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 sho