may be used in-lieu-of the required 12 inch bed of sand or pea gravel if speed levelers are used on all outlets. The drop between inlet and outlet inverts shall be at least two inches. A baffle is required at the inlet side of the box when the slope of the pipe from the septic tank to the box exceeds ½ inch per foot or when siphon dosing is used. A partially truncated short sanitary tee with the base toward the inlet open or containing perforations may be used as a baffle since it minimizes short-circuiting and enables absorption field gases to flow back to the septic tank and thence up the soil slack. When such short sanitary tees are used, a minimum of one inch clearance between the underside of the distribution box cover and the top of the sanitary tee shall be provided to permit venting of absorption facility gases.

The inverts of box outlets shall be at least two inches above the bottom of the box to prevent short circuiting and reduce solids carry-over. Use of adjustable outlet levelers is recommended in distribution boxes.

Distribution boxes may be constructed in place or purchased prefabricated. When concrete is used to construct boxes, it shall have a minimum compressive strength of 2,500 psi at 28 day set. Prefabricated boxes may be constructed of concrete, fiberglass or plastic. The boxes shall be installed in conformance with the manufacturer's instructions in addition to the above-noted requirements.

Non-perforated pipe shall be used to connect the distribution box to the absorption facility. The non-perforated pipe shall have a minimum slope of 1/32 inch per foot and be of tight joint construction on undisturbed earth or properly bedded throughout its length.

Serial Distribution

Serial distribution comprises flooding and sequential failure of absorption trenches on sloped sites with the uppermost trench failing first. Serial distribution is acceptable for use with dosing systems only. It is the least desirable of all methods since individual laterals cannot be periodically rested and settled sewage is not uniformly distributed to all laterals.

Connections between distribution lines shall be non-perforated pipe of tight joint construction on undisturbed earth. Connections between successive pairs of distribution lines (i.e., 1 and 2, 2 and 3, etc.) shall be as far from each other as practicable to prevent short circuiting. The invert of the connection pipe exiting each trench shall be at least 12 inches below existing and final grade. The invert of the connection pipe exiting the uppermost trench shall be at least 4 inches lower than the septic tank or aerobic unit outlet invert.

Drop Manholes

Drop manholes are used on sloping sites to reduce the velocity of flow to distribution lines. Drop manholes are frequently used when the slope of the septic tank effluent pipe or non-perforated distributor pipe is ≥ 10 percent. The inverts of all outlets (i.e., direct connections to distribution lines) within each drop manhole shall be at the same elevation to assure uniform distribution at a given contour line. The use of outlet levelers is recommended. Inverts of outlets should be at least two inches above the bottom of the manhole to prevent short-circuiting and reduce solids carry-over. The drop between inlet and distributor inverts will routinely exceed two inches. The drop between inlet and overflow (i.e., direct connection to the next drop box or distribution box) shall be at least one inch and the slope of the connection pipe shall be at least 1/32 inch per foot. The invert of the overflow should be at least 1 ¼ inches above the outlet inverts.

Drop manholes maximize flow to the uppermost absorption trenches and produce sequential trench failure with the uppermost trenches failing first. System longevity can be improved by periodically resting any of the upper (i.e., not the lowest) laterals by replacing adjustable outlet levelers with plugs for a six month period.

Baffles at the inlet end of the manhole and approximately four inches from the inlet are required in drop manholes to prevent short circuiting and assure uniform flow to distribution lines. A modified sanitary tee as described in the distribution box section and depicted in Figure 12 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) may be used in drop manholes.

Pressure Distribution and Dosing

Pressure distribution utilizes a sewage effluent pump to convey septic tank/aerobic unit effluent through a pipe network and-into the soil. The volume discharged in each cycle will exceed the volume available in the pipe network and will be discharged from perforated pipe under pressure. Pipe used in pressure distribution shall have a minimum diameter of one inch and a maximum diameter of three inches. The ends of all pipes shall be capped. Perforated pressure distribution lines shall be installed level. Only pumps designated by the manufacturer for use as sewage effluent pumps shall be used. Pressure distribution rumps shall be selected to maintain a minimum pressure of one psi (2.3 feet of head) at the downstream end of each distribution line during the distribution cycle.

Pressure distribution or dosing permits rapid distribution of septic tank/aerobic unit effluent throughout the absorption system followed by a rest period during which no septic tank/aerobic unit effluent enters. Periodic application of wastewater to absorption facilities is accomplished by means of a pump or siphon installed in a dosing tank (see Figures 13 and 14 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). These methods assure that absorption facilities are fully and uniformly utilized. The maximum length of absorption lines used in conjunction with these methods shall be 100 feet (i.e., 100 feet for an end manifold distribution network as shown in Figure 15 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and 100 feet in each direction from a central manifold distribution network as shown in Figure 16 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). Dosing or pressure distribution is recommended for all absorption systems since it promotes improved treatment of wastewater and system longevity as compared to systems lacking dosing or pressure distribution.

Pressure distribution systems should be designed to minimize headloss due to friction in the distribution network. Excessive headloss in the distribution network causes unequal application of settled sewage to the absorption facility. Distribution lateral pipe should be designed to limit differences in flows at the distal orifice and the supply end orifice (i.e., manifold end) to ten percent of the distal orifice flow (e.g. 0.1 gpm and ≤ 0.11 gpm). Distribution manifold pipe should be designed to limit the difference in heads between the distal and supply ends to ten percent of the distal end head (e.g., 1.0 and 1.1 psi).

Pump chambers shall be equipped with an audible or visual alarm to indicate pump malfunction (see Figure 14 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). Pump chambers shall not be equipped with an overflow. Pump chambers should be equipped with a downward facing screened vent riser to reduce the concentration of explosive and/or toxic gases within the chamber. Pump chambers shall be sized to provide a minimum reserve storage capacity of one day's design flow above the alarm level. The purpose of the reserve storage capacity is to prevent a nuisance from occurring (i.e., backup of sewage) and enable continued use of sanitary facilities while the malfunctioning pump is rapidly repaired/replaced. Use of household sanitary facilities should be minimized (i.e., practice water conservation) during electrical power outages or periods of pump failure in all homes served by pump chambers since wastewater can be generated but the effluent pump will not function. Special care is warranted for households served by public water supply since water service may remain available during power outages (i.e., a solenoid valve on the water supply service line may be warranted to discontinue water service during electrical power outages). The volume of wastewater pumped per dosing cycle should never exceed the daily design flow to prevent overdosing. The volume pumped must be controlled following electrical power outages or pump malfunction (i.e., when storage buildup has occurred). The volume pumped can be controlled (a) manually via an on-off switch on the pump coupled with the calculated maximum time of pumping or (b) automatically via an overriding timer switch built into the pump controls. A minimum two hour rest period should occur between pumping cycles (i.e., before the pump is reactivated following a daily design flow dosage). Duplex pumps with individual audible or visual alarms may be used in-lieu-of a single effluent pump and one day's reserve storage capacity. Immediate repair/replacement is necessary whenever either of the pumps malfunctions. High groundwater conditions or shallow depth to bedrock will frequently cause duplex pump installations to be selected in-lieu-of a single pump with one day's storage. Pump stations installed below the maximum ground water table are subject to the buoyancy effect of the displaced volume of the station. Any buoyancy effect shall be addressed in the design of pump stations to prevent damage to the inlet and outlet sewers and the station. Pump chambers and connecting plumbing shall be watertight to prevent ground water contamination in the vicinity of the pump station and infiltration of ground water into the sewerage system (i.e., short circuiting the designed sewage pumping cycle due to the admixture of ground water with sewage).

Where the discharge pipe is not buried below the frost line, the pipe should be drained between doses. Draining the pump discharge line into the pump storage tank between doses can be accomplished by: (a) using a solenoid valve controlled discharge with the solenoid valve being open when the pump is off and closed when the pump is operating, (b) eliminating the check valve at the pump if the pump/motor is not subject to damage by operating in reverse, or (c) providing a "weephole" in the pump discharge line downstream from the pump check valve. Use of rigid foamed plastic insulation in the trench above the discharge line assists in preventing freezing of discharge lines within the frost zone if the lines are used daily during winter.

In time, the distal end of distribution laterals may become partially clogged with suspended and settleable solids that flowed out of the septic tank and pump chamber. Sewage fungal growths, which slough off pump chamber surfaces and distribution pipes, also increase the clogging effect. Special provisions for periodically flushing (i.e., cleaning) distribution laterals should be incorporated into the design of pressure distribution systems for wastewater. Flushing should occur when septic tanks are pumped.

Distribution laterals may be designed with individual valves near the distribution manifold end to enable maintenance personnel to direct flow to individual laterals for flushing. Valves should be installed in low profile boxes to be easily accessible. Turn-ups (i.e., 90 degree elbow and vertical riser) may be installed at the distal ends of laterals to accommodate flushing and cleaning. Schedule 40 pipe is recommended for the pressure pipe network including the turn-ups. Turn-ups should be protected with sleeves of larger diameter pipe and both should terminate as near grade as possible. Turn-ups must be capped and teflon tape should be used on the riser threads to prevent leakage.

Flushing can be accomplished by pumping water through each individual lateral and thence through an attached hose back to the inlet end access manhole of the pumped out septic tank. The pump chamber should be filled with water to the high water level immediately prior to activating the pump for lateral flushing. The low water level pump stop sensor should be operational during lateral flushing to prevent damage to the pump. Lateral flushing should continue until return water is relatively free of large solids. Trucked in water is the preferred source of flushing water for filling the pump chamber. If a hose connected to the household water system is used to fill the pump chamber, the hose shall not contact the chamber contents to avoid contamination of the water supply (i.e., an air break must be maintained between the discharge end of the hose and the pump chamber).

Except for absorption bed systems, conventional on-site treatment systems do not normally require pressure distribution. Dosing may be required for: (1) large wastewater flows in slowly permeable soils; (2) alternative systems where even distribution is critical to the performance of the system; (3) site conditions where a gravity system cannot provide even distribution; or, (4) systems with a total absorption trench length exceeding 500 feet. When pressure distribution or dosing siphons are used, a design professional should be engaged to design the system, supervise construction, prepare an operating manual, and implement start-up.

Dosing involves the use of a pump or siphon to move the effluent into the pipe network. Discharge from the perforated pipe is by gravity. Perforated distribution lines shall be installed as level as possible. The volume of effluent in each dose should be 75% to 85% of the volume available in the pipe network. The use of manually operated siphons or pumps is not acceptable. For conventional absorption fields, siphons are preferred. They are operated hydraulically and by gravity flow, have no moving parts, and will operate during electrical power outages.

Dosing chambers are tanks which store wastewater effluent from septic tanks/aerobic units and periodically discharge via a siphon or pump to art absorption field or sand filter. In absorption fields, single dosing units are required when the total trench length exceeds 500 feet. Alternate dosing units are required when the length exceeds 1.000 feet. Alternate dosing devices have the capability of dosing two separate sections of the same system by having two siphons or pumps. When duplex pumps are used in-lieu-of a single pump with one day's reserve storage capacity in alternate dosing, duplex pumps with individual alarms shall replace each single pump. Safety precautions applicable to inspecting septic tanks are also applicable to inspecting dosing chambers. In sand filters, dosing is required whenever the filter contains 300 or more lineal feet of laterals or 900 or more square feet of filter area.

Since dosing siphons are equipped with an overflow pipe, any reserve storage capacity serves no useful purpose. Dosing siphons should be inspected periodically to assure that the wastewater level in the storage chamber is within its normal operating range (i.e., bottom of bell to below the overflow). An audible or visual alarm is recommended to indicate, that the siphon chamber is overflowing. Dosing siphons should be equipped with a downward facing screened vent riser to reduce the concentration of explosive and/or toxic gases within the unit.

Upon installation, new siphons should be primed with water. If hydraulic bell siphons are used, they can be tested for leaks by covering with water and inspecting for air bubbles. Float switches controlling pumps should be tested and adjusted for correct discharge level. Pumps and floats must be readily accessible for servicing. Pumps and control devices within a dosing tank shall be of an explosion proof design. Manufacturer's directions must be carefully followed in the installation of siphons and pumps. A design professional should supervise the installation of siphons and pumps and should certify to the reviewing authority that installation was in accord with approved plans and manufacturer's recommendations. Storage tank, pump station, or dosing tank access covers located at or above grade should be lockable to prevent entry by unauthorized persons, especially children. Storage tank, pump station, or dosing tank access covers should be located at or above grade to facilitate operation, maintenance, repair or replacement of equipment as required. Access covers should exclude precipitation from the tank/station interior and site grading should convey surface runoff away from access covers.

Conventional gravity absorption systems utilize four inch diameter perforated pipes with ¼ to ½ inch diameter holes. Pipe for siphon dosing is sized to conform with the volume of the dose and can range from three to six inches in diameter based upon the volume of each dose. Pipe used in pressure distribution shall have a minimum diameter of one inch and a maximum diameter of three inches. The volume per lineal foot of pipe in the one to six inch diameter range is shown in Table 4 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Pressure distribution pumps shall be selected to maintain a minimum pressure of one psi (2.3 feet of head) at the downstream end of each distribution line during the settled wastewater distribution cycle. The ends of all pipes in pressure distribution and dosing systems shall be capped.

A distribution box should be used for systems incorporating dosing to evenly distribute settled sewage to all perforated distribution lines. A distribution box shall not be used for pressure distribution.

Subsurface Treatment Systems

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General Information

All effluent from septic tanks or aerobic units shall be discharged to a subsurface treatment system.

Although septic tanks and aerobic units improve the quality of raw sewage, the effluent contains pollutants and harmful organisms and is not suitable for direct discharge to surface waters or ground waters. Subsurface treatment systems are designed to filler and oxidize most dissolved and suspended solids in the septic tank/aerobic unit effluent. Absorption fields placed in loam soils will remove most of the phosphorus in septic tank or aerobic unit effluent. If the absorption trenches are kept shallow (see Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook as recommended, the vegetative cover root system can penetrate and take up some of the phosphorus and nitrogen during the growing season. Hence, the possibility of phosphates and nitrates moving any significant distance through soil to the ground water table and contributing significant quantities of nutrients that might reach a lake or other impoundment and accelerate its eutrophication can be greatly minimized. This is particularly so when considered in relation to the phosphorus and nitrogen contribution from surface runoff, storm water, and direct wastewater discharges to lakes and streams. To promote adequate removal of these nutrients, pollutants, and pathogenic organisms, at least two feet of usable soil shall exist between the bottom of absorption trenches and the highest ground water level, rock, or other impermeable strata. A three foot separation is required between the bottom of a seepage pit and these boundary layers. Certain ecologically critical areas may dictate the imposition of greater separation distances. Examples include locating absorption facilities above limestone, karst or shale recharge areas for ground water aquifers (i.e., especially where wells and/or springs are sources of water supply). Absorption systems should not be constructed directly over visible cracks, crevices, depressions, sinkholes, etc., in such formations to protect the aquifer. The depth of usable soil between the bottom of absorption facilities and the recharge rock should be at least four feet. An alternative method of protecting bedrock aquifers comprises installation of a six inch clay barrier on the in situ soil/rock beneath the proposed absorption area and extending radially as noted below. On slopes of less than one percent, the clay layer covered with at least one foot of usable soil (i.e., one to 60 minutes/inch) should extend 100 feet radially from the toe of the absorption area including the projected expansion area. On slopes of one to 15 percent, the clay layer covered with at least one foot of usable soil should extend radially from the toe of the absorption area including the projected expansion area 100 feet in the downslope direction, 25 feet parallel to contours, and 20 feet in the upslope direction. At least four feet of usable soil should be installed above the clay layer in the proposed absorption area including the projected expansion area. Fill slopes shall not exceed one vertical to three horizontal. A design professional should supervise the above-noted system construction and certify to the reviewing authority that construction was in accord with approved plans. Site requirements for design of individual wastewater treatment systems are shown in Table 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Surface water should be diverted from the vicinity of subsurface sewage treatment systems by grading and construction of diversion berms or ditches upqradient of all systems on sloped sites.

