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Septic effluent treatment in the ground (subsurface wastewater treatment): these model septic design regulations discuss the means of disposition of septic effluent using a conventional septic leach field (drainfield, absorption bed are synonyms).
We provide tables of required septic drainfield trench length and describe septic treatment field construction and materials. Distances from the leach field to other site features, design criteria based on soil percolation test results, absorption trench length requirements, and materials to be used are all discussed.
This document uses the New York State wastewater treatment standard for individual household septic systems (Appendix 75-A) to provide an example of state regulated design and installation of both conventional tank and leach field septic systems and alternative septic system designs, including raised septic systems, septic mound systems, intermittent sand filter septic systems, and evaportion-transpiration septic systems.
Here and in this article series we will discuss or provide the following:
Table of Septic Drainfield Trench Lengths Determined by Soil Percolation Rate and Daily Wastewater Input Flow.
Materials to be used for drainfields, gravel. Guidelines for use of seepage pits in septic systems.
Design criteria for gravelless septic drainfields; design of cut and fill septic systems.
Design criteria for deep septic trench & shallow septic trench systems. Specifications and construction details for drainfields, leachfields, pressure-dosing absorption fields.
See SEPTIC DRAINFIELD SIZE for details of septic drainfield sizing and specifications, including An Alternate Table for Determining Septic Drainfield Size.
Also see SEPTIC SOIL & PERC TESTS Perc Tests or Deep Hole Testing for details about how to conduct soil percolation tests.
Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References."
Model: New York Septic Regulations NYS-A.8, Subsurface Treatment, Effective Date: 12/01/90
All effluent from septic tanks or aerobic tanks shall be discharged to a subsurface treatment system. Surface discharge of septic tank or aerobic unit effluent shall not be approved by the Department of Health or a local health department acting as its agent.
[DF NOTE: This section discusses the design requirements for septic absorption fields, also called leach fields, drain fields, drainfields, or conventional soil absorption systems.]
(i) The minimum distances that absorption fields shall be separated from other facilities are shown in Figure 1 and Table 2.
(ii) A minimum of four feet of useable soil shall exist above bedrock and groundwater with a minimum separation of two feet to the lowest part of any trench.
(iii) Absorption fields shall not be built under driveways, parts of buildings or under above-ground swimming pools or other areas subject to heavy loading. Surface waters shall be diverted from the vicinity of the system.
(i) The required length of absorption trench is determined from Table 4A based upon the percolation test results and confirmed by the soil evaluation. The maximum trench width for design purposes shall be 24 inches. Only 24 inches shall be allowed for absorption area calculations. Where trenches exceed 24 inches in width, calculations of absorptive area shall be based on a width of 24 inches.
(ii) Adjacent trenches shall be separated by at least four feet of undisturbed soil. Individual trenches shall be constructed parallel to the ground contours with trench bottoms as near level as possible. They need not be perfectly straight but abrupt changes in direction shall be avoided.
Specifications for the required length of septic system drainfield trenches based on input flow rate and soil percolation rate are given in the table below.
Required Septic Absorption Trench Length in Feet
Septic Wastewater Effluent Input Flow Rate (Gallons per Day) 
|220 gpd||260 gpd||300 gpd||330 gpd||390 gpd||450 gpd||440 gpd||520 gpd||600 gpd||550 gpd||650 gpd||750 gpd||660 gpd||780 gpd||900 gpd|
|1 - 5||92||108||125||138||162||187||184||216||250||230||270||312||275||325||374|
|6 - 7||110||130||150||165||195||225||220||260||300||275||325||375||330||390||450|
|8 - 10||123||145||167||184||217||250||245||290||333||306||360||417||367||433||500|
|11 - 15||138||162||188||207||244||281||275||325||375||344||406||469||413||488||563|
|16 - 20||158||186||214||236||279||321||315||372||429||393||464||536||472||557||643|
|21 - 30||184||217||250||275||325||375||367||433||500||459||542||625||550||650||750|
|31 - 45||220||260||300||330||390||450||440||520||600||550||650||750||660||780||900|
|46 - 60||245||290||333||333||433||500||489||578||667||612||722||833||734||867||1000|
|Dosing not required
|Dosing system or alternative design is required if the total drainfield (soakbed / leachfield) trench length is 500 feet or more in length.|
Notes to the Septic Drainfield Trench Length Table
 Original source: New York State NYS75-A.8 Table 4A. Updated 12 January 2015.
