Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR
SEPTIC CARE INSTRUCTIONS
SEPTIC D-BOX INSTALL REPAIR
SEPTIC DRAINFIELD FAILURE DIAGNOSIS
SEPTIC DYE TEST PROCEDURE
SEPTIC FAILURE SIGNS
SEPTIC INSPECTION & TEST GUIDE
SEPTIC LIFE EXPECTANCY
SEPTIC SUPPLIES & PARTS
SEPTIC SYSTEM DESIGN ALTERNATIVES
SEPTIC SYSTEM DESIGN BASICS
SEPTIC SYSTEMS, HOME BUYERS GUIDE to
SEPTIC SYSTEM SAFETY WARNINGS
SEPTIC TREATMENTS & CHEMICALS
SEWAGE BACKUP PREVENTION
SEWAGE EJECTOR / GRINDER PUMPS
SEWER GAS ODORS
SEWER LINE REPLACEMENT
SINKHOLES, WARNING SIGNS
SOAKAWAY BED FAILURE DIAGNOSIS
SULPHUR & SEWER GAS SMELL SOURCES
TOILETS, INSPECT, INSTALL, REPAIR
TRAPS on PLUMBING FIXTURES
TREATMENTS & CHEMICALS, SEPTIC
VIDEO GUIDES: Septic Videos
WATER CONSERVATION MEASURES
WATER SOFTENERS & CONDITIONERS
WATER SUPPLY & DRAIN PIPING
WATER, WELLS, WATER TANKS: TESTING GUIDE
WASHING MACHINES & SEPTIC SYSTEMS
WASTEWATER TREATMENT BASICS
WINTERIZE A BUILDING
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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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.
[Click to enlarge any image]
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.
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]
[Click to enlarge any image]
Our second drainfield sketch (left, USDA) shows a slightly different view, in this case an isometric cross-section drawing of a septic drainfield trench.
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.
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.
[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
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.
Notes to the Septic Drainfield Trench Length Table
 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.
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.]
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.
TABLE 4B - SOIL APPLICATION RATES [Click to enlarge any image]
(3) Materials used for Septic Drainfields
(4) Construction of Septic Drainfields
[See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
Gravelless Septic Absorption Systems
Because they omit gravel, the soakaway bed or drainfield trench dimensions for no-rock or gravelless septic systems are different than for conventional drainfields. If you are working on a design for GRAVELLESS SEPTIC SYSTEMS continue reading or use the web-links at SEPTIC SYSTEM DESIGN ALTERNATIVES navigate the online septic systems book or our other online documents.
(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
(3) Construction specifications for deep trench septic absorption systems
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
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.
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:
CONTACT us to suggest alternative septic system designs and specifications.
(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:
(3) Design criteria for cut and fill septic systems
(4) Construction of cut and fill septic systems
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
(3) Design Criteria for absorption bed septic systems
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
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.
Continue reading at SEPTIC DRAINFIELD SHAPE or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
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.
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.
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
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.
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
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 the "More Reading" links at the bottom of this article ) 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
Todd I'm unsure what the flood easement means in your case but two conditions give us concern:
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
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
Good question, Anon.
For an RV Trailer Park, using government design manuals  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.
Question: ok to add to existing leach lines to add capacity after tree roots clogged lines?
(June 23, 2014) email@example.com said:
can i add to my existing leach lines because i think a tree entered one of my lines i have not opened my ground up yet
Certainly, provided your site has adequate space and the addition meets clearance or property setback requirements, adding additional drainfield capacity is generally a good move. You might want first to try to diangose the trouble more accurately. If a tree is close to an existing drainfield line and the new line is not quite some distance away, the new line is likely to become invaded and blocked as well - unless you remove the tree and stump.
Questions & answers or comments about septic drainfield or soakaway bed size or capacity requirements & design.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References