The specific type of subsurface system used depends largely upon soil and site conditions. The most common systems are absorption fields/beds and seepage pits. Alternative systems (e.g., raised, mound, intermittent sand filter, etc.) may also be used under certain conditions. Care is essential when installing any system 10 assure protection of water supplies, ground water and surface water.

Discharges to surface waters are not acceptable for any new individual household system. Replacement and upgrading of some existing failing systems may however necessitate use of a surface discharge with all appropriate controls (e.g., SPDES permit, seasonal, or year-round disinfection, monitoring, reporting, etc.), rather than a conventional or alternative system. The Department of Environmental Conservation has jurisdiction over all sewage discharges to surface waters and that agency or its agents may approve such discharges. Discharges to the ground surface or roadway ditches are prohibited and are considered a public health nuisance or public health hazard.

Absorption System Location

Careful selection of the absorption system location will minimize the chance of future malfunction. An important consideration in absorption system location is the possibility of future connection to public sewers. When systems cannot be constructed in front of a home, the home's internal plumbing should be designed to facilitate the sewer connection in the future. Installation of a dry house sewer at the time of home construction will eliminate a costly future sewer connection and is highly recommended whenever future public sewering is anticipated.

The daily discharge of hundreds of gallons of septic tank effluent at each household poses a potential threat of pollution. Absorption systems should be located far from wells and water courses to minimize the chance of contamination and to facilitate repairs and regular maintenance. The minimum distances that absorption systems shall be separated from other facilities are shown in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Figures 1 and 2 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The separation distances apply to the proposed absorption system and its proposed future expansion. These distances allow for additional treatment of the wastewater in the soil prior to reaching any ground water use location.

Circumstances may warrant the pumping of treated sewage to a suitable location prior to final treatment in order to achieve maximum separation. In such cases, consideration should also be given to designing a system that is easily accessible for maintenance and repairs. Pressure distribution or dosing can and should be incorporated in systems that require pumping to reach suitable locations.

Consideration should also be given to prevent future home improvements from interfering with the operation of the absorption system. Impermeable surfaces and surfaces subject to heavy loads, such as driveways, sidewalks, portions of buildings, parking lots, or swimming pools (i.e., above-ground or in-ground) shall not be constructed upon or in absorption fields. Where paving over seepage pits, gallies and other absorption devices is necessary due to limited absorption system space, the downstream portion of the absorption facility shall be equipped with a downward facing screened vent riser to assure access for air to the absorption facility and the absorption facility shall be designed to withstand the load(s) imposed.

Subsurface Drainage Facilities

Subsurface drainage facilities, such as curtain drains, vertical drains or underdrains may be installed to control shallow lateral ground waterflow or perched water tables in the vicinity of existing or proposed subsurface sewage treatment facilities as depicted in Figures 18 through 18D (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Separation distances between subsurface drainage facilities and sewage treatment components in level terrain should equal Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) values "To Stream, Lake, Watercourse (b), or Wetland" to prevent short circuiting to the watercourse receiving the subsurface drainage facility discharge. Short circuiting of wastewater from absorption facilities to drainage facilities must be avoided. The ground water collection portion of subsurface drainage facilities on sites with ≥ 5% slope should be at least 15 feet upslope of wastewater absorption facilities to provide effective ground water dewatering and prevent short circuiting of wastewater to the subsurface drainage system. If the difference in elevation between the bottom of the ground water collection facility and the top of the aggregate in the uppermost wastewater absorption facility exceeds ten feet, the minimum horizontal separation should be 1.5 times the vertical difference for ≥ 5% sloped sites. The horizontal separation should be increased at least five feet for each one percent reduction in slope of the site (i.e., ≥ 20 feet for 4%, ≥ 25 feet for 3%, ≥ 30 feet for 2%, and ≥ 35 feet for 1%) to 1%. The horizontal separation should be 100 feet for < 1% slope. The surface outlet of a subsurface drainage facility on sloped sites should be at least 20 feet downslope of wastewater absorption facilities when the outlet flows fewer than 183 days per year and 100 feet when the outlet flows more than 182 days per year (i.e. forms a watercourse.)

Drainage of artesian fed water tables or slow-moving, unconfined water tables are not recommended. Subsurface drainage system design is addressed in (a) the 1973 U.S. Department of Agriculture Soil Conservation Service text titled "Drainage of Agricultural Land" (available from Water Information Center, Inc., 125 East Bethpage Road, Plainview, New York 11803, 430 pages) and (b) the September 1987 U.S. Department of Agriculture Soil Conservation Service booklet titled "Drainage Guide for New York State." Drainage design technical assistance may be requested from County Soil Conservation Service staff regarding compatibility between soils to be drained and the specific type of drain selected. Technical assistance may also be requested from soil scientists, design professionals and local health department personnel.

Curtain drains may be installed upslope of proposed absorption facilities on sloped sites to intercept and control high ground water. Non-perforated, watertight pipe installed on in situ soil bedding at least ten feet from the absorption facility should be constructed to convey the collected ground water to the ground surface as depicted in Figure 18A (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The surface outlet should be protected from water/soil erosion and animal entry as depicted in Figure 18B (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The upstream end of all perforated and non-perforated segments of curtain and/or footing drains may be fitted with capped cleanouts to facilitate future cleaning. Cleanouts are most apt to be needed when non-granular soils are drained/dewatered.

Subsurface drainage aggregate (i.e., washed number 2 stone or gravel) in granular soils should be surrounded by permeable geotextile (preferably non-woven) to prevent siltation and plugging of the aggregate, perforated drain pipe and non-perforated drain pipe as depicted in Figure 18C (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The useful life of subsurface drainage facilities in granular soils is markedly increased by proper installation of permeable geotextile surrounding the aggregate. Aggregate should surround the perforated drain pipe and extend above the existing high water table to avoid ground water bridging over the subsurface drainage system.

Subsurface drainage in soils with a silt fraction of 40% or more (i.e., 0.002 to 0.05 mm) should be accomplished with a washed coarse sand and aggregate envelope surrounding the perforated drain pipe as depicted in Figure 18D (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Permeable geotextile should not be used to fully surround the coarse sand and/or aggregate in soils with a high silt fraction to avoid plugging the geotextile.

Wastewater short circuiting from absorption facilities to watercourses via household foundation drains should also be avoided by appropriate separation distances. Use of impermeable barriers such as clay or plastic sheeting should be considered to prevent short circuiting when absorption facilities must be located close to foundation drains, which should be considered as curtain drains. The recommended minimum separation distance between wastewater absorption facilities and downslope curtain and/or footing drains is 100 feet to prevent short-circuiting of wastewater to the ground surface when curtain and/or footing drains are located in the general flow path of wastewater absorption facilities. This separation distance may be reduced to no less than 50 feet when the soil percolation rate is five to 60 minutes per inch and the minimum vertical separation distance from the bottom of any absorption trench to high ground water, bedrock, or impermeable soil is four feet. The recommended minimum separation distance is not applicable to footing drains located above seasonal high ground water.

The effectiveness of subsurface drainage systems should be determined via periodic monitoring during the wet season (i.e., March 15 - June 30) following installation of the system. Plans for wastewater absorption systems in areas requiring ground water lowering should not be approved until the effectiveness of the required subsurface drainage system(s) has been demonstrated if any of the three following conditions exist: (a) The ground water slope is less than five percent; (b) The soil percolation rate is slower than 30 minutes per inch; (c) The bottom of the proposed curtain drain/underdrain is not in contact with the impermeable strata causing the high ground water condition.

Washed sand and aggregate drainage facilities, which are not surrounded by permeable geotextile, should be covered with a permeable geotextile to minimize entrance of backfill soil into the drainage system. Backfill should comprise finely textured soil to minimize entrance of surface water into the subsurface drainage system. Breathers or vents may be needed for proper functioning of long curtain drains or underdrains.

Minimum slope for four inch diameter drainage pipe is generally 0.004 feet per foot. The minimum pour distance (i.e., drain outlet invert to normal low water surface or outlet channel bottom) should be one foot for drain pipe slopes up to 4%.

The drain outlet should comprise at least eight feet of rigid, non-perforated conduit (i.e., metal or schedule 40 PVC or equal). At least two-thirds of the length of the outlet conduit shall be installed in soil and backfilled with at least two feet of soil to prevent leakage, slippage, or freezing.

Only perforated drainage pipe with perforations completely around the pipe wall should be used. Wastewater distribution pipe should not be used for drainage systems. Trees and shrubs should not be present near the drainage collection system to avoid root interference. Root systems of many trees and shrubs extend beyond the tree/shrub drip line (i.e., tips of branches).

Choice of Treatment Systems

Site conditions limit treatment system choices. Site requirements for design of individual wastewater treatment systems are shown in Table 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Absorption fields comprise the recommended treatment in well drained areas where upper soils possess adequate percolation. Parallel distributor lines/trenches served by a distribution box are commonly used in relatively flat areas while drop manholes and serial distribution laterals are frequently used where land slopes between 10 and 15 percent. At any site, individual absorption field trenches shall be constructed parallel to the ground contours.

An absorption bed, (see Figure 19 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)) though less desirable than trenches due to reduced sidewall area, may be considered on some sites. Construction of the bed can be quite difficult. A backhoe can straddle a trench during absorption field trench construction and leave the infiltrative surfaces undisturbed in terms of compaction. Construction equipment shall not be permitted to operate within the area designated for construction of an absorption bed. The sidewall area of the bed should be maximized for the system to perform properly. For these two reasons, a bed wider than 20 feet will not be considered. If a system is designed with multiple beds, a minimum separation of ten feet should be provided between the sidewalls of the beds (i.e., ten feet of undisturbed soil).

The least desirable of the conventional systems is the seepage pit. Seepage pits do not provide even distribution of septic tank/aerobic unit effluent over the design absorption area and pit depth reduces the opportunity for oxygen exchange at the active infiltrative surface. This leads to reduced treatment of wastewater as compared to absorption field trenches and progressive failure of the system. Seepage pits may be used where the soil encountered at the proposed pit depth has suitable percolation and a lens of impervious sailor impermeable upper soil eliminates consideration of absorption fields. Pits should not be used where drinking water is obtained from shallow wells or where subsoil is a coarse sand and/or gravel.

High ground water, periodic flooding, unsatisfactory percolation test results, inadequate permeable soil depth to bedrock or impermeable soils comprise reasons for discouraging or prohibiting use of conventional absorption systems. Alternative systems have been developed to overcome some of these constraints. Large lots, which allow substantial separation distances between systems and wells, watercourses, etc., and meet specific site and slope constraints, may be developed using alternative systems. Special consideration should be given to alternative systems or special designs when replacement of an existing failing individual sewage treatment system occurs to minimize failure recurrence and prevent recurrence of a public health hazard/nuisance. Waivers may be required as noted below.

New construction should routinely meet all standards. Department regulations do provide for issuance of a specific waiver in an individual situation because of a hardship or other circumstance that makes it impractical to comply with a standard. Not all land is suitable for development for residential purposes using individual sewage treatment systems. Specific waivers are required for new construction of engineered systems not listed in Appendix 75-A as well as deviations from Appendix 75-A standards unless a general waiver or local waiver had been issued to the approving municipal authority.

Although State Department of Health regulations do not require specific waivers for correction or replacement of existing failing individual sewage treatment systems, some county health units may require the issuance of a specific waiver for correction or replacement of existing failing systems. Such corrections or replacements should comply with Appendix 75-A standards if possible. Tables 13 and 14 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) provide guidance in evaluating and correcting periodic/continuous system failure.

Fill needed to provide enhanced treatment as per County regulations (e.g., a five feet separation between the bottom of absorption facilities and impermeable soil/bedrock/high ground water rather than two feet as per Appendix 75-A) is subject only to County policies and procedures rather than Appendix 75-A for shallow trench, raised or mound systems.

Materials

Non-perforated watertight pipe shall be used between the distribution box and the trenches and be at least two feet in length (i.e., minimum separation between distribution box and trenches). Only perforated distributor pipe shall be used in the trenches. Four inch diameter pipe is recommended for all gravity systems. Perforated pipe shall be made of rigid or corrugated plastic and be labeled as fully meeting ASTM standards for septic systems. Corrugated plastic pipe delivered in coils is not to be used unless provision is made to prevent the recoiling or movement of the pipe after installation. Pressure distribution pipe shall have a minimum diameter of one inch and a maximum diameter of three inches. Pipe for siphon dosing is sized to conform with the volume of the dose and can range from three to six inches in diameter based upon the volume of each dose. All distributor pipe whether gravity, pressure or dosing should have perforations of ¼ inch minimum diameter. In gravity distribution systems, distributor pipe should be laid at a slope between 1/16 inch and 1/32 inch per foot.

Pressure distribution lines and dosing distributors should be installed level. A typical gravity absorption trench detail is shown in Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The ends of distributor pipes should not be interconnected since it reduces operational flexibility of resting individual lines as needed and frequently results in unequal distribution of settled effluent to individual trenches (i.e., the lower trench will receive more settled effluent than the higher trench). A typical lot layout for absorption field systems is shown in Figure 1 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Aggregate used in absorption fields shall be washed gravel or crushed stone ¾ - 1 ½ inches in diameter. Larger diameter or finer substances (e.g., crushed shells) are unacceptable.

The aggregate shall be covered with a material that prevents soil from entering the aggregate after backfilling, yet must permit air and moisture to pass through. The preferred material for covering the aggregate is a permeable geotextile. A geotextile is extremely strong and does not promptly degrade like untreated building paper, hay or straw. Untreated building paper or a four inch layer of hay or straw is also acceptable for covering aggregate when permeable geotextiles are unavailable. Polyethylene and treated building paper are relatively impervious and shall not be used.

Absorption Field Systems

General Information

An absorption field is a system of narrow trenches partially filled with a bed of washed gravel or crushed stone ¾ to 1 ½ inches in diameter (i.e., aggregate) through which a perforated distribution pipe is laid.

Two types of absorption field layouts are in common use: a distribution box connected to parallel absorption laterals for slopes not exceeding ten percent and drop manhole or serial absorption systems for slopes up to 15 percent. If a distribution box connected to parallel absorption laterals is used for slopes between ten and 15 percent without use of speed levelers, the first length of non-perforated distribution line leaving the box shall have the same slope for all laterals. Speed leveler devices are recommended for all distribution boxes.