 Conditions that require more than 1000 feet of septic drainfield trench must have an alternative dosing system design.
The table assumes that the soil absorption trench is two feet wide. Other drainfield trench dimensions are given in the text below.
Wastewater design flow data based on plumbing fixture types is given at NYS Appenndix A-75 Table 1: Septic System Design Flow Rate Data found at SEWAGE FLOW & DESIGN FLOW ESTIMATES
More Notes to the Septic Drainfield Trench Length Table above
This table is provided in a more detailed and up-to-date form at SEPTIC DRAINFIELD SIZE tables given in the live link found in note  just below.
 Original source: New York State NYS75-A.8 Table 4A.
 Conditions that require more than 1000 feet of septic drainfield trench must have an alternative dosing system design.
 See SEPTIC DRAINFIELD SIZE for An Alternate Table for Determining Septic Drainfield Size and for New York State NYS75-A.8 Table 4A - Required Septic Absorption Trench Length in Feet versus Soil Percolation Rate & Wastewater Flow Rate . an updated version of the table shown just above.
 See SEPTIC SOIL & PERC TESTS Perc Tests or Deep Hole Test for Soil Absorption Rate Testing - how to conduct soil testing for percolation rate determination
 See SEWAGE FLOW & DESIGN FLOW ESTIMATES for NYS Appenndix A-75 Table 1: Septic System Design Flow Rate Data
Application Rates for non-standard septic effluent design flows, table of application rates in GPD per sq.ft. and soil percolation rate are given by the table shown in image form above and in more-legible data just below.
Table 4B Sewage Effluent Application Rates for Non-Standard Design Flows
|Soil Percolation Rate: Minutes / Inch||Septic Effluent Application Rate: Gallons / Day /Sq.Ft.|
|1 - 5 min/inch||1.2 gpd/sq.ft. of trench bottom|
|6 - 7||1.00|
|8 - 10||0.9|
|11 - 15||0.8|
|16 - 20||0.7|
|21 - 30||0.6|
|31 - 45||0.5|
|46 - 60||0.45|
|61-120||0.20 gpd / sq. ft. is permitted in some jurisdictions (E.g. NY) - cf MOUND SEPTIC SYSTEMS|
Soil with a percolation rate of less than 1 minute / inch is not suitable for a conventional septic system [presumably because no treatment can occur - Ed.]
(i) Perforated distributor pipe shall be used in the trenches. Solid (non-perforated) pipe shall be used between the distribution box and the trenches. Perforated pipe shall be made of rigid or corrugated plastic and be labeled as fully meeting ASTM standards for use in 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.
(ii) Aggregate shall mean washed gravel or crushed stone 3/4 - 1 1/2 inches in diameter. Larger diameter material or finer substances and run-of-bank gravel are unacceptable.
(iii) The aggregate shall be covered with a material that prevents soil from entering the aggregate after backfilling, yet must permit air and moisture to pass through. The preferred material for covering the aggregate is a permeable geotextile. Untreated building paper or a four inch layer of hay or straw is acceptable. Polyethylene and treated building paper are relatively impervious and shall not be used.
(i) Trench locations and depths should be marked by stakes before the trenches are excavated. The natural surface shall not be significantly disturbed. If the site is re graded or similarly disturbed, the soil shall be allowed to stabilize and new percolation tests conducted.
(ii) The trench depth shall be as shallow as possible, but not less than 18 inches. At least six inches of aggregate is placed below the distribution line and two inches above the line. The earth cover over the aggregate should not exceed 12 inches in order to enhance natural aeration and nitrogen uptake by plant life.
