Septic Drainfield Design: Septic Size Requirements Guide
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Septic drainfield design & size determination: this article explains how we choose the size of a septic leachfield or soakaway bed or drainfield. We discuss rules of thumb used to set the size of a conventional septic drainfield. Septic drainfields, also called leach fields, absorption beds, soil absorption systems, soakaway beds, and leaching beds, perform the functions of septic effluent treatment and disposal
in onsite wastewater treatment systems, conventionally called "septic systems". Here we discuss several different conventional soil absorption systems: absorption fields:
conventional trench, deep trench, shallow trench, cut-and-fill, and gravelless septic systems. Then we discuss septic absorption beds, and seepage pits.
Basic septic system designs are discussed beginning at SEPTIC SYSTEM DESIGN BASICS. Advanced & alternative septic treatment methods are discussed beginning at SEPTIC SYSTEM DESIGN ALTERNATIVES. Also see SEPTIC DRAWINGS.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How to Determine Septic Absorption Field Size
A conventional septic tank performs roughly 45% of the sewage treatment or less at a private home served by a septic system. The rest of the wastewater treatment and ultimately the liquid disposal occurs in the drainfield.
Also see 75-A.8 Subsurface treatment of effluent, absorption field requirements, leach field design criteria for example regulations describing the construction of septic drainfields.
LEACH FIELD SIZE - Septic Leach Field or Septic Absorption Field Size: How large does the absorption field need to be?
The size of the absorption field needed (in square feet of area, presumably
also unencumbered by trees, driveways, buildings, etc.) can range considerably depending on the soil percolation rate. Sketch at left - USDA.
A lot with a good percolation rate or "perc" of perhaps one inch of percolation
in three minutes might require about 4500 square feet for a typical three bedroom home. If the same home were
built where there was a poor a soil percolation rate of an hour per inch, 9000 square feet or more might be required
for the absorption area.
Typically, septic leach fields (synonyms: drainfield, leach bed, soakaway bed, absorption bed) are built by placing perforated effluent distribution pipes in a field or bed of gravel. The field is a series of trenches that may be up to 100-feet long and 1 foot to 3 feet in width, separated by six feet or more, depending on local requirements, and sometimes constructed leaving space between the original lines to install replacement leach lines when needed. - paraphrasing USDA.
Drainfield size and location also have to take into account local zoning - setback requirements from property borders, setbacks from
streams, wetlands, wells, water supply lines, and other encumbrances.
Septic Drainfield Design Basics: septic trench line specifications
Below we summarize the basics of septic drainfield design, followed by more detailed septic field design specifications for common types of drainfields or soakaway beds.
In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield. [See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
- Perforated septic drainfield pipes are
placed in gravel-filled trenches 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).
- Drainfield trench piping slope specifications: 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.
The sketch at left shows a cross section of a septic drainfield or soakaway field or leach field trench - the common name varies by geographic location and custom.
- Septic drainfield pipe hole or perforation position: up or down? As you see in the sketches shown here, the septic leaching bed perforated pipe is placed on a bed of gravel and then covered with more gravel. Do not place the pipe at the bottom of the trench before any gravel is placed therein.
We like to place the pipe with perforations at the 5 o'clock and 7 o'clock positions so that we're not simply storing and collecting sludge and effluent in the pipe bottom; by not placing the perforated septic drainfield pipe with one of its rows of holes facing directly down (at 6 o'clock position) we defer the clogging of the pipe effluent drain openings.
Our second drainfield sketch (left, USDA) shows a slightly different view, in this case an isometric cross-section drawing of a septic drainfield trench.
- Septic drainfield trench depth specification: A typical septic drainfield trench is 18 to 30 inches in depth, or per the USDA, 2 feet to 5 feet in depth. IN some climates such as northern Minnesota we have observed drainfield trenches placed much deeper - unfortunately meaning that while effluent disposal may be protected from freezing, effluent treatment may be marginal.
- Septic drainfield trench width specifications: typically septic trenches are 8 to 12 inches wide in some applications, or 18" to a maximum of 36" wide in traditional, conventional septic drainfield designs. (Regardless of actual drainfield trench width, for design purposes each linear foot of drainfield line is considered to to provide one square foot of drainage are
- Septic drainfield trench spacing: The trenches are dug about 6 feet apart on center (center of pipe to center of next pipe) which allows,
in good design, space for a set of replacement trenches to be placed between the original ones when the first set fails.