Sites with at least one foot of unsaturated permeable soils and slopes not exceeding 20% may be modified by grading (i.e., cut and/or fill) to meet the maximum 15% slope requirement. The proposed absorption area and future expansion area soils shall be stabilized prior to conducting percolation and deep hole tests to determine compliance with all boundary conditions. Ground water monitoring should be performed periodically from March 15 to June 30 following soil stabilization to determine high ground water in fill at modified sites. Before site modification begins, it is recommended that officials of the local municipality having jurisdiction be contacted to determine if plans/permits are needed (e.g., an erosion and sediment control plan or an environmental assessment form). Stabilized soil means the site shall be allowed to settle naturally for a period of at least six months and include at least one freeze-thaw cycle. If the site to be modified and any fill comprise only permeable granular material (i.e., sand or sandy loam similar to fill material for mound systems with a percolation rate ≤ 30 minutes per inch), stabilization may be achieved by mechanical compaction in approximately six inch lifts to the approximate density of the undisturbed underlying granular soil. Excessive compaction of many soils produces impermeable soils rather than the desired usable soils. Appropriate ditches, berms, and drains (i.e., curtain, vertical, underdrains) shall be installed to control surface drainage and ground water in the vicinity of absorption fields on sloped sites. Sites with existing slopes exceeding 20 percent should not normally be considered for residential development using individual sewage treatment systems. Development following mining represents one exception (i.e., conversion of hillsides, eskers. etc., to relatively level sites). Site modification activities to reduce slope to 15 percent or less should only be conducted during relatively dry periods to minimize soil compaction and smearing. The 15 percent slope limitation for construction of subsurface sewage treatment systems applies only to the portion of the lot to be used for the proposed absorption facility plus the proposed future expansion, not the entire lot. Site modifications to control ground water, alter slope to ≤ 15%, alter drainage or increase separation distances to boundary conditions should be conducted under the supervision of a design professional.

Many health department staff have experienced satisfactory absorption facility performance from very carefully constructed absorption trenches in situ soil with slopes of > 15 to 20 percent when special conditions are met. Said construction may be considered in-lieu-of site modification (i.e. ≤ 20 percent slope to ≤ 15 percent slope) to minimize site disturbance, compaction and erosion provided all of the following conditions are met:

  1. The in situ soil exhibits a percolation rate of one to sixty minutes per inch.
  2. Design flow is in accord with Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).
  3. Septic tank sizing is in accord with Table 3 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).
  4. Absorption trench design is in accord with Table 5 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).
  5. Absorption trenches are constructed parallel to contours.
  6. Absorption trench bottoms are constructed essentially level.
  7. Absorption trenches are constructed 18 to 30 inches below grade.
  8. A minimum horizontal separation distance between parallel absorption trenches of 10', 9', 8', or 7' is provided when the minimum vertical separation distance from the bottom of any absorption trench to high ground water, bedrock, or impermeable soil is 2', 3', 4' or 5' respectively.
  9. Required separation distances from wastewater system components noted in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) are provided.

Site Requirements

The minimum distances that absorption fields shall be separated from other facilities are shown in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Figure 1 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). A minimum of four feet of usable soil shall exist above bedrock, impermeable strata, and ground water with a minimum separation of two feet to the lowest part of any trench. Absorption fields shall not be built under driveways, parts of buildings or under above-ground swimming pools or other areas subject to heavy loading. Surface waters shall be diverted from the vicinity of the system. All proposed sites should be evaluated in terms of slope, wetlands, watercourses, rock outcroppings, impermeable strata, percolation and deep hole tests, location of wells in the vicinity, and property boundaries.

Design Criteria

The required length of absorption trench is dependent upon the daily flow rate and soil percolation test results which have been confirmed by site evaluation. Once the daily flow rate and soil percolation test results are known, total absorption field length is determined using Tables 5 and 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

The maximum length of absorption lines used in conjunction with gravity distribution shall be 60 feet. The maximum length of absorption lines used in conjunction with pressure distribution or dosing shall be 100 feet. All absorption lines in a system shall be approximately the same length except for serial distribution and drop manhole distribution. Absorption trenches should not be installed in wet soil to prevent undesirable smearing/compaction of the infiltrative soil surface. Trenches shall be installed parallel to ground contours and as shallow as possible. Trenches need not be perfectly straight but abrupt changes in direction are to be avoided. The sides and bottom of trenches shall be raked prior to placement of aggregate. The ends of all distributor pipes should be capped. Distributor pipe caps are required for systems using pressure distribution or dosing.

Sewage treatment efficiency is related to the area of the trench bottom and sidewall (known as the wetted perimeter) available for infiltration. A typical gravity absorption trench detail is depicted in Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). At least six inches of aggregate is placed under the distribution line and a minimum of two inches over the distribution line. The slope of the perforated distribution lines shall be 1/16 to 1/32 inch per foot. The aggregate shall be covered with permeable geotextile, untreated building paper, a four inch layer of hay, or a four inch layer of straw to prevent soil from entering the aggregate after backfilling and to provide for passage of moisture and gases. Earth cover above the aggregate shall be between six and 12 inches to enhance natural aeration and nitrate utilization by plant life. Trench depth should be as shallow as possible, but not less than 18 inches nor more than 30 inches. The inverts of distribution lines shall not be deeper than 24 inches below grade. The maximum trench width for design purposes shall be 24 inches and the required length of absorption trench for various flow rates and soil percolation rates for a 24 inches wide trench are depicted in Table 5 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Where trenches exceed 24 inches in width, calculations of absorptive area shall be based on a width of 24 inches. Application rates for non-standard design flows. (i.e., trenches less than 24 inches wide) are depicted in Table 6. Dosing or pressure distribution is recommended for all systems as it promotes better treatment of wastewater and system longevity.

Adjacent absorption trenches shall be separated by at least four feet of undisturbed soil as depicted in Figure 19 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Individual trenches shall be constructed parallel to ground contours with the trench bottom essentially level. Trenches need not be perfectly straight, but abrupt changes in direction should be avoided. Maintaining a uniform slope for the entire length of each perforated distributor line is very difficult when the lines are not installed straight.

Construction

Construction of absorption facilities shall not occur when the ground is frozen or the soil moisture content is high. If a fragment of soil from approximately nine inches below the surface can easily be rolled into a ribbon instead of crumbling, the soil moisture content is too high for construction purposes. Obviously, some silt or clay must be present for a ribbon to be formed.

Trench locations and depths should be marked by stakes before trenches are excavated. The natural surface shall not be significantly disturbed. If the site is regraded or similarly disturbed, the soil shall be allowed to stabilize (i.e., settle naturally for a period of at least six months, including at least one freeze-thaw cycle) and new percolation and deep hole tests conducted. Heavy equipment use in the designated absorption system area should be minimized to prevent soil permeability reduction due to compaction plus possible trench cave-in and distributor pipe misalignment/breakage.

After a final grade check, trench excavation may begin. Trench depth shall be as shallow as possible, but not less than 18 inches. Trenches shall be excavated to design depth with bottoms practically level.

Following excavation, the trench bottoms should be graded by hand. The bottoms of the trenches should be checked by a transit, engineer's level, or carpenter's level to assure that each is practically level. Trench bottoms and sidewalls should be immediately raked after final grading and at least six inches of aggregate placed in the trenches.

Gravity distributor pipe shall be carefully installed at a slope of 1/16 to 1/32 inch per foot. Pressure or dosed distributor pipe shall be installed level. Grades shall be determined by an engineer's level, transit or carpenter's level. An easy method of measuring the 1/16 inch per foot slope for distributor pipe is to tape a ¼ inch block to one end of a straight four foot board. By placing the blocked end with the block facing down on the downstream top of the distributor pipe, a carpenter's level can be used to determine the 1/16 inch per foot slope. A 1/8 inch block similarly used would result in a 1/32 inch per foot slope. Another method used to establish the grade of distributor pipe is by batter boards as shown in Figure 20 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Aggregate shall be installed in the trenches to a depth of at least two inches above the top of the distributor pipes as depicted in Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Additional aggregate may be required to bring the top of the aggregate to within six to 12 inches of final grade as shown in Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

After the upper aggregate is placed in the trenches, the geotextile, untreated building paper, hay or straw is to be immediately installed on the aggregate and the trench backfilled with excavated soil. If trenches cannot be immediately backfilled, the trenches should be temporarily covered with an impervious material, such as treated building paper to prevent sidewall collapse and siltation into the aggregate.

The earth backfill is to be mounded slightly above the original ground level (i.e., not compacted) to allow for settling. Following settlement, the entire area should be graded without the use of heavy equipment and seeded with grass. Heavy equipment shall not enter the absorption facility area or the proposed expansion area after the subsurface sewage treatment system has been constructed.

Gravelless Absorption Systems

General Information

Gravelless absorption systems are generally proprietary products, which allow septic tank/aerobic unit effluent to infiltrate soil in the absence of installed aggregate. These systems were developed where aggregate was not economically available or to increase the soil-infiltrative area in constructed trenches. Wastewater infiltrates soil between aggregate particles in an aggregate filled trench. An aggregate-soil masking phenomenon occurs since wastewater cannot flow through aggregate particles into the soil.

Gravelless absorption products include leaching chambers, galleys, flow diffusers, and large diameter (i.e., at least eight inches) corrugated and perforated pipe surrounded by a permeable geotextile. Leaching chambers, flow diffusers and galleys provide increased soil infiltration area for trench bottoms. Large diameter corrugated and perforated pipe surrounded by a permeable geotextile provides some increased soil infiltration area for trench sidewalls. The outer corrugations may create soil masking due to soil pressure upon the corrugations. Excavated trenches containing these products are backfilled with soil excavated to form the trenches (i.e., aggregate is not placed around the products). Representative gravelless systems are shown in Figures 21, 22 and 23 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). These systems are classified as conventional sewage treatment systems with specific site, design, and construction constraints.

Geotextile surrounded corrugated and perforated pipe must be at least eight inches in diameter and the leaching chambers, flow diffusers and galleys must be at least two feet wide to be equivalent to one linear foot of conventional (24 inches wide) aggregate filled absorption trench.

Site Requirements

These systems are restricted to sites that have a design percolation rate of one to 45 minutes per inch and a slope not exceeding 15 percent. In addition, the vertical and horizontal separation distances 'oted in figures 1 and 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met.

Design Criteria

One linear foot of properly sized gravelless systems shall be equal to one linear foot of conventional (24 inches wide) aggregate filled absorption trench. The local health department having jurisdiction shall be contacted prior to construction regarding the acceptability of specific products for use as a gravelless absorption system.

Construction

The applicable portions of construction of absorption field systems noted previously (i.e., soil moisture, heavy equipment use, excavation, grading, hand raking trench bottoms/sidewalls, and backfilling trenches) shall be followed. In addition, gravelless absorption systems shall be installed in conformance with the manufacturer's instructions because of the proprietary design of some products.

Deep Absorption Trenches

General Information

Deep absorption trenches may be used where sites contain at least four feet of permeable soil (i.e., one to 60 minutes per inch percolation rate) overlain by one to five feet of impermeable soil (i.e., greater than 60 minutes per inch percolation). Conventional absorption field systems may be used when the overlaying impermeable soil is no more than one foot deep provided backfill above the aggregate comprises only permeable soil. A cut and fill system may also be used where a deep trench system may be used. The cut and fill system is the recommended choice since shallow trenches provide improved treatment and enhanced oxygenation of the infiltrative soil surface (i.e., gravel to permeable soil interface).

As shown in Figure 24 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook), trenches are excavated at least two feet into the permeable soil. Careful excavation is necessary to assure that impermeable overburden does not remain in the trench bottom. The trenches shall be backfilled to 30 inches below the surface with aggregate or coarse sand to promote aeration and utilization of the entire infiltrative permeable soil surface (i.e., permeable soil - sand/aggregate interface). A conventional absorption field system (i.e., trenches with distribution lines and aggregate) is constructed in the upper 30 inches of the trenches. The inverts of distribution lines shall not be deeper than 24 inches below grade. Diversion of surface runoff around the absorption area by means of ditches or berms is required uphill of all sloped sites.

Site Requirements

These systems are used on sites where a usable layer of soil (i.e., at least four feet deep) is overlaid by one to five feet of impermeable soil and the slope does not exceed 15 percent. In addition, the vertical and horizontal separation distances noted in Figures 1 and 24 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met.

Design Criteria

At least four feet of usable soil (i.e., percolation rate of one to 60 minutes per inch above ground water, bedrock, or impermeable strata) shall be present below the one to five feet of impermeable soil. The required length of absorption trench shall be determined from Table 5 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate and percolation test results of the underlying permeable soil.

Construction

Trenches are excavated at least two feet into the usable soil and backfilled with aggregate or coarse sand to a level 30 inches below grade. The trenches shall be constructed parallel to surface contours and the trench bottoms shall be as level as possible. A conventional absorption field system shall be constructed in the upper 30 inches of the trenches. On sloped sites, a diversion ditch or berm shall be constructed on the uphill side of the absorption area to prevent surface runoff from entering the trenches. The soil placed above the aggregate in the trenches shall have a percolation rate faster than 60 minutes per inch. Original surface material (i.e. overlaying impermeable soil) shall not be used as backfill above the aggregate in the trenches.

Shallow Absorption Trenches

General Information

Shallow absorption trenches are constructed parallel to ground contours with the bottom as level as possible and located at or below original ground surface level. These systems may be used where sites contain at least two feet but less than four feet of permeable soil (i.e., one to 60 minutes per inch percolation rate). Diversion of surface runoff around the fill area by means of ditching/berming is required uphill of all sloped sites. Heavy equipment shall not enter the 'absorption area. Fill material shall have approximately the same percolation rate as the underlying permeable soil. Fill material, including a six inch topsoil layer, shall not be more than 30 inches above original grade, shall extend at least six feet horizontally from and at the same depth as the trench sidewall in fill, and shall have a maximum slope of one vertical to three horizontal at the edge to intersect with the original ground surface. A conventional absorption field system (i.e., trenches with distribution lines and aggregate) is constructed in the fill and original soil as shown in Figures 25 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Fill more permeable than the on-site permeable soil shall not be utilized to avoid short-circuiting of wastewater to the surface of the ground.

Site Requirements

These systems are used where there is at least two feet but less than four feet of usable soil. Vertical separation distances between trench bottoms and boundary conditions (i.e., bedrock, ground water, and impermeable strata) as depicted in Figures 17 and 25 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met. In addition, the horizontal separation distances noted in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Figure 1 shall be met. Note (c) in Table 2 is applicable to these systems whenever the bottom of trenches is less than six inches below the surface of existing in situ soil.

Design Criteria

A minimum two feet separation must be maintained between the bottom of each trench and all boundary conditions (i.e., bedrock. ground water, and impermeable strata). The bottom of each trench must not be above the original ground surface and should preferably be at least six inches below original grade. At least two percolation tests shall be performed within the proposed absorption area with the bottom of the test holes at the depth of the proposed trenches or at six inches below grade if the bottom of the proposed trenches will be between grade and six inches below grade. The slowest percolation rate observed shall be used to design the absorption facility. Fill material shall have a permeability similar to but not more permeable than the underlying in situ usable soil. The depth of fill material, including a six inch topsoil layer, shall not exceed 30 inches above the original ground elevation. A conventional absorption field system (i.e., trenches with distribution lines and aggregate) as depicted in Figures 17 and 25 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) is designed and constructed in the fill and original soil. The required length of absorption trench shall be determined from Tables 5 or 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate and percolation test results of the on-site permeable soil. Percolation test results of the in situ fill material (i.e., at the borrow pit shall be used to assure that the permeability of the fill material is compatible with the on-site soil permeability.

