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• How to determine the septic drainfield size needed

Drainfield trench/line specifications In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield. Pipes are placed across the slope line of sloped property (so that all of the effluent doesn't simply rush down to and leak out at the end of the drain line pipe). While some experts describe the bottom of these trenches as "level" in practice they are dug to slope slightly, perhaps 1/8" per foot or less. A typical trench is 18 to 30 inches in depth, and 8 to 12 inches wide. The trenches are dug about 6 feet apart which allows, in good design, space for a set of replacement trenches to be placed between the original ones when the first set fails. The maximum length of a trench is typically about 150 feet but I've found installations that were three times that length.

The sketch above shows a cross section of a typical drainfield trench, and places below the trench the critical biomat as well as other septic field design areas and considerations. (Source US EPA who in turn obtained the drawing from Ayres Associates)

Where lot space does not permit drainfield trenches such as I've just described, a septic engineer may specify that seepage pits or galleys are to be installed. These fit in a smaller space since a single pit may be 6' to 8' in diameter. But the depth to which effluent is being delivered (4' or more) means that the sewage effluent is unlikely to be fully treated by a biomass. These systems may successfully "dispose" of effluent but they are probably not adequately "treating" it.

The Biomat: The formation, clogging, and measures to protect and extend the life of the biomat, or organism layer below and around soil absorption system effluent discharge piping is discussed at Septic System Absorption System Biomat Formation as a subchapter of this text.

Detailed Specifications for Septic Absorption Field Designs

The following specification for septic drainfields or leaching beds, of various types, designs, and depths, is from: New York State Appendix 75-A.8 Subsurface treatment, of New York's Wastewater Treatment Standards for Individual Household Systems.

(a) General Information

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.

Leach Fields - (b) Absorption Field Systems - Conventional Septic Leach Fields

[DF NOTE: This section discusses the design requirements for septic absorption fields, also called leach fields, drain fields, drainfields, or conventional soil absorption systems.]

(1) Site requirements for Septic Drainfields

(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.

(2) Design criteria for Septic Drainfields

(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.

Septic Drainfield Trench Length Specifications

This table specifies the necessary length of a conventional septic drainfield trench as a function of the soil percolation rate and the anticipated daily wastewater flow from the building served.

TABLE 4A - REQUIRED LENGTH OF ABSORPTION TRENCH (Based Upon Two (2) Feet Wide Trench)

Septic Drainfield Soil Application Rates for Non-Standard Wastewater Flow Quantities

This table specifies the allowable wastewater application rate into the soil of a conventional septic system drainfield as a function of the soil percolation rate for percolation rates between 1 minute per inch to 60 minutes per inch. Soils with a percolation rate of less than 1 minute per inch should not be used for a conventional septic drainfield.

Readers will note that this table considers only the dimensions of the bottom of the drainfield trench in considering the effective soil absorption area. Typically a conventional drainfield trench is 2 ft. wide, so the effective absorption area is simply 2 ft. x field-length in ft.

More Reading:
Soil Percolation Tests Perc Tests or Deep Hole Test for Soil Absorption Rate Testing - how to conduct soil testing for percolation rate determination

(3) Materials used for Septic Drainfields

(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 labelled 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.

(4) Construction of Septic Drainfields

(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 regraded 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. Trenchs 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 misalign and break pipe.

(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.

Gravelless Septic Absorption Systems

If you scrolled down in this document to look for information on Gravelless Septic Absorption System construction, that subchapter is published at Gravelless Systems. Please continue reading by using this link, or use the web-links at page-left to navigate the online septic systems book or our other online documents.

Deep Trench Systems - (d) Deep Septic Absorption Trench Systems

(1) Site Requirements for deep trench septic systems

These are used on sites where an useable layer of soil is overlaid by three to five feet of impermeable soil.

(2) Design Criteria for deep trench septic systems

(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.

(3) Construction specifications for deep trench septic absorption systems

(i) Trenchs 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.

Shallow Trench Systems - (e) Shallow Septic System Absorption Trenchs

(1) Site Requirements for shallow septic system absorption trenches

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.

(2) Design criteria for shallow septic absorption trenches

(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.

(3) Construction of Shallow Septic Absorption Trenchs

(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.

Cut and Fill Systems - (f) Cut and Fill Septic Systems

(1) A cut and fill septic system is an absorption trench system installed on sites where impermeable soil overlays a permeable soil.

(2) Site Requirements for cut and fill septic systems

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.

(3) Design criteria for cut and fill septic systems

(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).

(4) Construction of cut and fill septic systems

(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.

Absorption Bed Systems - (g) Absorption Bed Septic Systems

(1) General

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 leachline.

(2) Site Requirements for absorption bed septic system

(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.

(3) Design Criteria for absorption bed septic systems

(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 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 bed shall be between 18 and 30 inches below original ground level.

(iv) Laterals 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 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

(4) Construction of absorption bed septic systems

(i) Heavy construction equipment shall be kept outside the proposed bottom area of the bed.

(ii) The required 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 bed.

(iv) The 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 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.

Seepage Pits - (h) Seepage Pits used for septic system effluent disposal

If you scrolled down in this document to look for information on Seepage Pit construction, that subchapter discussing the design and use of seepage pits for onsite wastewater disposal is published as a separate web page: Seepage Pits. Please continue reading by using this link, or use the web-links at page-left to navigate the online septic systems book or our other online documents.

More Reading:

More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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