- Septic drainfield trench length: The maximum length of a trench is typically about 150 feet but we have found installations that were three times that length. Some writers opine that the maximum septic trench line is 100 feet. A realistic answer is, it depends - on site and soil conditions.
- Septic drainfield pipe diameters: the usual minimum septic drainfield pipe inside diameter is 4"
- Septic drainfield gravel specification: typically, washed gravel, 3/4" to 2 1/2" (pretty big) gravel is placed for a depth of 12" under the drain line piping ("septic drain tile" in some references). The pipe is then covered by additional gravel to a depth of at least 2" over the top of the septic drain line before backfill is added. Some septic installers place a geotextile over or around the gravel and pipe to reduce clogging by dirt infiltration from the septic trench backfill.
- Septic drainfield pipe or drain tile connections: modern drainfields constructed using perforated plastic pipe are comprised of pipe sections that are physically connected together: one end of each pipe is expanded to slip over the diameter of its mating section. Older traditional drainfields made of other perforated piping were constructed by laying perforated pipe end to end, with abutting pipe ends spaced 1/4" apart and protected from backfill soil by a layer of roofing felt.
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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.
Septic Drainfield Inspection Ports
A high quality septic drainfield or leach bed design includes inspection ports or pipes that permit inspection of the condition of the field. Vertical pipes are placed, usually at the end of each drainfield trench or section or at critical or suspect areas near the drainfield to permit monitoring of liquid levels in the drainfield trenches. If a few ports are included outside the drainfield area they will aid distinguishing between a drainfield suffering local effluent saturation from an area groundwater problem that also impedes the drainfield operation.
Some installers trim the inspection ports to ground level, sealing each with a removable cap to permit inspection of the drainfield condition without interfering with mowing.
Detailed Specifications for Septic Absorption Field Designs
The following specification for septic drainfields or leaching beds, of various types, designs, and depths,
is adapted and expanded from: New York State Appendix 75-A.8 Subsurface treatment, of New York's Wastewater Treatment Standards for Individual
Household Systems. We also include excerpts and references from other U.S., Canadian, and other authorities at state and provincial levels, and from the US EPA Onsite Wastewater Treatment Systems Manual which is available free from the US EPA and is provided in a linked-to copy at the references section of this article.
(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 of Septic Drainfield Trench Lengths Determined by Soil Percolation Rate and Daily Wastewater Input Flow
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.
P
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a
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R
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e
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Septic Wastewater Effluent Input Flow Rate (Gallons per Day) [1]
|
Minutes /
Inch |
2 Bdrms |
3 Bdrms |
4 Bdrms |
5 Bdrms |
6 Bdrms |
| |
260
gpd |
300
gpd |
390
gpd |
450
gpd |
520
gpd |
600
gpd |
650
gpd |
750
gpd |
780
gpd |
900
gpd |
| |
Required number of feet of absorption trench, assuming a 2-ft wide conventional gravel trench |
| 1 - 5 |
108 |
125 |
162 |
187 |
216 |
250 |
270 |
312 |
325 |
374 |
| 6 - 7 |
130 |
150 |
195 |
225 |
260 |
300 |
325 |
375 |
390 |
450 |
| 8 - 10 |
145 |
167 |
217 |
250 |
290 |
333 |
360 |
417 |
433 |
500 |
| 11 - 15 |
162 |
188 |
244 |
281 |
325 |
375 |
406 |
469 |
488 |
563 |
| 16 - 20 |
186 |
214 |
279 |
321 |
372 |
429 |
464 |
536 |
557 |
643 |
| 21 - 30 |
217 |
250 |
325 |
375 |
433 |
500 |
542 |
625 |
650 |
750 |
| 31 - 45 |
260 |
300 |
390 |
450 |
520 |
600 |
650 |
750 |
780 |
900 |
| 46 - 60 |
290 |
333 |
433 |
500 |
578 |
667 |
722 |
833 |
867 |
1000[2] |
| |
Dosing not required
(but recommended) |
Dosing system or alternative design is required |
Notes to the Septic Drainfield Trench Length Table
[1] Original source: New York State NYS75-A.8 Table 4A.
[2] Conditions that require more than 1000 feet of septic drainfield trench must have an alternative dosing system design.
[3] See SEPTIC DRAINFIELD SIZE for details of septic drainfield sizing and specifications.
An Alternate Table for Determining Septic Drainfield Size
The following is adapted from
our engineer's article summarizing "How Big Should the Septic Leach Field Be" found at HOW BIG SHOULD THE LEACH FIELD BE?.