Construction

Generally, sites with large trees, numerous small trees, or large boulders are unsuitable for a shallow absorption trench system because of the difficulty in preparing the site and the reduced infiltration area beneath the system. In areas which are suitable, all trees and stumps shall be cut at grade and removed. Other vegetation (i.e. brush, vines, weeds, grass) shall be cut as close to grade as possible and removed. All leaves, limbs and boulders above grade shall also be removed. Root structure below grade should not be removed. Rototilling or soil scarification with construction equipment is not recommended.

Heavy equipment shall be kept out of the absorption area. Grade stakes may be used to delineate the limits of fill and prevent over-excavation of absorption trenches. Fill material shall be carefully placed within the absorption area. The edge of the fill material shall be tapered from at least six feet beyond any trench to original grade at a slope no greater than one vertical to three horizontal. On sloped sites a diversion ditch or berm shall be constructed on the uphill side of the fill material to prevent surface runoff from entering the fill. The shallow absorption trench system is constructed in the fill material and upon or in existing in situ soil. Construction of trenches at least six inches into existing in situ soil is preferred to utilize a stabilized sidewall infiltrative surface.

Cut and Fill Systems

General Information

A cut and fill system is an absorption trench system installed on sites where impermeable soil overlays usable soil (i.e., one to 60 minutes per inch percolation rate). These systems are generally used where the impermeable overlaying soil is one to five feet deep. Conventional absorption field systems may be used when the overlaying impermeable soil is no more than one foot deep and usable soil is placed above the aggregate. The overlaying impermeable soil is removed from the proposed absorption field area (i.e., extending al least five feel beyond any proposed absorption trench) and replaced by permeable soil comparable to the underlying soil as depicted in Figure 26 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Careful excavation is necessary to assure that the usable underlying soil is not made unusable through compaction and impermeable overburden does not remain in the bottom of the excavation (i.e., on top of the permeable underlying soil). A conventional absorption field system (i.e., trenches with distribution lines and aggregate) is constructed in the upper 18 to 30 inches of the permeable fill/underlying soil. If the bottom of all trenches are not in or at the permeable underlying soil (i.e., the bottoms are in fill), the fill must undergo stabilization and testing prior to constructing the trenches. Stabilization may be achieved by natural settlement for at least six months including at least one freeze-thaw cycle. If the underlying permeable soil and the comparable permeable fill comprise only granular material (i.e., sand and sandy loam similar to fill material for mound systems), stabilization may be achieved by mechanical compaction in six inch lifts to the approximate density of the undisturbed underlying granular soil. On sloped sites, a diversion ditch or berm shall be constructed on the uphill side of the fill area to prevent entrance of surface runoff.

Although deep trench systems may be used at sites with four feet of usable soil overlaid by one to five feet of impermeable soil, cut and fill systems are the recommended choice since shallow trenches provide improved treatment and enhanced oxygenation of the infiltrative soil surface (i.e., gravel to permeable soil interface).

Site Requirements

These systems may be used where all the following conditions exist:

  1. Soil with a percolation rate slower than 60 minutes per inch, such as clay or clay loam, overlays a usable soil with a percolation rate of one to 60 minutes per inch.
  2. At least three feet of usable soil is present beneath the overlaying impermeable soil.
  3. All minimum vertical and horizontal separation distances noted in Figures 1 and 26 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met.

Design Criteria

The overlaying impermeable soil shall be removed and replaced with a soil having a percolation rate comparable with the underlying usable soil. The excavation method selected should assure that the usable underlying soil is not made unusable through compaction. A conventional absorption field system (i.e., trenches with distribution lines and aggregate) as depicted in Figures 17 and 26 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) is designed for the upper 18 to 30 inches of the permeable fill/underlying soil. The required length of absorption trench shall be determined from Tables 5 or 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate and percolation test results of the permeable fill or underlying soil (i.e., whichever has the lower permeability). Stabilization of the fill is required prior to conducting percolation tests in the fill and constructing trenches if the bottoms of all trenches are not in or at the underlying usable soil. Percolation test results of the in situ fill material (i.e., at the borrow pit) shall be used to assure that the permeability of the fill material is compatible with the on-site soil permeability.

Construction

The area excavated and filled must provide at least a five foot buffer in each direction beyond the trenches. The soil placed above the aggregate in the trenches shall have a percolation rate faster than 50 minutes per inch. Original surface material (i.e., overlaying impermeable soil) shall not be used as backfill above the aggregate in the trenches. The surface area of the fill system must be mounded and graded to enhance runoff of precipitation from the absorption system and seeded to grass. On sloped sites. a diversion ditch or berm shall be constructed on the uphill side of the absorption area to prevent surface runoff from entering the fill.

Site Modification for Too Rapid Percolation Rate Soils

Where soils exhibit a percolation rate faster than one minute per inch and all horizontal and vertical boundary conditions are met, the site may be modified via a special cut and fill system. All soil bounded by two feet from the proposed absorption trenches (i.e., horizontally and vertically) shall be removed and blended with fine sand or sandy loam and replaced in six inch layers with mechanical compaction to the approximate density of the on-site soil. Percolation tests of the stabilized blended soil coupled with proposed daily flow rates shall be used to select the total lineal footage of distribution pipe needed. The blended soil percolation rate shall be in the five to 50 minutes per inch range. Conventional absorption trenches as depicted in Figure 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be constructed in the blended soil. Blended soil shall be used for trench backfill above the aggregate and permeable geotextile, untreated building paper, hay or straw. The vertical and horizontal separation distances noted in Figures 1 and 17 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met.

Replacement granular soil with a percolation rate of five to 60 minutes per inch range may be used to fill the excavation in place of blended soil. All other conditions noted above shall be met.

Absorption Bed Systems

General Information

An absorption bed system operates on a principle similar to the absorption trench except that several laterals, rather than just one, are installed in a single excavation. This reduces the effective sidewall infiltration area per lineal foot of lateral/distributor/leachline. These systems require the use of pressure distribution or siphon dosing and are limited to sites with a maximum slope of eight percent and at least four feet of usable soil with a percolation rate of one to thirty minutes per inch. Careful excavation is necessary to assure that the bottom and sidewall areas are not made unusable through compaction. Heavy construction equipment shall be kept outside the proposed bottom area of the bed. Diversion of surface runoff around the bed area by means of ditches or berms is required uphill on all sloped sites.

Site Requirements

A bed system may be built in soils with a percolation rate between one and 30 minutes per inch. A bed shall not be built where the soil evaluation indicates silty loam, clay loam, or clay. Slope of the site shall not exceed eight percent. The length of the bed shall be as parallel as possible with the ground contours. Bed systems are more practical on sites that are long (i.e., parallel to ground contours) and narrow with a minimal slope. All minimum vertical and horizontal separation distances noted in Figures 1 and 27 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met. Usable soil at the bed site must be at least four feet deep.

Design Criteria

Pressure distribution is required for the installation of an absorption bed system. The local health department having jurisdiction may allow the use of siphon dosing on specific sites. The maximum width of the bed shall be 20 feet. The maximum length of each lateral for a pressure manifold shall be 100 feet and for a siphon dosing system shall be 75 feet. Use of a center manifold system enables the distributor lengths to be doubled (i.e., from 100 feet to 200 feet and from 75 feet to 150 feet). Distribution boxes are not to be used with a pressure distribution system.

The depth of the bed shall be between 18 and 30 inches below original ground level (i.e., as measured at the midpoint of the width of the bed).

Distribution laterals shall be spaced five feet center to center. Two and one-half feet shall be provided between the laterals and the bed sidewalls (i.e., for both length and ends of laterals) as depicted in Figure 27 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

The maximum bed size is 20 feet wide by 205 feet for pressure distribution with a center manifold and 155 feet for siphon dosing with a center manifold. The maximum bed size is 20 feet wide by 105 feet for pressure distribution with an end manifold and 80 feet for siphon dosing with an end manifold.

Pressure distribution lines should be one inch to three inches in diameter with all downstream ends capped. Pumps for the pressure distribution system should be designed to provide one pound per square inch (2.3 feet of head) at the downstream end of the distribution laterals during bed dosing. Pipe for siphon dosing is sized to conform with the volume of the dose and can range from three to six inches in diameter based upon the volume of each dose. The downstream ends of all distributor laterals shall be capped. Dosing frequency should be selected based upon the absorption bed soil type as noted in Table 7A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). A minimum dosing frequency of 3/day is recommended based upon the design flow rate.

The volume discharged during each cycle of a pressure distribution system will exceed the volume available in the pipe distribution network and will be discharged from the pipe under pressure. The volume discharged during each cycle of a siphon dosing system should be 75% to 85% of the volume available in the pipe distribution network.

The required bed bottom area shall be calculated from the application rates shown in Table B.

Construction

The location of the proposed bed should be marked with stakes before construction begins. Heavy construction equipment shall be kept outside the proposed bottom area of the bed to avoid compaction of the soil (i.e., reduction in permeability). The required bed bottom area is excavated to the design depth as level as practical. The bottom and sidewalls of the bed shall be hand raked to reduce soil (i.e., infiltrative surface) smearing. Bottom levelness shall be checked with a transit, engineer's level or carpenter's level. The level bed shall be covered with a six inch layer of aggregate (i.e., ¾ to 1 ½ inch washed gravel or crushed stone).

The distribution laterals are installed level upon the six inch layer of aggregate. Additional aggregate shall be installed across the entire bed to two inches above the top of the distribution laterals.

The entire bed area of aggregate shall be covered with a permeable geotextile to permit movement of fluids and repel movement of backfill soil into the aggregate. Untreated building paper or a four inch layer of hay or straw may be substituted if a permeable geotextile is unavailable. The permeable geotextile shall be covered with permeable soil backfill, which should be mounded slightly above original grade. After settlement has occurred (i.e., at least six months including at least one freeze-thaw cycle), the area should be graded by hand to enhance surface drainage off the bed and seeded with grass. To enhance system performance, the seeded bed area should be routinely mowed. Use of lawn fertilizer on the bed area is not recommended.

Seepage Pits

General Information

A seepage pit is a covered pit with an open-jointed or perforated lining through which septic tank effluent infiltrates into the surrounding soil. These devices are sometimes called a leaching pit or leaching pool and incorrectly called a cesspool. A typical seepage pit detail is depicted in Figure 28 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used. Absorption fields provide better distribution of septic tank/aerobic unit effluent to the infiltrative soil surface, enhanced natural aeration of the infiltrative surface, and reduced probability of ground water contamination than seepage pits. Seepage pits may be used where the available land is insufficient for construction of shallow systems or where relatively impermeable shallow surface soils overlay deep usable soils. Cut and fill systems and deep trench systems are recommended for the latter condition.

Since shallow absorption systems provide better treatment than seepage pits, the bottom of seepage pits must be at least three feet above ground water, bedrock or impermeable strata. Large aggregate sized ¾ to 2 ½ inches (i.e., washed gravel or crushed stone) is used on the bottom of the pit and as an annular ring surrounding the pit liner. Larger sized aggregate (i.e., more than 2 ½ inches) which results in increased soil infiltrative surface masking (i.e., settled sewage cannot flow through the aggregate particles into the soil) and increased settlement of soil outside the annular ring from soil movement into the larger aggregate voids, is not recommended.

A distribution box shall be used to convey septic tank/aerobic unit effluent to more than one seepage pit. Pits shall not be connected in series. When multiple pits are used, all should be approximately the same size. Each pit should receive approximately the same flow of septic tank/aerobic unit effluent. On sloped sites, a diversion ditch or berm should be constructed uphill from the pit(s) to prevent surface runoff from entering the pit(s). Series of pits should be installed parallel rather than perpendicular to ground contour lines to minimize ground water mounding. Use of shallow pits is recommended in place of deep pits to enhance natural aeration of the soil infiltrative surface.

Site Requirements

If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used. All of the other conventional absorption systems are recommended over seepage pits. A minimum three foot vertical separation must exist between the bottom of any pit and the high ground water level, bedrock; or impervious strata as depicted in Figure 28 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Sufficient area must be available to provide three times the effective diameter between pits (i.e., undisturbed soil between pit excavations) and all required horizontal separation distances from any pit as shown in Figure 2 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met.

Design Criteria

The required number and size of seepage pits for a given household shall be determined from Tables 9 and 10 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate and percolation test results of the on-site permeable soil. Pits should be as shallow as possible to enhance natural aeration of the soil infiltrative surface. Pit depth is frequently controlled by the depth to ground water, bedrock, and impermeable strata. If relatively uniform soil is encountered, two percolation tests should be made for each pit: one at the halfway depth and the other at the bottom of the pit. The two results are averaged to obtain the applicable percolation rate. If different soil layers are encountered at the proposed pit sidewall area, a percolation test shall be conducted in each permeable layer and the applicable pit design percolation rate shall comprise the weighted average of each test result based upon the depth of each permeable layer. The required "effective seepage pit sidewall area" can be determined from Tables 9 and 10 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate and the applicable percolation rate. The "applicable percolation rate" should only be used where the impervious soil layers (i.e., percolation rate slower than 60 minutes/inch) comprise small lenses rather than extensive/broad layers. The three foot vertical separation of the pit bottom above impermeable strata should apply to the entire effective sidewall area of a pit.

No allowance for infiltration area shall be made for the bottom area of a pit or the sidewall surface area of impervious soil layers (i.e., percolation rate slower than 60 minutes/inch). The preferred soil percolation rates for seepage pits range from five to 60 minutes per inch. The required effective seepage pit sidewall infiltrative area is shown in Table 9 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) for various design flows and sidewall percolation rates.

The effective diameter of a pit is the diameter of the large aggregate-infiltrative soil surface cylinder (i.e., the diameter of lining plus the annular ring of large aggregate) as shown in Figure 28 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The effective depth of a pit is the vertical distance from the invert of the inlet to the bottom of the pit with the thickness of any intervening impervious layers deducted. The required effective seepage pit area for a given household can be obtained by varying:

  1. The number of seepage pits
  2. The shape (usually cylindrical) of the pit, and
  3. Seepage pit dimension (i.e., effective depth, diameter, length, width, etc.).

Table 10 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shows the effective sidewall absorption area for various sized cylindrical seepage pits.

Perforated or open-jointed linings may be precast concrete, cast-in place concrete or constructed in place with unmortared hollow cinder or concrete blocks. Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended. Material with comparable structural strength, determined in accordance with commonly accepted sewage facility construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced.

Precast rings with large perforations all around are widely used for seepage pits due to their structural soundness, durability, and low cost of installation. Often advertised as "dry wells", these precast rings are usually available in various diameters with approximately two to four foot heights. The sections are manufactured with ship lap joints to facilitate stacking. Footing rings are also readily available when needed.

If standard building block is used to construct the lining, eight inch block should be used with the cells (holes) horizontal and facing the nearest pit sidewall.

Seepage pit covers shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be precast concrete or cast-in-place concrete and shall be reinforced. A manhole and removable cover with an opening of at least 20 inches in the shortest dimension shall be provided (see Figure 28 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)).

Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.

Laterals conveying effluent from septic tanks or distribution boxes to seepage pits shall be non-perforated (i.e., watertight), at least four inches in diameter, and installed with a minimum slope of 1/8 inch per foot.

Seepage pits shall not be connected in series. A distribution box shall be used to convey settled sewage to more than one seepage pit.