Trenches shall be excavated to design depth with bottoms practically level. Heavy equipment shall be kept away from the field because the weight may permanently alter soil characteristics due to compaction, cause trench cave-ins, and/or cause misalignment and broken pipes.
Septic System Trench Depths on Sloped Terrain
|Percent Slope||Trench Depth||Soil Cover Depth|
|0 - 10% Slope||30 inches||12 - 18 inches|
|10 - 20% Slope||42||18|
|20 - 30% Slope||48||30|
The setbacks in this table are intended to provide eight horizontal feet distance from the downhill side of the top of the drainfield aggregate to the natural slope. [sic]
The soil absorption area of the trench shall be computed by using the bottom area of the absorption trench. Individual septic drainfield trenches shall be no longer than 100 ft.
(iii) Trench bottoms are to be raked and immediately covered with at least six inches of aggregate.
(iv) Any smeared surfaces on the trench walls are to be raked. Distributor lines are carefully placed on the aggregate and covered with aggregate to a depth of at least two inches over the top of the pipe. Additional aggregate may be required to bring the top of the aggregate to within six to 12 inches of the surface.
(v) In gravity distribution systems, the pipe shall be carefully sloped at between 1/16 inch and 1/32 inch per foot. Grades shall be determined by an engineer's level, transit or carpenter's level.
(vi) After the upper aggregate is placed, the geotextile, untreated building paper, hay or straw is to be immediately installed and the trench backfilled with native soil. If the trenches cannot be immediately backfilled, they should be temporarily covered with an impervious material such as treated building paper to prevent sidewall collapse and siltation into the aggregate.
(vii) The earth backfill is to be mounded slightly above the original ground level to allow for settling and after settlement the entire area should be graded without the use of heavy equipment and seeded with grass.
(1) In one type of system, a large diameter corrugated plastic pipe (eight inches or greater in diameter) surrounded by a hydrophilic geotextile is installed in an excavated trench using only the original soil as backfill. Other systems utilizing products such as galleys, flow diffusers or leaching chambers can be installed without aggregate backfill. One linear foot of these products shall be equivalent to one linear foot of conventional (24 inch wide) absorption trench.
These systems shall be used on sites that have been classified as having a design percolation rate of one to 45 minutes per inch, and meet the vertical and horizontal separation distances in Table 2 which is shown in 75-A.4 Soil and site appraisal for Septic Systems.
The local health department having jurisdiction shall be contacted prior to construction regarding the acceptability of specific products for use as a gravelless distribution system.
Gravelless distribution systems shall be installed in conformance with the manufacturer's instructions because of the proprietary design of some products.
These are used on sites where an useable layer of soil is overlaid by three to five feet of impermeable soil.
(i) There shall be at least four feet of useable solid beneath the impermeable layer.
(ii) The required length of absorption trench is determined from Table 4A based upon percolation tests conducted in the underlying soil.
(i) Trenches are excavated at least two feet into the useable layer and backfilled with aggregate or coarse sandy material containing a low percentage of fines more permeable than the underlying material to a level 30 inches below the original ground surface.
(ii) An absorption trench system as described in Section 75-A.8(b) is constructed in the upper 30 inches of the excavation.
These systems are used where there is at least two feet but less than four feet of useable soil and/or separation to boundary conditions.
(i) A minimum two foot separation must be maintained between the bottom of each trench and all boundary conditions.
(ii) The bottom of each trench must not be above the original ground surface.
(iii) Material of the same permeability as the underlying original soil shall be used as fill material. The depth of the fill shall not be greater than 30 inches above the original ground elevation.
(iv) An absorption trench system as described in Section 75-A.8(b) is designed using the percolation of the underlying original soil.
(i) Heavy equipment shall be kept out of the absorption area.
(ii) Fill material is carefully placed within the absorption area.