Determining the required size of a leach field is a bit more complicated. The first thing to consider is the nature of the soil in which the leach
field is to be constructed. Because water has to be absorbed in the soil, we need to know how fast it can be absorbed. This is called the percolation
rate and is expressed as the time it takes for water in a test hole to decrease in level by one inch (minutes/inch).
We must also know the type of
soil and whether seasonal changes in the natural level of groundwater will interfere with the satisfactory operation of the system. Seasonal
groundwater must be more than four feet from the bottom of the leach field trenches. Judgments regarding the soil conditions and percolation rates are
best left to a professional. If the soil percolates very quickly, (less than one minute per inch) or very slowly (greater than 60 minutes per inch) it
will not be possible to install a standard leach field in the existing soil.
We must now determine the amount of water that has to be absorbed each day. As with the septic tank sizing, there are also "rules of thumb" that can be
used to find out how much water must be absorbed each day for each bedroom in the house (expressed as gallons per day per bedroom).
For older houses
(built before 1979) we must allow 150 gallons per day (gpd) per bedroom. For houses where the toilets are limited to no more than 3.5 gallons per
flush and the faucets and showerheads are limited to 3 gallons per minute or less, we must allow 130 gpd per bedroom. For houses with water-saving
toilets that use only one gallon per flush we allow 90 gpd per bedroom. The required flow rate is found by multiplying the appropriate flow by the
number of bedrooms (in this case, we do not have to count a garbage disposal as a bedroom).
Knowing the rate at which water can be absorbed by the soil (the percolation rate) and the flow rate (in gallons per day), we can use the
following table to calculate how many square feet of absorption field is needed. [Readers will notice that this table is similar to but less detailed than our typical state or board of health table above at Table 4A.]
What is the Required Size of the Septic Leach Field? |
Absorption
Percolation
Minutes per Inch |
Allowable
Application Rate - Gallons
per Day per Square Foot |
| 1 - 5 |
1.2 |
| 6 - 07 |
1.0 |
| 8 - 10 |
0.9 |
| 11 - 15 |
0.8 |
| 16 - 20 |
0.7 |
| 21 - 30 |
0.6 |
| 31 - 45 |
0.5 |
| 46 - 60 |
0.45 |
Soil with a percolation rate less than 1 minute per inch or more than 60 minutes per inch is unsuitable for a conventional system. |
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 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.
(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 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
mis-align 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.
[See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
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 SEPTIC 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) 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.
Shallow Trench Systems - (e) Shallow Septic System Absorption Trenches & Septic Drainfield Trench Construction Specifications
(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 System Absorption Trenches
(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.
Question: Can I Backfill the Septic drainfield Trenches with Wet Soil during Field Construction?
I recently installed leaching chambers and before I could backfill the trenches it rained for a couple of hours, is it ok to use the wet backfill to fill the trenches? - B.D.
Reply:
OPINION-DF: A competent onsite septic field construction inspection by an expert may find additional concerns that need to be addressed to assure a long drainfield life. That said, here are some things to consider:
- Provided you are installing the required gravel under and around the drainfield piping, backfilling with wet soil may not be a problem
- We say "may not be" because of not the so much the soil itself, as soil is wet after backfill when it rains, but rather because depending on trench depth and how the operator runs the equipment, driving over some wet soils (depending on soil composition itself), or piling deep amounts of wet soil in a trench may compact the soil more than it would have been compacted if filled dry.
- Our opinion is that because of its added weight, filling to depths more than 30" might compact soils unnecessarily.
- But a much more serious concern would be driving the backhoe or other construction equipment over the trenches, makes for a high risk of excessive soil compaction, especially driving the equipment over the trenches.
- So soil that is not clay, and drainfield trenches that are less than 30" deep from top of gravel (over the pipes) to surface, mean you may be ok.
But driving the earth moving equipment around over the drainfield trenches themselves (or in the future driving anything over the drainfield) risks damage to the fields by soil compaction or even crushing and breaking the drainfield piping. Don't do it - backfill with care, driving equipment in the space between the drainfield trenches rather than over them.
CONTACT us to suggest alternative septic system designs and specifications.
Cut and Fill Septic Drainfield 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 leach line.
(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 - below
Absorption Bed Septic System Required Bottom Area
TABLE 5 ABSORPTION BED SEPTIC SYSTEMS -- REQUIRED BOTTOM AREA
PERCOLATION RATE APPLICATION RATE
MINUTES/INCH GALLONS/DAY/SQ. FT.
------------------ ------------------
1 - 5 0.95
6 07/13/2010 - 07 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. [See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
(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 Pit Construction - (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.