The separation distance between the outside edges of seepage pits shall be at least three times the effective diameter of the largest pit. This separation distance is measured across the undisturbed soil between pit excavations.

When multiple pits are used, each pit shall have an equal effective sidewall absorption area to facilitate equalization of flow to the total infiltrative surface area. The distribution box shall provide equal flow to each pit. On sloped sites, a diversion ditch or berm should be constructed uphill from the pit(s) to prevent surface runoff from entering the pit(s).

Construction

The pit shall be excavated in accordance with its design dimensions (i.e., depth and sidewall area) and the bottom shall be made as level as possible. Bottom and sidewall areas shall be raked to minimize smearing and enhance infiltration. If ground water is encountered during pit excavation, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum ground water and adjustments made to the proposed pit dimensions (i.e., redesign the system and maintain minimum pit separation distances). Impermeable overburden shall not remain in the bottom of an excavated pit.

At least a six inch layer of large aggregate (¾ to 2 ½ inch size) shall be placed over the entire pit bottom and leveled. A 12 inch layer of large aggregate placed over the entire pit bottom is recommended. If footing rings are needed they shall be installed on the leveled bottom layer of large aggregate. Perforated rings shall be installed upon the footing rings or directly upon the leveled bottom layer of large aggregate. Additional rings shall be installed as needed. The annular space between the perforated rings and the pit sidewall shall be filled with large aggregate to the bottom of the inlet pipe elevation. The watertight four inch minimum diameter inlet pipe shall be installed with a minimum slope of 1/8 inch per foot. The annular ring of large aggregate shall be covered by a permeable geotextile, untreated building paper, or a four inch layer of hay or straw to prevent soil from filling the aggregate voids.

A reinforced concrete structurally sound pit cover capable of supporting 300 pounds per square foot at its weakest point shall be installed. A manhole and removable cover with an opening of at least 20 inches in the shortest dimension shall be installed and its top shall be located six to 12 inches below grade. A manhole cover location marker (i.e., treated lumber or concrete post) should be installed above the top of the cover to finished grade for future maintenance. Permeable soil shall be used as backfill to fill the remaining excavation to grade with some mounding to allow for settlement. The upper six inches of the permeable backfill may be topsoil to facilitate future seeding to grass. On sloped sites, a diversion ditch or berm should be constructed uphill from the pit(s) to prevent surface water from entering the pit(s).

Alternative Systems

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General Information

The treatment systems addressed in the previous subsurface treatment section are classified as conventional systems and may be used on sites with adequate soil percolation and vertical/horizontal separation distances to boundary conditions. At many sites that are not suitable for conventional systems, consideration can be given to the construction of alternative systems to assure proper treatment of sewage rather than to restrict use of land. However, there will be sites that are not suitable for residential development using conventional or alternative systems. Such sites require public sewers and treatment for residential development to avoid creation of public health hazards/nuisances.

Sites compatible with development using alternative systems generally require detailed evaluation, complicated design, and costly construction and maintenance. Plans for alternative systems described in this section must be designed by a licensed design professional and approved by the local health department. Plans for alternative systems approved by SHD staff (e.g., District Office staff) should be filed with the local code enforcement officer to assure construction in accord with approved plans. The design professional shall certify to the local health department that the site/soil evaluation and the plans meet the minimum requirements of these standards. Construction must be supervised by the design professional, and certification of construction in conformance with the approved plans shall be provided by the design professional to the local health department. Any of these alternative system requirements may be waived by a local health department which provides that particular service(s).

Alternative designs not addressed in this section may be considered by the local health department on a limited experimental basis or for replacement systems on difficult sites. Plan preparation, site/soil evaluation, design, and construction certification noted previously shall be provided for alternative systems. Performance monitoring of alternative systems is highly recommended.

The most commonly used alternative systems include raised systems, mounds and intermittent sand filters. All alternative systems shall be preceded by an appropriate septic tank or aerobic unit. All distribution lines shall be installed parallel to the contours of the site.

Placement of fill material or construction of absorption facilities in fill should not occur when the soil moisture content is high. If a fragment of soil from approximately nine inches below the surface (i.e., borrow pit, fill placement site, or stabilized fill) can easily be rolled into a ribbon instead of crumbling, the soil moisture content is too high for placement/construction. Obviously, some silt or clay must be present for a ribbon to be formed.

A minimum of 50 percent expansion of the absorption area is required for all new conventional or alternative systems and 100 percent expansion is recommended. Future maintenance should include periodic flow diversion from the replacement system to the original system and back for long-term system operation.

Raised System

General Information

A raised system comprises a conventional absorption trench system constructed in stabilized permeable fill placed above the original ground surface on a building lot.

Site Requirements

A raised system may be used where site investigation demonstrates all of the following conditions exist:

  1. There is at least one foot of usable original soil with a percolation rate between one and 60 minutes/inch above any impermeable soil layer, bedrock or maximum high ground water level.
  2. Slope of the ground surface at the proposed absorption facility plus at least 50 percent expansion shall not exceed 15 percent.
  3. All minimum horizontal and vertical separation distances as described in Figures 29 and 30 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) must be maintained for the proposed system and 50 percent absorption facility expansion.

If any site has been modified to meet condition (2) above, it shall be evaluated after soil stabilization has occurred.

Design Criteria

Design of a raised system shall be based upon the design flow rate and the results of a site investigation including topography, slope, ground water elevation, depth to impermeable soils and bedrock, rock outcroppings, soil permeability, vegetation, land drains, etc., plus any necessary site modifications. Percolation and deep hole tests are required. A raised system is depicted in Figures 29 and 30 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

At least two percolation tests shall be performed at the one foot below grade depth of in situ soil at the proposed location of a raised system to verify the in situ soil is usable (i.e., 1 to 60 minutes per inch). At least one deep test hole ( ≥ 4 feet) shall be dug at the proposed location of a raised system to verify boundary conditions. At least one percolation test and one deep hole test should be performed at the proposed location of the raised system expansion area.

Percolation tests shall be conducted in the in situ fill material at the borrow pit and in the stabilized fill material at the construction site. Conducting percolation tests of in situ borrow pit soils provides a good indication of eventual stabilized fill percolation rates. The slower percolation test results of the in situ borrow pit soils and the construction site stabilized soil shall be used for design of an absorption trench system. Construction of the absorption trench system in the filled area is identical to that of a conventional system (See Figures 13 thorugh 17, 19, 29 and 30 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)) including laterals being installed parallel to contour lines for the native soil prior to installing fill.

The total quantity and dimensions of fill required to be placed at the construction site shall be based upon the design of the basal area plus a minimum 20 foot taper. The taper shall be sloped no greater than one vertical to three horizontal.

The basal area is defined as the total area beneath the absorption trenches extending 2.5 feet in all directions from the outer edge of all trenches. The minimum size of the basal area shall be calculated using 0.2 gpd/sq. ft. for the residence being considered. Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be used to determine daily flow for the residence. Use of slowly permeable soils for the fill material will result in a trench system requiring an enlarged basal area.

Sufficient stabilized soil with a percolation rate between 5 and 30 minutes per inch is required to maintain at least two feet separation between the proposed bottom of the absorption trenches and any boundary condition such as ground water, bedrock, clay or other impermeable soil or formation. Preferred fill soil comprises sand or sandy loam with a percolation rate of 5 to 10 minutes per inch to minimize construction and operation problems. Very permeable and slowly permeable fill materials should be avoided to prevent surface exposure of inadequately treated sewage from too rapid or impeded wastewater movement through the fill. If the in situ unsaturated soil has a percolation rate faster than one minute per inch, sufficient stabilized soil with a percolation rate between 5 and 30 minutes per inch is required to maintain at least two feet separation between the proposed bottom of the absorption trenches and in situ soil.

The edge of the fill material beyond the basal area shall be tapered at a slope no greater than one vertical to three horizontal with a minimum taper length of 20 feet. All minimum horizontal separation distances as described in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be measured from the outer edge of the taper since the fill is considered a system component.

The fill must be stabilized prior to conducting final percolation tests and construction of the trench absorption system. Soils whose permeability characteristics could change significantly upon stabilization (i.e., soils containing clay and/or silt) shall be allowed to settle naturally for a period of at least six months and include one freeze-thaw weather cycle. Soils whose permeability characteristics will not change significantly upon stabilization (i.e., sand and sandy loam similar to soils required for mound systems with a percolation rate of 5 to 30 minutes per inch) may be mechanically compacted or allowed to settle naturally as indicated above. Mechanical compaction shall be achieved via track type machines (e.g., bulldozer or front end loader with downward blade/bucket pressure) or steel wheeled roller. Mechanical compaction shall be accomplished in shallow lifts (i.e., approximately six inches) to the approximate density of the undisturbed borrow pit soil. Compaction must be carried out carefully to avoid creating layers of different density.

The absorption trench system shall incorporate an automatic dosing device, siphon or pump, or pressure distribution unless: (a) the system is to be installed under the jurisdiction of a local health department which has a program incorporating site evaluation, system design approval, and construction inspection/certification; and, (b) a minimum of two feet of fill material with a percolation rate of five to 30 minutes/inch shall be placed between the bottom of the trenches and the existing ground. When an automatic dosing device, siphon or pump, or pressure distribution is used, dosing frequency shall be in accord with Table 7A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate (e.g., ≤110 gal/dose for sandy loam fill and a 330 gpd design flow).

Curtain drains may be used on the uphill side of proposed fill areas on sloped sites to intercept and control ground water where high ground water levels exist. Non-perforated pipe constructed to convey ground water from the perforated drain pipes to the ground surface should be installed on in situ soil bedding at least five feet from the toe of the slope of the fill material. Excavated soil shall be used as backfill around and above the non-perforated pipe. The surface outlet shall be protected from water/soil erosion and animal entry.

On sloped sites, a diversion ditch or curtain drain shall be constructed uphill from the fill to prevent surface runoff from entering the fill. The topsoil surface of the fill shall be graded to enhance runoff of precipitation. Short circuiting of wastewater to the nearby drainage systems shall be avoided.

Construction

Preparation of the site on which the fill is to be located and placement of fill on the site are critical to proper operation of the absorption system. Extreme care must be taken to assure that construction techniques do not compromise the integrity of the system. Heavy construction equipment must not be allowed within the area of the system.

Generally, sites with large trees, numerous small trees, or large boulders are unsuitable for a raised system because of the difficulty in preparing the surface and the reduced infiltration area beneath the system. In areas which are suitable, all trees and stumps shall be cut at grade and removed. Other vegetation (i.e., brush, vines, weeds, grass) shall be cut as close to grade as possible and removed. All leaves, limbs and boulders above grade shall also be removed. Root structure below grade should not be removed. Rototilling or soil scarification with construction equipment is not recommended. The underlying soil shall be undisturbed although the surface may be plowed with at least a double bottomed blade/furrow plow and the furrow turned upslope.

After the site has been cleared and plowed, all traffic shall be excluded. Fill material can be deposited on the site with a front end loader or pushed on from the side, preferably the upslope side, using a track type machine with at least six inches of fill beneath the tracks. At no time should ruts be made in the plowed area.

Fill should be placed on the site immediately after it is prepared to avoid undesirable changes to the native soil (i.e., traffic, compaction, erosion, etc.). The fill shall be stabilized as described under "design criteria" above. After stabilization, the absorption trench system shall be constructed as noted in the "Design Criteria."

After the absorption trenches have been constructed in the stabilized fill (i.e., including backfilling trenches), the entire surface of the absorption system including the tapers shall be covered with a minimum of six inches of topsoil, mounded to enhance runoff of precipitation from the system (i.e., ≥ 1% slope) and seeded to grass.

Appropriate curtain drains and diversion ditches shall be constructed uphill from the absorption system on sloped sites to prevent ground water from interfering with absorption system operation or surface runoff from entering the fill.

Mounds

General Information

A mound system is a soil absorption system that is elevated above the natural soil surface in suitable fill material. Mounds are a variation of the raised system utilizing sandy fill material without requiring a stabilization period prior to construction of the absorption bed/trenches. On sites with permeable soils of insufficient depth to groundwater or creviced or porous bedrock for a conventional absorption system, the fill material in the mound provides the necessary treatment of wastewater. The overall size of a mound system will usually be substantially smaller than a raised system. The size reduction is achieved by improved solids retention in the required multi-compartment septic tank or tanks in series coupled with required pressure distribution (i.e., uniform distribution of settled effluent within the absorption bed/trenches.)

Installation of water saving devices (i.e., faucets. shower heads, toilets) is recommended for any residence using a mound system to minimize wastewater flow. When mound systems are proposed as replacement systems for existing homes, water saving fixtures should be retro-fitted in the household plumbing.

Site Requirements

A mound system may be used where site investigation demonstrates all of the following conditions exist:

  1. There is at least two feet of naturally occurring soil above bedrock.
  2. The maximum high ground water level must be at least one foot below the natural ground surface.
  3. The percolation rate of the naturally occurring soil shall be between one and 120 minutes per inch in the upper foot of soil.
  4. Slope of the original ground surface at the proposed absorption facility plus 50 percent expansion shall not exceed 12 percent.
  5. All minimum horizontal and vertical separation distances as described in Figures 31, 32 and 33A (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and Tables 2 and 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) must be maintained for the proposed system, and 50 percent absorption facility expansion.

Design Criteria

Design of a mound system shall be based upon the design flow rate and the results of a site investigation including topography, slope, ground water elevation, depth to impermeable soils and bedrock, rock outcroppings, soil permeability, vegetation, land drains, etc., plus any necessary site modifications. Site modification is not acceptable to attain the required maximum 12 percent slope for the absorption area plus at least 50 percent expansion (i.e., the natural site must have a slope not exceeding 12 percent). Percolation and deep hole tests are required. At least one deep test hole at least 4 feet deep or to bedrock shall be dug at the proposed location of the primary mound system to verify boundary conditions. Mound systems are depicted in Figures 31 through 33C (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

The design professional shall consult with the local health unit having jurisdiction regarding the method for detailing the hydraulic design.

The basal area of a mound system is defined differently than a raised system. The basal area for a mound system on level ground includes all the area beneath the absorption trenches or bed and the area under the tapers (i.e., basal area = [LW] in Figure 33B (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). On a sloping site (i.e., with a slope of one to 12 percent), the basal area includes only the area under the absorption trenches/bed and the lower or downhill taper (i.e., basal area [(math formula B+L divided by 2) (C) + AB] in Figure 33B (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook)). All minimum horizontal separation distances depicted in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met and shall be measured from the outer edge of the taper since the fill is considered a system component. All minimum vertical separation distances depicted in Figure 33A (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met and dimension D in Figure 33A (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) must be at least one foot. The required basal area is dependent upon the daily flow rate and soil percolation test results of naturally occurring soil. Settled sewage application rates depicted in Table 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be used when the percolation rate is between one and 60 minutes per inch. A settled sewage application rate of 0.2 gallons/day/square foot shall be used when the percolation rate is 61 to 120 minutes per inch. Percolation test holes shall be approximately 12 inches deep to determine the percolation rate of the upper foot of naturally occurring soil. Percolation tests shouId be conducted near each end of tile proposed mound and the proposed 50 percent expansion area. The slowest percolation test results (i.e., worst case observed within the selected basal area) shall be used to design the basal area required. At least one deep hole test should be performed at the proposed location of the mound system expansion area to verify boundary condition separation.