(iii) The edge of the fill material shall be tapered at a slope of no greater than one vertical to three horizontal. On sloped sites a diversion ditch shall be placed on the uphill side to prevent runoff from entering the fill.
(iv) The absorption trench system is constructed in the fill material, extending into the existing natural soil.
(1) A cut and fill septic system is an absorption trench system installed on sites where impermeable soil overlays a permeable soil.
Cut and fill septic systems may be used where all the following conditions are found:
(i) A soil with a percolation rate slower than 60 minutes per inch, such as clay or clay loam, overlays a useable soil with a percolation rate faster than 60 minutes per inch;
(ii) At least three feet of useable soil is available beneath the tight soil;
(iii) All minimum vertical and horizontal separation distances can be maintained as described in Table 2.
(i) It shall provide for the removal of the overlaying unusable soil and replacement by soil having a percolation rate comparable with the underlying soil;
(ii) An absorption trench system is designed as described in Section 75-A.8(b).
(iii) The required length of absorption trench is based upon the percolation of the underlying soil or the fill material, whichever has the slower percolation (lower permeability).
(i) The area excavated and filled must provide at least a five foot buffer in each direction beyond the trenches.
(ii) The material placed above the trenches shall have a percolation rate faster than 60 minutes per inch.
(iii) Original surface material shall not be used as backfill above the trenches.
(iv) The surface area of the fill system must be mounded and graded to enhance the runoff of rainwater from the system and seeded to grass.
An absorption bed system operates on a principal similar to the absorption trench except that several laterals, rather than just one, are installed in a single excavation. This reduces the effective sidewall infiltration area per linear foot of lateral or leach line.
(i) A bed system may be built in soils with a percolation rate between one and 30 minutes per inch. A bed shall not be built where the soil evaluation indicates silty loam, clay loam, or clay.
(ii) Slope of the site shall not exceed eight percent.
(iii) Bed systems are more practical on sites that are long and narrow with a minimal slope.
(iv) All vertical and horizontal separation distance requirements shall be met.
(i) Pressure distribution is required for the installation of an absorption bed system. The local health department having jurisdiction may allow the use of siphon dosing on specific sites.
(ii) The maximum width of the absorption bed shall be 20 feet. The maximum length of each lateral from a pressure manifold shall be 100 feet. Utilizing a center manifold system, a bed may then have a maximum length of 200 feet. Laterals for siphon dosing systems in beds are limited to 75 feet.
(iii) The depth of the leachfield bed shall be between 18 and 30 inches below original ground level.
(iv) Laterals for the drainfield shall be spaced five (5) feet apart. Two and one-half feet (2 1/2') must be provided between the laterals and the sidewalls. In the maximum width of 20 feet, only four laterals may be installed.
(v) Using pressure distribution with a center manifold, a drainfield bed system shall have maximum dimensions of 205 feet by 20 feet.
(vi) The required bed bottom area shall be calculated from the application rates shown in Table 5.
TABLE 5 ABSORPTION BED SEPTIC SYSTEMS -- REQUIRED BOTTOM AREA PERCOLATION RATE APPLICATION RATE MINUTES/INCH GALLONS/DAY/SQ. FT. ------------------ ------------------ 1 - 5 0.95 6 - 7 0.80 8 - 10 0.70 11 - 15 0.60 16 - 20 0.55 21 - 30 0.45 30+ Not Acceptable
(i) Heavy construction equipment shall be kept outside the proposed bottom area of the bed.
(ii) The required drainfield or leach field bed bottom area is excavated as level as practical. The bottom and sides of the excavation are hand raked to reduce soil smearing.
(iii) After excavation, a six inch layer of aggregate is placed across the bottom of the drainfield bed.
(iv) The drainfield laterals are laid level on the aggregate and covered with aggregate to a level two inches above the top of the pipe.
(v) The entire leachfield bed area is covered with a permeable geotextile. Untreated building paper or a four inch layer of loose hay or straw may be substituted if a permeable geotextile is unavailable.