Frequently Asked Questions (FAQs) about septic drainfield, leach bed, or soakaway bed design, size, & capacity requirements
Question: which are the best types of pipes to use in a conventional drainfield/leachfield?
I could not find which type of pipes are better to use for a drainfield/leachfield. I have seen 3 different types at lowes.
1. PVC pipe with 3 rows of holes on the bottom.
2. Black flexible corrugated pipe with 2 rows of holes on the bottom
3. Black flexible corrugated pipe that has small slits instead of holes and it is all the way around the circumference of the pipe.
I am not sure which to get and the price is about the same for all 3 of them. - D Smith 6/25/11
Reply:
All of these will work if you are using gravel-filled trenches, and will work longer and better if you install an effluent filter at the septic tank outlet so as to avoid pipe clogging. In gravelless systems the galleys may use more narrow slits (and in some designs also a covering of geotextile fabric) to avoid soils washing INTO the galley or piping.
In general I avoid installing piping with holes facing "UP" in conventional drainfield trenches.
Question: how effective will my drainfield be?
If I built a system 22x36 with 3 long 30' and 8' between them and the 8' on both sides of the center being leach line also how much effective field would I have? Ed 10/16/11
Reply:
I'm sorry, Ed, but I don't think there is a straight forward answer to your question of how much more effective a drainfield will be with your specifications - first of all, what are we comparing it to ? And what are the soil characteristics? What is the design load or wastewater volume to be disposed-of? You don't say. The design and its efficacy depend on various characteristics that you haven't stated.
I do agree that leaving space between the leach lines is a good design that allows for future field replacement between the existing trenches.
Question: how do I figure out the size of an existing drainfield?
i am trying to determine the size of an existing conventional drainfield. how do i go about that? - Alan H. 1/17/12
Reply:
Alan, on determining the size of an existing septic drainfield, if you are unable to find as-built plans or a site map for the septic fields, you will want to review the advice given at SEPTIC DRAINFIELD LOCATION (article links listed at Related Topics ) on how to find the location of the drainfield. The rough size and shape of that area will be as close as you'll get without some digging and probing.
Question: how will a flood easement affet my septic system ?
my septic and leach field have been in for 30 yrs. working great. the state of PA wnt to expand their flood easement over the leach field. Will this cause a problem if i need to replace it 5yrs down the road? Example getting permits etc. - Todd 3/3/1012
Reply:
Todd I'm unsure what the flood easement means in your case but two conditions give us concern:
1. any vehicle traffic passing over your leach field is likely to damage if not destroy it, leading to a need for costly repairs and perhaps relocation of the field
2. If the flood easement means that your leachfield is subject to actual flooding, you know that the fields will not function in any sanitary way when saturated.
Question: our septic alarm keeps going off if we use more than 400 gallons
I just moved to a brand new house and that has a septic. I have septic first time in my life. I have some guest due to family occasion. Alarm goes off every other day. I am told that septic tank pumps 400 gallons a day. If we use more than 400, it sets off alarm. We have 1250 gallon tank and other tank is 1000 gallons that pumps water to drainfield. I have a big family so I will have guest several times a year. What should I do? Can I pump it manually e.g. total 800 gallons a day until guests are gone? or Should I buy a small pump e.g. 2 hp and pump water out on a small ditch? Can I do that? What is the solution? What is maximum gallons I can have pumped? My drain field is 6 ft deep and about 500 total length. - Jay 5/7/12
Reply:
Jay, it sounds from your note as if the septic system was designed to handle a daily maximum wastewater volume of 400 gallons per day. I'd start by asking a local wastewater system engineer or installer who is familiar with yoru system design to assure that everything is currently working properly since if not, some repair action is needed.
Next, in my OPINION if your regular usage is exceeding the design capacity of the system, you'll need to address that problem by a combination of steps to minimize water usage to lighten the load on the system combined with an expansion of system capacity. To do anything else might seem to "work" in the short run, but actually the result is discharging untreated or insufficiently treated wastewater into the environment - contaminating groundwater, potentially contaminating local wells etc., and risking a near term system failure.
Question: how do we size a drainfield for commercial properties?
What is the required size for a septic system for 70 units - RV Trailer Park 6/1/12
For commercial property with no bedrooms how is the size of the drainfield determined? - Anon 7/18/12
Reply:
Good question, Anon.