The recommended separation distance between mound systems (i.e., toe of slope of fill to toe of slope of fill) perpendicular to ground contours is at least 30 feel. Heavy construction equipment shall not be allowed within the basal area and a recommended minimum 20 feet wide area downslope of the basal area which acts as a dispersal area for the mound.

Percolation tests for the fill material shall be conducted at the borrow pit in areas representative of the soil to be obtained. Only soils with a percolation rate between five and 30 minutes per inch shall be used for the fill material. Sands with greater than ten percent by weight finer than 0.074 mm (i.e., Number 200 sieve) must be avoided. At least 25 percent of the material by weight shall be in the range of 0.50 to 2.0 mm. Less than 15 percent of the material by weight shall be larger than a half-inch sieve. A sieve analysis is recommended and may be necessary to verify compliance with the soil specifications. The 0.05 mm material requirement is a numerical error and inconsistent with the commonly used U.S. Sieve Series.

A pressure distribution network shall be required and the distribution lines shall be installed parallel to the contours of the site. The width of the system (i.e., up and down slope) shall be kept to a minimum and, in no case, shall the absorption area be wider than 20 feet (i.e., outside edge of bed or trench to outside edge of bed or trench) as depicted in Figures 31 and 32 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). Distribution lines shall not exceed four per mound and shall be spaced as depicted in Figures 31 and 32. A four distributor mound shall not be expanded later to a six distributor mound to satisfy a needed 50 percent absorption system expansion. In a distribution network using a central manifold, distribution lines shall have a maximum total length of 200 feet (i.e., end cap to end cap) as depicted in Figure 16 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). In a distribution network using an end manifold, distribution lines shall have a maximum length of 100 feet (i.e., manifold pipe to end cap) as depicted in Figure 15 (Figures can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook). The overall size of absorption facilities in a mound (i.e., bed or trenches) shall not exceed 20 feet by 205 feet for a central manifold or 20 feet by 105 feet for an end manifold.

Figures 33A, 33B and 33C are referenced for calculating mound dimensions. The letter dimensions are explained below.

  1. = absorption bed/trench total width and shall not exceed 20 feet.
  2. = absorption bed/trench total length and shall not exceed 205 feet for a central manifold or 105 feet for an end manifold.
  3. = downslope setback and shall be the larger of (1) approximately three times the height of the mound at the downslope edge of tile absorption facility (i.e., [3][E + F+ G]); or, (2) the dimension calculated from percolation tests on the naturally occurring soil plus flow rates to meet the required basal area. The required basal area for a level-site is [WL] and for a sloped site is [(math formula B+L divided by 2) (C) + AB].
  4. = depth of fill at the upslope edge of the absorption facility between the top of the plowed surface and the bottom of the absorption facility and shall be at least one foot.
  5. = depth of fill at the downslope edge of the absorption facility between the top of the plowed surface and the bottom of the absorption facility and shall be equal to [D + (slope of site)(A)).
  6. = E for level sites.
  7. = depth of aggregate in the absorption facility and shall be at least 9.5 inches (i.e., ≥ six inches of aggregate plus ≥ 1.5 inches of distribution pipe diameter plus ≥ two inches of aggregate).
  8. = depth of silty/clayey loam cap plus topsoil cover at the upslope and downslope edges of the absorption facility and shall be at least one foot (i.e., ≥ six inches of silty/clayey loam and ≥ six inches of topsoil).
  9. = depth of silty/clayey loam cap plus topsoil cover at the width center of the absorption facility and shall be at least 1.5 feet (i.e., ≥ one foot of loam cap plus ≥ six inches of topsoil).
  10. = upslope setback and shall be at least:
    [3(D + F + G)] - [Slope of Site][(3)(D + F+ G)) =[1 - Slope of Site][(3)(D + F + G)).
  11. = sideslope setback and shall be at least three times the total height of the mound (i.e., [3][(math formula D+E divided by 2)+ F+H]).
  12. = total length of mound and equals [B + 2K). The length shall be parallel to the site contours.

     W. = total width of mound and equals [A + C+J). The width shall be perpendicular to the site contours.

The silty/clayey loam cap shall extend beyond the aggregate at least 1.5 feet with the top surface having a slope not exceeding 1 vertical to 3 horizontal.

Mound dimensions shall be consistent with Figures 33A through 33C and meet or exceed those required by the health unit having jurisdiction.

The required basal area for a relatively level site is [WL) and for a sloped site (i.e., one to 12 percent slope) is [(math formula B+L divided by 2)(C) + (AB)).

The required absorption area of the bed or trenches shall be based upon the daily flow rate, the percolation rate of the in situ fill material (i.e., at the borrow pit) and settled sewage application rates depicted in Tables 5, 6 or 8 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook).

Design professionals should refer to pages 278 through 296 of the 1980 EPA Design Manual for Onsite Wastewater Treatment and Disposal Systems for design of the required pressure distribution network (i.e., including hydraulic design equations, nomographs, charts, tables, and diagrams) and/or Appendix A. Dosing frequency shall be in accord with Table 7A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) based upon the design flow rate. Appendix A "Comprises an example solution for mound design as per Appendix 75-A.

The absorption system aggregate shall be completely covered with a permeable geotextile to prevent infiltration of soil into the aggregate.

A dual chamber septic tank or two tanks in series in addition to the dosing tank (i.e., pump storage) shall be provided. A gas baffle or other outlet modification that enhances solids retention in the tank(s) is recommended.

Construction

Preparation of the site on which the mound is to be located, placement of fill on the site, construction of the absorption bed/trenches, placement/grading of the cap, grading the exposed fill, and grading/seeding the top soil are critical to proper operation of the absorption system. The in situ soil beneath the mound basal area must be capable of absorbing the filtered effluent of the mound fill. Extreme care must be taken to assure that construction techniques do not compromise the integrity of the mound system. Heavy construction equipment must not be allowed within the fill area of the system or immediately downslope of the system. The downslope area provides effluent dispersal for the mound system. Placement of fill material or construction of absorption facilities in fill shall not occur when the soil moisture content is high as explained in the general information for alternative systems.

Generally, sites with large trees, numerous small trees, or large boulders are unsuitable for a mound system because of the difficulty in preparing the surface and the reduced infiltration area beneath the system. In areas which are suitable, all trees and stumps shall be cut at grade and removed. Other vegetation (i.e., brush. vines. weeds, grass) shall be cut as close to grade as possible and removed. All leaves, limbs and boulders above grade shall also be removed. Root structure below grade should not be removed. Rototilling or soil scarification with construction equipment is not recommended. The proposed mound area shall be plowed to a depth of seven or eight inches with a double bottomed blade/furrow plow and the furrow turned upslope.

After the site has been cleared and plowed, all traffic shall be excluded. Fill should be placed on the site immediately after it is prepared to avoid undesirable changes to the plowed native soil, (i.e., traffic, compaction, erosion). Fill material can be deposited on the site with a front end loader or pushed on from the side (i.e., preferably the upslope side) using a track type machine with at least six inches of fill beneath the tracks. At no time, should ruts be made in the plowed area.

The fill material shall cover the entire mound area. The required bed or trenches shall be constructed in the fill. Construction of the absorption bed/trenches in the fill material shall be in accord with Figures 19, 27, 31, and 32 including distribution laterals being installed parallel to contour lines for the native soil prior to installing fill. The bottom of the bed/trenches shall be level. The bottom and sidewalls of the bed/trenches shall be raked prior to installing aggregate.

Pressure distribution laterals shall be installed on at least six inches of aggregate and be covered by at least two inches of aggregate. The aggregate in the bed/trenches shall be completely covered with a permeable geotextile to prevent infiltration of line soil into the aggregate.

The tapered slopes of the fill shall not exceed one vertical to three horizontal. A clayey loam or silty loam cap as depicted in Figures 31, 32 and 33A shall be installed over the entire absorption area (i.e. six inches deep along the two outer edges parallel to the contours and one foot deep at the centerline and parallel to the two noted parallel edges.) The loam cap shall be tapered to approximately 18 inches beyond the absorption area and mounded to enhance runoff of precipitation (i.e. ≥ 1% slope.) The entire mound including the tapers shall be covered with six inches of topsoil and seeded to grass.

Appropriate curtain drains and diversion ditches shall be constructed uphill from the mound on sloped sites to prevent ground water from interfering with absorption system operation or surface water from entering the mound.

Intermittent Sand Filters

General Information

Intermittent sand filtration comprises the intermittent application of settled wastewater to a bed of granular material which is underdrained to collect and discharge filtered effluent. Septic tank or aerobic unit effluent is intermittently spread across the surface of a bed of sand via perforated distribution lines in aggregate. Collector pipes in aggregate beneath the sand and a three inch layer of 1/8 to ¼ inch diameter crushed stone or washed gravel collect filtered wastewater for additional treatment. Sand filter effluent shall be discharged to a subsurface absorption facility (i.e., downstream absorption mound or modified shallow trench system).

Intermittent sand filter and downstream absorption systems should only be used on large lots. These systems are not intended for use where the surface soil/rock is impermeable since the absorption system would exhibit continuous weeping. Absorption systems may exhibit some weeping of double filtered wastewater (i.e., by the sand filter and the absorption system) during the wet season. Hence, the absorption systems should be located distant from residences and property lines.

Site Requirements

An intermittent sand filter and absorption mound system may be used where site investigation demonstrates all of the following conditions exist:

  1. All horizontal separation distances shown in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) must be maintained for the proposed sand filter plus the downstream absorption system with proposed 50% expansion. All minimum horizontal separation distances depicted in Table 2 shall be measured from the outer edge of the taper/sand filter since the fill/sand is considered a system component.
  2. The bottom of the collector pipes shall be at least two feet above the maximum high ground water elevation.
  3. An environmental assessment must indicate that development of the site with a sand filter and downstream absorption system is consistent with the overall development of the area and will cause no adverse environmental impacts.
  4. The natural slope of the site of the downstream mound must not exceed 12%.
  5. The percolation rate in the upper six inches of in situ soil at the proposed mound site (i.e., a six inch deep percolation test hole) should be no slower than 120 minutes/inch. The percolation rate for a 12 inch deep percolation test hole has no limitation (i.e., it may be much slower than 120 minutes/inch).

An intermittent sand filter and modified shallow trench system may be used where site investigation demonstrates all of the following conditions exist:

  1. Above-noted conditions (1), (2) and (3).
  2. The natural slope of the downstream modified shallow trench must not exceed 15%.
  3. The percolation rate in the upper six inches of in situ soil at the proposed downstream modified shallow trench system site (i.e., a six inch deep percolation test hole) should be no faster than 1 minute/inch and no slower than 60 minutes/inch. The percolation rate for a 12 inch deep percolation test hole has no limitation (i.e., it may be much slower than 60 minutes/inch).

Design Criteria (Sand Filter)

Design of an intermittent sand filter and downstream absorption system shall be based upon the design flow rate and the results of a site investigation including topography, slope, ground water elevation, rock outcroppings, soil permeability, vegetation, land drains, etc., plus any site modifications. An intermittent sand filter is depicted in Figures 34 and 35. A six-inch layer of relatively impermeable clay shall be placed under and along the sides of a proposed sand filter being located upon any rock, shale, etc., surface to prevent short circuitinq of wastewater to an aquifer.

Septic tanks installed upstream of an intermittent sand filter shall have dual compartments or two tanks in series. Use of a gas deflection baffle on the septic tank outlet is strongly recommended. A gas deflection baffle is required when a garbage grinder can reasonably be expected at the time of construction or in the future.

A dosing siphon or pump shall be used to apply septic tank/aerobic unit effluent to a sand filter with 300 lineal feet or more of distributors or 900 square feet or more of filter area. Gravity distribution may be used for smaller filters. Pressure distribution pumps shall be selected to maintain a minimum pressure of one psi (2.3 feet of head) at the downstream end of each distribution line during the distribution cycle.

Perforated distributor lines shall be installed three feet center to center and 1.5 feet from the filter sidewall. The downstream end of each pressure distributor shall be capped. The downstream end of each gravity or dosing distributor may be capped or vented (i.e., via an individual or common vent). Perforated distributor lines for gravity systems shall be four inches in diameter and have a slope of 1/16 to 1/32 inch per foot. Perforated pressure distribution lines shall be one to three inches in diameter and installed level. Perforated distributor lines for dosed systems shall be three to four inches in diameter and installed level. When soil cover above the filter (i.e., above the permeable geotextile and aggregate surrounding the distributors) exceeds 24 inches, the downstream end of distributors (i.e., gravity flow) or the upstream end of collectors (i.e., pressure distribution) should be manifolded and connected to a downward facing screened vent to provide oxygen to the wastewater-sand interface.

Four inch diameter perforated collector pipes beneath the filter shall be centered between distributor pipes, have a maximum center to center spacing of 12 feet, have a maximum spacing from the filter sidewall of six feet, and have a slope of 1/16 to 1/8 inch per foot. The soil surface beneath the filter shall be sloped toward the trenches in which the underdrains are laid. The trenches beneath the underdrains shall have a slope of 1/16 to 1/8 inch per foot.

Open joint pipe with joint covering may be used in lieu of perforated pipe for distributors or collectors except for pressure distributors.

The distribution system incorporating pressure or dosing shall be designed to dose the filter at least three times daily based upon the design flow rate. The volume of each dose should be approximately 75 percent of the volume of the distribution lines when dosing is used.

The sand media shall have an effective grain size of 0.25 to 1.0 mm (i.e., sieve sizes number 60 to slightly larger than number 20). A log-log plot of mechanical sand analysis is used to determine effective grain size. Effective grain size is the dimensions of that mesh screen which will permit 10 percent of the sample to pass and will retain 90 percent. If nitrification is not required by the local health department, the effective grain size shall be in the range of 0.5 to 1.0 mm (i.e., sieve sizes slightly smaller than number 30 to slightly larger than number 20.) All sand shall pass a ¼ inch sieve.

The uniformity coefficient of the sand shall not exceed 4.0 (i.e., D60/D10 ≤ 4.0 where D60 is the size of sieve opening at which 60% of the sand particles pass through and 40% is retained and D10 is the size of sieve opening at which 10% of the sand particles pass through and 90% is retained).

The maximum allowable daily septic tank/aerobic unit effluent application rate to the sand filter shall be 1.15 gal/day/sq. ft. when pressure distribution or dosing are used and 1.0 gal./day/sq.ft. for gravity distribution.

Design Criteria (Downstream Absorption Facility Option)

A modified shallow trench system may be used in-lieu-of a downstream mound to provide subsurface dispersal of sand filter effluent when at least six inches of in situ unsaturated soil with a percolation rate of 1- 60 minutes/inch exists. Sufficient soil similar to the in situ permeable soil shall be provided to assure a trench depth of at least 18 inches with a minimum one foot depth of aggregate filled sidewall contacting permeable soil. Trenches shall be designed upon the estimated quantity of effluent reaching the sand filter collector pipe and an application rate of 1.2 gal./day/sq. ft. for two feet wide trenches. Trenches shall be located at least 20 feet from drainage ditches and should be as long as possible to minimize multiple parallel trenches. Distributor length shall not exceed 60 feet for gravity flow and 100 feet for pressure distribution. The finished slope of the grass seeded fill shall not exceed one vertical to three horizontal.