A seepage pit, sometimes called a leaching pit, leaching pool, or incorrectly a cesspool, is a covered pit with an open-jointed or perforated lining through which septic tank effluent seeps into the surrounding soil.
[DF NOTE: the term drywell might be used for a seepage pit disposing of septic effluent in this case; by contrast, a cesspool holds both solid waste and septic effluent. I use the term "seepage pit" to refer to a pit used to dispose of septic effluent which originated as blackwater, the solids having been retained in the septic tank. I use the term "drywell" to refer to a pit used to dispose of graywater (greywater) which originated in sinks, laundry facilities, or showers. While their uses and implications of their presence at a property are quite different, the actual construction details of a seepage pit or a drywell are about the same.
See Cesspools for more in-depth information about those systems.
See DRYWELL DESIGN & USES for more in-depth information about those systems.]
(i) If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.
(ii) A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.
(i) The required "effective seepage pit area" is obtained from Tables 6 and 7 which are shown below.
(ii) No allowance for infiltration area is made for the bottom area of a seepage pit or the surface area of impervious soil layers (percolation rate slower than 60 minutes/inch).
(iii) The effective diameter of a seepage pit includes the diameter of the lining plus the added diameter provided by the annular ring of aggregate. Any area surrounding the liner with rock smaller than 2 1/2 inches in size shall not be included as part of the effective diameter.
(iv) Effective depth of a seepage pit is measured from the invert of the seepage pit inlet to the floor of the pit, with the thickness of impervious layers deducted.
[Click to enlarge any image or table]
(v) Seepage pit linings may be pre cast concrete, cast-in-place concrete, or built in place with un mortared hollow cinder or concrete blocks.
Concrete shall have a minimum compressive strength of 2,500 psi and 3,000 psi is recommended. Material with comparable structural strength, determined in accordance with commonly accepted sewage construction standards, principles or practices, may be allowed on an individual basis to prevent unreasonable hardship, provided public health is not prejudiced.
(vi) The separation between the outside edges of seepage pits shall be three times the effective diameter of the largest pit. This separation is measured as the undisturbed soil between pit excavations.
(vii) Seepage Pits shall be designed with sufficient structural stability to withstand lateral soil forces as well as vertical loads.
(i) Laterals leading to each seepage pit must be at least four inches in diameter with a minimum slope of 1/8 inch per foot.
(ii) Seepage pits shall not be connected in series. A distribution box shall be required where more than one seepage pit is installed.
(iii) The seepage pit excavation is to be raked to minimize sidewall smearing that may occur and reduce infiltration capacity. If groundwater is encountered, the pit shall be backfilled with the original soil to a level at least three feet higher than maximum groundwater and adjustments made in the pit dimensions.
(iv) The seepage linings are placed upon a concrete block, poured concrete, or pre cast footing and surrounded by a six inch minimum annular ring of large aggregate (2 1/12 - 4 inches in size).
(v) The rock [used to provide improved drainage around a seepage pit] is covered to prevent soil from filling the void spaces. Building paper, a four inch thick layer of hay or straw may be used.
(vi) The seepage pit cover shall be structurally sound and capable of supporting 300 pounds per square foot at the weakest point. Covers may be pre cast concrete or cast-in-place and shall be reinforced. A manhole with an opening of at least 20 inches in the shortest dimension shall be provided.
Continue reading at SEPTIC DRAINFIELD SIZE for complete septic design tables or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see SEPTIC SOIL & PERC TESTS Perc Tests or Deep Hole Testing for details about how to conduct soil percolation tests.
Or see SEPTIC SYSTEM DESIGN BASICS - home
Or see DICTIONARY of SEPTIC SYSTEM & WASTEWATER TERMS for a more complete dictionary of septic system and wastewater terminology
Or see SEPTIC CODES & REFERENCES
Or see SEPTIC & SEWAGE TREATMENT CODES & REFERENCES - home
Or see U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS - start of this series
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