For an RV Trailer Park, using government design manuals [1][5] as a guide, we estimate that daily wastewater volume will be 75 to 125 gallons pe day per person, with a typical volume of 100 gpd. The drainfield will be designed to handle (100 gallons x number of visitors) per day.
For a commercial property with no bedrooms, the wastewater design volumes vary enormously depending on the type of facility. More details are at COMMERCIAL SEPTIC DESIGN
Sizing for commercial drainfields and septic tanks) is more difficult than for residential installations. Residential designs start with a simple assumption of the number of occupants and asn average daily wastewater volume (common is 150 gallons/bedroom or 75 gallons per day per person, though some sources use larger numbers). We often hear complaints from people who say "we are just two people living in a four-bedroom home, why does our septic system have to be designed to handle eight people (4 bedrooms x 2)?" A sensible answer is that the wastewater system should be designed to handle the number of occupants that the building is designed for.
But commercial installations vary widely in the wastewater volume used per person per day depending on the type of facility, the number of visitors to it, how long they stay there, and what activities they pursue. For example a gas station at a turnpike may have thousands of visitors per day, many of whom use the toilet facilities - that's why we stop at a rest stop - even though the typical length of visit is relatively short.
The US EPA Wastewater manual as well as some U.S. state DEC/DEP wastewater specifications guidelines have published a series of tables of ranges of wastewater production for different types of facilities per visitor or user along with other sources of possible usage volume (such as number of parking spaces).
In a separate article COMMERCIAL SEPTIC DESIGN [in process] we provide excerpts from that larger body of information.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- [1] US EPA Onsite Wastewater Treatment Systems Manual [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems,
Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers.
U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
- [2] "International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
- [3] "Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental
Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
- [4] Manual of Septic Tank Practice, US Public Health Service's 1959.
- [5] "Installers Manual for Conventional Onsite Domestic Wastewater Treatment and Disposal Systems", Department of Environmental Conservation, Division of Environmental Health Drinking Water and Domestic Wastewater Program, Alaska Department of Environmental Conservation, 1 Aug 2000, Anchorage Offices, 555 Coredova, Anchorage AK 99501, Tel: 907-269-7500. retrieved 17 July 2012, original source: http://dec.alaska.gov/water/wwdp/onsite/pdf/Certified_Installer%27s_Manual.pdf [Copy on file as Alaska_Certified_Installer's_Manual.pdf].
Notice: [Quoting]
This document contains information regarding the installation of onsite se3wer systems for single-family and duplex residences. It must be used by Certified Installers and homeowners who are subject to 18 AAC 72. Additional requirements are included in 18 AAC 72. If there is a conflict between the provisions of this manual and 18 AAC 72, 18 AAC 80, or other state regulations, the regulations language controls. AEDC offices.
The regulations cited above for Alaskans can be found at http://www.alaska.gov/dec./deh/water/ci.htm
- [6] Soil Percolation Tests soil perc testing guide and instructions
- [7] Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
- [8] Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. Readers should also see Soil Percolation Tests and for testing an existing septic system, also see Dye Tests
- [9] Wells and Septic Systems, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF.
Quoting an Amazon description:Here's all the information you need to build a well or septic system yourself - and save a lot of time, money, and frustration. S. Blackwell Duncan has thoroughly revised and updated this second edition of Wells and Septic Systems to conform to current codes and requirements. He also has expanded this national bestseller to include new material on well and septic installation, water storage and distribution, water treatment, ecological considerations, and septic systems for problem building sites.
- [10] Design Basics for Septic Systems: Choosing Septic Tank Size, Leach Field Size - basic septic system volume and absorption system design guides.
- [11] How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
- [12] HOW BIG SHOULD THE LEACH FIELD BE? - an engineer's view of leach field sizing
- [13] Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
- [14] Sketches of the Septic System Components Private Sewage Disposal Systems - Septic Drawing Library
- [15] Percolation Testing Manual, CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf
- [16] Soil Test Pit Preparation, fact sheet, Oregon DEQ Department of Environmental Quality, original source www.deq.state.or.us/wq/pubs/factsheets/onsite/testpitprep.pdf The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or see http://www.oregon.gov/DEQ/
- [17] Thanks to reader Michael Roth for technical link editing 6/29/09.
- [18] Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
- [19] Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
- [20] Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
- [21] Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
- [22] 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.)
- [23] The Plumbers's Handbook, Howard C. Massey, Craftsman Book Company; Rev Sub edition (April 1998), ISBN-13: 978-1572180567 includes septic system design basic sketches and specifications.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP;
Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
- Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP;
Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.
TABLE 4B - SOIL APPLICATION RATES