Design Criteria (Downstream Mound)

Effluent from the sand filter collector pipes shall be discharged to a mound that is constructed above the original ground surface. The size of the mound shall be based upon the estimated quantity of filtered effluent reaching the sand filter collector pipes and an application rate of 1.2 gal/day/sq. ft. to mound trenches with pressure distribution regardless of the underlying soil percolation rate (i.e., in a 12 inch deep percolation hole). A percolation rate no slower than 120 minutes/inch is necessary in the upper six inches of in situ soil (i.e., a six inch deep percolation hole) to prevent rapid mound failure.

Some of the septic tank/aerobic unit effluent applied to the sand filter undergoes evapotranspiration and percolation into the soil beneath the sand filter. The estimated quantity of effluent reaching the sand filter collector pipes shall not be less than 85% of the daily design flow rate to the sand filter. If the percolation rate at the bottom of the sand filter (i.e., the soil beneath the sand filter) is approximately 120 minutes/inch, a design flow figure of 85% may be used. If a percolation rate of ≥ 180 minutes/inch occurs at the bottom of the sand filter, a design flow figure of 95% may be used.

The site for a proposed downstream mound plus 50% future expansion shall not exceed 12% slope for naturally occurring soil. Downstream mounds shall be similar to Figures 33B, 33C, and 36. Fill material for a downstream mound shall consist of medium sand with a percolation rate tested at the borrow pit, between five and 30 minutes/inch. All minimum horizontal and vertical separation distances as described in Figure 36 and Tables 2 and 3A (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be met for the proposed downstream mound system and 50% absorption system expansion. The six inch layer of permeable soil should extend 100 feet radially from the downstream mound in the direction of flow. Since downstream mounds may exhibit some weeping during the wet season, the mounds should be located distant from the residence and at least 100 feet from any property line.

A pressure distribution network is required and the distribution laterals shall be installed parallel to the contours of the site. The width of the system (i.e., up and down slope) shall be no wider than 20 feet (i.e., outside edge of trench to outside edge of trench) as depicted in Figure 36. Construction concerns previously noted for mound systems are applicable to downstream mounds.

Requirement differences between mounds and downstream mounds (i.e., following a sand filter) include: (a) trench or bed design vs. trench design only; (b) filtered wastewater application rate to mound trenches using pressure distribution (i.e., variable for settled wastewater vs. 1.2 gal./day/sq. ft. for filtered wastewater); (c) design flow rate (i.e., 100% vs. ≥ 85%); (d) minimum height of usable sandy fill between plowed grade and the bottom of the absorption trenches (i.e., one foot vs. two feet); (e) minimum depth of usable in situ soil with a percolation rate of one to 120 minutes/inch (i.e., one foot vs. six inches); and, (f) basal area wastewater application rate to the upper one foot of plowed in situ soil (i.e., variable vs. nonapplicable).

If a specific waiver is granted to allow use of gravity distribution to a downstream mound (i.e., in lieu of required pressure distribution) due to hardship and special circumstance, recommended design modifications include: (a) the filtered wastewater application rate to mound trenches shall be in accordance with Tables 5 or 6 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) and, (b) the perforated distribution lines shall not exceed 60 feet in length.

Construction (Sand Filter)

Preparation of the site on which the sand filter is to be located shall include clearing of excess vegetation (i.e., including tree/stump/root and brush removal) and excavation/grading.

Following excavation/grading, perforated collector pipes shall be installed in aggregate (i.e., washed gravel or crushed stone ¾ to 1 ½ inches in diameter) at a slope of 1/16 to 1/8 inch per foot. At least two inches of aggregate shall surround the lower half of the four inch diameter collector pipes. At least four inches of aggregate shall underlay the entire system (i.e., above the graded soil and above the top of the collector pipes) and its upper surface shall; be as level as possible.

A three inch layer of 1/8 to ¼ inch diameter washed gravel or crushed stone shall be spread across the entire bed of aggregate to minimize movement of sand media into the aggregate and collector pipes.

A minimum of 24 inches of approved sand (i.e., E.S. = 0.25 to 1.0 mm and all sand passing a ¼ inch sieve; U.C. ≤ 4.0) shall be placed above the three inch layer of 1/8 to ¼ inch stone or gravel. The sand should be settled by flooding before aggregate is placed upon the sand bed.

A two inch layer of aggregate shall be placed upon the entire sand bed. Perforated distribution pipe shall be installed on the aggregate in accordance with Table II. Additional aggregate shall be installed to provide a minimum depth of four inches across the entire sand surface and at least two inches above the distribution pipes. Figures 34 and 35 depict the recommended uniform eight inch layer of aggregate containing distributor pipes across the entire sand surface.

Permeable geotextile, untreated building paper, or four inches of hay or straw shall be placed over the entire bed of aggregate to prevent the infiltration of fine soil into the sand media.

The entire surface of the filter shall be covered with six to 12 inches of topsoil mounded to enhance runoff of precipitation from the filter (i.e., ≥ 1% slope) and seeded to grass.

Appropriate diversion ditches or berms shall be constructed uphill from the sand filter on sloped sites to prevent surface runoff from entering the sand filter.

Construction (Downstream Absorption Facility Option)

Construction of a modified shallow trench system should be in accord with construction criteria for shallow trench systems. Modified shallow trenches should be constructed in the shallow fill at least six inches into the in situ permeable and unsaturated soil.

Construction (Downstream Mound)

Construction of a downstream absorption mound shall be in strict accord with all conditions noted in "Construction of Mounds" in the previous section of alternative systems.

Evaporation-Transpiration (ET) and Evapo-Transpiration Absorption (ETA) Systems

General Information

ET systems rely upon the upward movement of moisture through (a) the system soil (i.e., capillary action) plus evaporation into the atmosphere and (b) surface vegetation during the growing season plus transpiration into the atmosphere. ETA systems also use the absorption capabilities of the in situ soil plus the system soil and are therefore not entirely dependent upon evaporation and transpiration. Pages 300 through 309 of the 1980 EPA Design Manual titled "Onsite Wastewater Treatment and Disposal Systems" should be thoroughly reviewed before designing any ET or ETA system for use in New York State.

Research sponsored by EPA and other organizations indicates that these systems function properly on a year-round basis only in arid and semi-arid climates such as is experienced in the western and southwestern United States. The temperate New York climate resuIts in annual precipitation of 30 to 40 inches. Precipitation upon the porous soils of ET/ETA systems can infiltrate into the system and impact system loading. ET/ETA systems may be feasible for summer homes occupied only during the growing season (i.e., the usual period of maximum evaporation and transpiration). ET/ETA systems should not be considered for new construction except on extremely large lots. These systems exhibit a very low level of reliability even when carefully designed and installed. Homeowners should be advised that these systems may have a short useful life and/or require frequent maintenance or repair. Use of water-saving devices is highly recommended when ET/ETA systems are installed as a replacement 10 an existing failing system.

Site Requirements

All conventional and alternative systems previously discussed, except an intermittent sand filter followed by an absorption mound (i.e., Sections 75-A.8(a) through 75-A.9(c) of Appendix 75-A), must be determined to be unacceptable for the planned building site.

An expansion area equal to or greater than 50% of the required basal area shall be available on the site. A 100% expansion area is recommended. Slopes shall not exceed 15% where ET/ETA systems plus proposed 50% expansion areas are to be located.

All minimum horizontal separation distances as described in Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) shall be maintained from the outer edges of the basal area and the designated expansion area. A minimum vertical separation of two feet shall be maintained between the bottom of any absorption trench in an ETA system and bedrock, impermeable soil, or high ground water.

An environmental assessment shall indicate that development of the site with an ET/ETA system is consistent with the overall development of the area and will cause no adverse environmental impacts. On sloped sites, a diversion ditch shall be constructed uphill from the ET/ETA system to prevent entry of surface runoff into the system. The topsoil surface of the ET/ETA system shall be graded to enhance runoff of precipitation.

Design Criteria

All ET/ETA systems must be preceded by properly sized septic tanks or aerobic units. Pressure distribution of settled effluent throughout the system is required. Absorption trench depth for ET/ETA systems shall not exceed 30 inches below the surface of the system.

In addition to the above-noted requirements, the designer must consider all of the items listed below and be able to document from reliable sources (i.e. National Weather Service, Soil Conservation Service) the parameters used and show that the net outflow from the system exceeds the net inflow without the exposure of sewage or partially treated sewage on the surface of the ground:

  1. Total annual precipitation (i.e., average and range for the area).
  2. Percentage of precipitation that will infiltrate into the system and the percentage of precipitation that can be expected to flow off the system (i.e., run-off).
  3. Annual land evaporation rate of the area.
  4. Vertical rise of water that can be expected in the system soil due to capillary action.
  5. Amount of transpiration expected from surface vegetation on the system during the growing season plus identification of the growing season.
  6. Permeability of the in situ soil underlying the system and projected system impacts upon ground water levels.

Construction

Construction techniques described previously for mound systems, where soil permeability may be decreased by poor construction practices, shall be used for ETA systems. An impervious liner supported above and below by a two inch layer of sand shall underlay all ET systems.

Large aggregate (¾ to 2 ½ inch size) shall surround the distributors. Permeable geotextile shall cover the large aggregate to prevent sand from settling into the large aggregate and filling the voids. Sandy fill shall exhibit a capillary rise equal to the depth of the system. A four inch layer of sandy loam or loamy sand shall cover the sand fill, be graded to help shed precipitation, and be seeded to grass. Appropriate curtain drains and diversion ditches shall be constructed to prevent ground water from interfering with all ETA system or surface water from entering an ET/ETA system.

Other Systems

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General Information

Other systems should be designed by a design professional. The design professional should certify to the local health department that the system was constructed in conformance with approved plans.

Residual wastes in these systems shall be removed periodically by a professional septage hauler and either taken to a sewage treatment plant or disposed of in an approved manner. The hauler must possess a valid DEC waste transporter permit to transport septage.

Holding Tanks

The use of holding tanks shall not be approved for new residential construction (i.e. full-time or part-time occupancy) except where occupancy of a dwelling is allowed while the wastewater treatment system is under construction. Holding tanks are only intended for use to meet the above noted exception or to correct existing failing wastewater treatment systems when no other alternative exists. Holding tanks are not deemed acceptable for long term use at year-round residences due to high maintenance costs and continuous operation attention.

Tank size shall be based upon a minimum of five days design flow or 1,000 gallons, whichever is greater, and meet the same construction standards as a septic tank except that the holding tank shall not have an outlet. All holding tanks shall be equipped with an alarm to indicate when pump out is necessary. A solenoid valve on the water service line to the household (i.e., upstream of all fixtures and immediately downstream of a hydropneumatic tank if one exists) should also be considered to avoid holding tank overflows. Water saving fixtures should be installed in any home being served by a holding tank to minimize pump out maintenance costs.

Waste management districts are recommended for areas with numerous holding tanks to provide continuous inspection and maintenance services.

Non-Waterborne Systems

General Information

The State Uniform Fire Prevention and Building Code (9NYCRR Subtitle S Sections 900.1(a) and (b)) requires wet plumbing (i.e., potable water plus sewerage) for all new residences. In accordance with Section 900.2(b), minimal required plumbing fixtures may be omitted for owner occupied single family dwellings if approved by the authority having jurisdiction. Health Department approval for said omission(s) shall be fully protective of public health and be in general harmony with the intent of Section 900.1 (i.e., provide satisfactory sanitary facilities). In some areas of the State where available water becomes insufficient to economically use flush toilets (i.e., even those with only 1.6 gallons per flush) or where a need or desire exists to conserve water, use of non-waterborne systems may be justified. In those cases, other household wastewater (i.e., greywater) from sinks, showers, tubs, and other fixtures must receive proper subsurface treatment (i.e., settling and soil absorption). Greywater systems shall be designed upon a flow of 75 gpd/bedroom and meet all the criteria previously indicated for treatment of household wastewater.

Composters

These units accept human waste into a chamber where composting of the waste occurs. Composter units shall be installed in accordance with the manufacturer's instructions. The units shall have a label indicating compliance with the requirements of National Sanitation Foundation (NSF) Standard 41 or equivalent. Only units with a manufacturer's warranty of five years or more shall be installed.

Composters accept only toilet wastes and kitchen food scraps coupled with supplemental additions of carbon-rich bulking agents such as planer shavings or coarse sawdust. Household cleaning products should not be placed in the unit. Failure to add adequate bulking agents or maintain adequate moisture can result in the composting pile becoming hard (and difficult to remove) or anaerobic. The composted humus contains numerous bacteria and may also contain viruses and cysts. Residual wastes (i.e., the composted humus) should be periodically removed by a professional septage hauler. If a homeowner chooses to personally remove the composted humus, it should be disposed of at a sanitary landfill or buried or well mixed into soil distant from food crops, water supply sources and watercourses. The humus comprises an admixture of recent additions and composted older additions and should be disposed of accordingly. Humus disposal sites shall meet Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) separation distances for sanitary privy pits.

Chemical and Recirculating Toilets

Chemical toilets provide a toilet seat located directly above a vault containing chemicals to disinfect and control odors from the wastewater. Recirculating toilets use chemicals as the toilet flush liquid. The wastes are separated from the liquid and discharged to an internal holding tank. The chemical liquid is reused for additional toilet flushing. The liquids used in these types of toilets do not completely disinfect the wastes so the residual waste products shall not be discharged to the surface waters or the ground surface. The reduced volume wastewater from recirculating toilets may be discharged to a larger holding tank but not to a subsurface absorption system due to the presence of chemical liquid. Residual wastes or spent chemical liquid in these systems shall be periodically removed by a professional septage hauler.

Incinerator Toilets

These units accept human waste into a chamber where the wastes are burned. The units have a very limited capacity and require a source of electricity or fuel to burn the wastes. The ash remains must be periodically removed and may be used similar to fireplace ashes (i.e., the ash remains are stable). The units must be installed in accordance with the manufacturer's instructions. These units routinely produce objectionable odors at the start of each incineration cycle as the temperature rises to its proper operating range. To avoid creating public health nuisances from objectionable odors, the separating distance from the unit exhaust stack to the nearest property line or other habitable buildings should be at least 500 feet.

Sanitary Privies

Sanitary privies may be of the pit, watertight vault, or removable watertight receptacle type. The toilet seat is located directly above the receiving pit/vault/receptacle. Pits shall be constructed in soil with the bottom of the pit at least two feet above bedrock and/or maximum high ground water. Table 2 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) separation distances shall be met.

The privy structure shall be fly and vermin tight with a self-closing door, screened vents, and screened exterior openings. Conventional toilet seats may be installed for user comfort.

Grading should prevent entrance of surface water into the pit. When excreta in the pit is within 18 inches of ground level, the pit contents should be removed/pumped out or covered with 18 inches of soil. The framed enclosure is usually constructed to facilitate pump out or relocation over a new pit.

Pump out of vaults or exchanges of receptacles shall be accomplished before the vessel is full and causes interference with use. Residual waste removal from sanitary privies shall be performed by professional septage haulers.

Engineered Systems

General Information

A wastewater treatment system of a type not addressed in this document may be allowed only through the issuance of a Specific Waiver by the local health unit having jurisdiction as provided for in 10 NYCRR Part 75 titled Standards for Individual Water Supply and Individual Sewage Treatment Systems.

Special Conditions

The proposed system shall be designed by a design professional.

An environmental assessment shall indicate that development of the site with the proposed system is consistent with the overall development of the area and will cause no adverse environmental impacts. The homeowner/purchaser shall be informed of the expected reliability or potential problems with the proposed system.

The design professional shall also supervise the installation of the system and certify in writing that the system was constructed in accordance with the approved plan and/or submits as built plans of the system.

New Products/System Design Interim Approval

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Based upon submission of engineering research and testing data indicating that certain products, design and performance are equivalent to 10 NYCRR Appendix 75-A standards, the Commissioner may grant interim approval for the use of systems, products or procedures differing from these standards.

Operation and Maintenance of Individual Onsite Wastewater Treatment Systems

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Onsite wastewater treatment systems will not function properly if left unattended. A household wastewater treatment system will function satisfactorily if it is properly located, designed, constructed and maintained. To assure that household wastewater treatment systems function satisfactorily, owners should observe the following practices for operating and maintaining their systems.

Operation

Detergents, kitchen wastes, laundry wastes, and household chemicals in normal amounts do not affect the satisfactory operation of household wastewater treatment systems. However, excessive quantities may be harmful.

Septic tank additives are not recommended. Additives are not necessary for the proper operation of household systems and may cause the sludge and scum in the septic tank to flow into the absorption facility, resulting in premature failure.

Garbage grinders substantially increase the accumulation of solids in the septic tank and may increase solids carryover to the absorption facility. They are not recommended for households with septic systems. Septic tank size must be increased and the outlet baffle must be equipped with a gas deflection device whenever a garbage grinder is installed. Installation of an outlet filter is also recommended.

All roof, cellar, and footing drainage, as well as surface water must be excluded from the system. Roof downspouts and precipitation runoff should drain away from absorption facilities.

Discharges from large volume fixtures and appliances (i.e. hot tubs, spas, whirlpool baths, etc.) should be limited to five gallons per minute to prevent solids from being washed out of the septic tank and into the absorption field. The volume of wastewater generated by these appliances/fixtures should be added to the total design flow calculated from Table 1 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) to properly design septic tanks/aeration units and subsurface absorption facilities.

The lifespan of household wastewater treatment systems can be prolonged by practicing water conservation. Check for leaky fixtures and defective toilet valves on a regular basis.

Avoid the disposal of cigarette butts, disposable diapers, feminine hygiene products, plastic, trash, etc. into household systems.

Never permit heavy equipment and vehicles to pass over the absorption facilities.

Inspection & Maintenance

Septic tank

The contents of the septic tank should be pumped out every two to three years, or whenever any of the following conditions apply:

  1. The total depth of sludge and scum exceeds one-third of the liquid depth of the tank;
  2. The bottom of the scum layer is within three inches of the bottom of the outlet baffle; or
  3. The top of the sludge layer is within ten inches of the bottom of the outlet baffle.

A pole wrapped with toweling and a four inch square board attached to the bottom can be used to measure scum and sludge clearances. Pump-out clearances also apply to any chamber in multi-compartment tanks and to any tanks in series.

If the tank is not cleaned periodically, solids flow into the absorption facility; rapid clogging occurs; premature failure follows and finally, the absorption facility must be replaced. Periodically pumping a septic tank is far less expensive than replacing an absorption facility. Pumping must be performed by a NYSDEC licensed septage hauler.

Septic tanks should be inspected annually to determine that the inlet and outlet baffles/tees are in place. All baffles, inlet and outlet piping should be inspected using a strong light. Repairs should be made if necessary. Do not enter septic tanks. Septic tanks may contain toxic gases caused by biological activity.

Some concrete baffles or sanitary tees deteriorate over time. Baffles/tees which have deteriorated and no longer perform as designed must be replaced. The gas deflection baffle should be inspected following the pump-out of the tank when it can be seen with the aid of a strong light.

Interior surfaces of the tank should be inspected for leaks and cracks using a strong light following pumpout of the tank. If a crack or leak below the liquid level cannot be repaired or sealed, the tank must be replaced. If differential (uneven) settlement is evident in the area of the septic tank, the tank must be properly replaced upon a stable foundation of regraded sand, pea gravel or crushed stone.

Distribution Devices

Distribution boxes should be inspected periodically to ensure equal flow to all absorption lines and to check for solids leaving the septic tank. The inlet and outlet pipes should be inspected to assure that all outlet inverts are at the same elevation. Distribution box baffles should be inspected to make sure they are properly positioned. Finding solids in a distribution box is indicative of hydraulic overload, improper septic tank outlet baffles, an undersized septic tank, and/or an overfull septic tank.

Distribution box speed levelers are recommended to ensure equal flow to all absorption lines. Speed levelers make it possible to correct unequal flow distribution with minimal effort. Distribution devices permit use of the entire absorption facility and/or periodic resting of portions of overdesigned absorption facilities (i.e., one or more additional lines/seepage pits). Periodic six months resting of each lateral or seepage pit is very advantageous to absorption facility longevity and is recommended. Periodic resting is easily accomplished by use of adjustable outlet levelers and plugs in distribution boxes.

A stake should be used to locate distribution box covers below grade. A sketch/plan indicating measured distances from permanent points (i.e., corners of house foundation, property stakes, telephone poles, etc.) to the covers should be retained and used by the owner. The cover of the distribution box should be kept locked at all times when it is located at or above grade.

Pump and Siphon Systems

Some septic systems incorporate dosing or pressure distribution systems. If a system uses siphon dosing or pressure distribution, the siphon or pump chamber must be inspected for proper operation.

Pump station and dosing tank access covers must be lockable to prevent entry by unauthorized persons, especially children. Pump stations and dosing tanks frequently contain toxic gases and must not be entered by individuals untrained in proper safe entry protocols. Only properly trained persons should ever attempt to enter or repair such facilities. Pump stations and dosing tanks must include downward facing and screened air vents, which should be checked periodically for blockage.

When inspecting pump systems, the sewage effluent pump and alarm should be checked for proper operation. If duplex pumps are used, both pumps and alarms should be checked. The pump chamber and plumbing connections should be checked for leaks and groundwater infiltration. Leaks should be repaired promptly. Float switches controlling pumps should be tested and adjusted for correct discharge level.

Siphon dosing systems should be inspected semiannually to assure the wastewater level in the dosing chamber is within the normal operating range (i.e., bottom of bell to below the overflow). The alarm should be checked for proper operation. Hydraulic bell siphons can be tested for leaks by covering the siphon with water and inspecting for air bubbles. The dosing chamber and plumbing connections should be checked for leaks or groundwater infiltration. Leaks should be repaired promptly.

Dosing and pressure distribution system designers and installers should provide owners with a manual which outlines proper operation and maintenance of the system. If the system is existing at a time of property transfer, a briefing with the previous owner of their experience with the system may be helpful.

Absorption Facilities

Proper operation and maintenance of individual household wastewater treatment systems increase system lifespan, optimize wastewater treatment and help to prevent system failures. System failures can pollute the environment and create public health nuisances. Keep tree roots away from the immediate area of the absorption field as these may clog the system. Keep swimming pools (above or inground) away from the absorption facility. Do not pave over an absorption facility.

The size of an existing system should be evaluated. Owners should consider increasing the size of an existing septic tank and absorption facilities when additions are made to homes (i.e., expansion bedrooms, installation of sewage generating appliances and fixtures, etc.)

Runoff to the absorption area should be eliminated by regarding surrounding areas. Groundwater diversion may also be necessary to alleviate high groundwater which interferes with the proper operation of the facility.

The absorption facility should be observed periodically for surface discharge or ponding or effluent. Such occurrences could indicate system failure and/or need for replacement.

Addressing System Failure

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Many systems fail (e.g., wastewater backup to plumbing fixtures, wastewater flow upon the ground surface, wastewater flow into water courses or wastewater flow into water supply sources such as wells or springs) due to: (a) siting, design, construction or maintenance deficiencies, (b) hydraulic overloading, or (c) exceeding the useful life of the system. Tables 13 and 14 (Tables can be found in the full Portable Document Format (PDF, 3.42MB, 147) copy of this handbook) provide guidance to evaluate and correct periodic or continuous system failure due to numerous causes.

Glossary (Definitions)

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Absorption Area
an area to which settled wastewater is distributed for infiltration into the soil.
Absorption Field
the area to which settled wastewater is distributed for infiltration into the soil by means of a network of pipes.
Absorption Trench
a long narrow area which includes a pipe for the distribution of septic tank or aerobic unit effluent.
Aeration
the process of transferring oxygen to water/wastewater by providing intimate contact between air and water/wastewater
Aerobic Treatment Unit
a system that provides for the biological decomposition of the organic portion of the wastewater by mechanical aeration of the wastewater.
Aggregate
washed gravel or crushed stone ¾ to 1½ inches in diameter. Number 2 stone or gravel meets this size requirement.
Application rate
the rate at which septic tank or aerobic unit effluent is applied to a subsurface absorption area expressed in gallons per day per square foot (gpd/sq.ft.) for design purposes.
Baffle
a flow deflecting device used in septic tanks and distribution boxes to inhibit the discharge of floating solids, reduce the amount of settleable solids that exit, and reduce the exit velocity of the wastewater.
Basal Area for a Mound
the total area beneath the absorption trenches, extending 2.5 feet in all directions from the outer edge of all trenches.
Building Drain
the lowest piping in a household drainage system which receives wastewater inside the walls of the building and conveys the wastewater three feet beyond the building wall to the building sewer.
Building Sewer
the portion of the wastewater drainage system which extends from the end of the building drain and conveys wastewater to the wastewater treatment system or sewer.
Cleanout
an opening providing access to sewage system components including building drain, building sewer, distribution box and septic tank.
Commissioner
the State Commissioner of Health.
Curtain Drain/Vertical Drain/Underdrain
a subsurface drain designed and constructed to control ground water intrusion into the wastewater treatment system or sewer.
Design Professional
a person licensed or registered in the State of New York and authorized by the State Education Law to design the systems described in 10 NYCRR Appendix 75-A.
Distribution Device
a device used to uniformly distribute settled wastewater to the absorption or filtration area.
Distribution Line (Distributor)
the perforated pipe used to distribute settled wastewater in the absorption or filtration area.
Diversion Ditch/Berm
a designed and constructed ditch/berm to control surface water intrusion into the wastewater absorption area on sloped sites.
Drinking Water
water whose physical, chemical, radiological, and biological quality is or is intended to be satisfactory for human consumption, food preparation
Effective Grain Size
a measure of the diameter of soil particles, when compared to a theoretical material having an equal transmission constant. It is the dimensions of that mesh screen which will permit 10 percent of the sample to pass and will retain 90 percent.
Gas Baffle
a device on the outlet of a septic tank which deflects gas bubbles away from the outlet and reduces the carryover of solid particles from the septic tank.
Greywater
all sewage or wastewater from a house except waste from flush toilets and urinals.
Ground Water
subsurface water occupying the saturation zone from which wells and springs are fed (i.e., water below the ground water table).
Heavy Equipment
all equipment which would result in the compaction of the design absorption area at a depth equivalent to the design depth of the distribution lines.
Infiltration
the flow or movement of water into the interstices or pores of a soil through the soil interface.
Invert
the floor, bottom, or lowest point of the inside cross section of a pipe or opening/slot/channel (i.e., flow opening between compartments in a multi-compartment septic tank).
Large Aggregate
washed gravel or crushed stone ¾ to 2½ inches in diameter. Number 3A and 3 stone or gravel meets this size requirement and Number 3 is preferred.
Local Health Department
a city, county, or part-county department of health or a State Department of Health District Office.
Pea Gravel
washed gravel or crushed stone 1/8 to ¼ inch in diameter. Number 1B stone or gravel meets this size requirement.
Percolation
the movement of water through the pores of a soil or other porous medium following infiltration through the soil interface.
Permeability
a measure of the rate of movement of liquid through soil.
Scum
the wastewater material which is less dense than water and floats on top of the water (i.e., especially in septic tanks).
Sewage
the combination of human and household waste with water which is discharged to the home plumbing system including the waste from a flush toilet, bath, shower, sink, lavatory, dishwashing or laundry machine, or the water-carried waste from any other fixture, equipment or machine.
Sludge
the wastewater material which is more dense than water and settles to the bottom in relatively quiescent areas (i.e., especially in septic tanks).
Stabilized Rate of Percolation
the rate corresponding to two consecutive equal or near equal percolation test results.
Uniformity Coefficient
a ratio of the diameter of soil particles obtained by sieving soil through standard U.S. Sieve Series meshes and dividing the mesh size of the sieve opening at which 60% of the particles pass through and 40% is retained by the mesh size of the sieve opening at which 10% of the particles pass through and 90% is retained; U.C. equals D60/D10.
Usable Soil
unless otherwise stated, a soil with a percolation rate of one to 60 minutes/inch with a compatible soil classification.
Wastewater
any water discharged from a house through-a plumbing fixture to include, but not be limited to, sewage and any water or wastewater from a device (e.g., water softener brine or garbage grinder waste) which is produced at the house or property.
Watercourse
a visible path through which surface water travels on a regular basis. Drainage areas which contain water only during and immediately following precipitation or snow-melt shall not be considered a watercourse.
Watershed
an area of drainage for a body of water that serves as a source of drinking water and for which watershed rules and regulations have been adopted by the Commissioner.
Well Head Area
the area surrounding a well which includes the cone of influence (i.e., where the drawdown of ground water causes ground water flow).
Wetland
an area of marshes or swamps which have been designated as such by the State Department of Environmental Conservation or other State agency (e.g., Adirondack Park Agency) having jurisdiction. Marshes or swamps that have not been so classified should not be treated as a wetland for design purposes.

Selected References

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The following represents a partial list of those documents/publications most influential to the authors.

  1. 10 NYCRR Part 75, "Standards for Individual Water Supply and Individual Sewage Treatment Systems," December 1, 1990.
  2. 10 NYCRR Appendix 75-A, "Wastewater Treatment Standards, Individual Household Systems," December 1, 1990.
  3. "EPA Design Manual, Onsite Wastewater Treatment and Disposal Systems," Publication EPA 625/1-80-012, October, 1980.
  4. "Waste Treatment Handbook, Individual Household Systems," New York State Department of Health, Division of Environmental Protection, Sixth Printing, 1985.
  5. "EPA Septic Systems and Ground Water Protection, A Program Manager's Guide and Reference Book," GPO Document No. 055-000-00256-8, July, 1986.
  6. "EPA Septic Systems and Ground Water Protection, An Executive's Guide," GPO Document No. 055-000-00256-6, July, 1986.
  7. "EPA Septic Tank Siting to Minimize the Contamination of Ground Water by Microorganisms," EPA Publication 440/6-87-007, June, 1987.
  8. "Recommended Standards for Individual Sewage Systems, 1980 Edition," A report of the committee of the Great Lakes - Upper Mississippi River Board of State Sanitary Engineers.
  9. "Design Standards for Wastewater Treatment Works, Intermediate Sized Sewerage Facilities," New York State Department of Environmental Conservation, 1988.
  10. Salvato, Joseph A., Jr., Environmental Engineering and Sanitation, Wiley Interscience, New York, 1972.
  11. "Drainage Guide for New York State," U.S. Department of Agriculture Soil Conservation Service, September 1987.
  12. "Drainage of Agriculture Land," U.S. Department of Agriculture Soil Conservation Service, Water Information Center, Inc., 1973.
  13. Great Lakes Upper Mississippi River Board (GLUMRB) of State Public Health and Environmental Managers Recommended Standards for Water Works, 1992.