SEPTIC SYSTEM DESIGN BASICS - Choosing Septic Tank Size, Drain Field Size, Basic Septic Design InspectAPedia® -
How to specify the size of a septic tank
How to specify the size of a septic drainfield
Septic system design basics
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This chapter outlines basic septic system design parameters such as finding the recommended septic tank volume and conventional
recommended onsite wastewater soil absorption system (leach field or drainfield) size,
along with some notes on how to calculate these from simple water usage and site conditions.
For more in-depth information about septic system design see our list of detailed septic design articles
at Septic System Design. Also see the EPA design manual at SEPTIC SYSTEM DESIGN MANUAL, ONLINE.
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."
This article is part of our series: Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.
SEPTIC SYSTEM DESIGN BASICS - Septic System Design:Choosing Septic Tank Size, Absorption System Size, Basic Design Notes
How to Use this Septic Design Guide
This document and the articles it recommends describe the planning and design of a basic or conventional septic system which uses a septic tank and soil absorption system. Septic system design parameters such as finding the recommended septic tank volume and conventional recommended leach field or drainfield size, along with some notes on how
to calculate these from simple water usage and site conditions are discussed here and in our basic design articles. Also take a look at the "Septic Publications" at "More Reading" below to see septic codes, septic design manuals, and onsite waste disposal standards links and articles.
The Basics of Septic System Design: The key articles shown in the page-left links under SEPTIC DESIGN BASICS will allow you to answer basic septic system design questions.
If you have no idea what the basic parts of a septic system are, you should take a look at the take a look the "Home Buyers Detailed
Guide" listed under "More Reading".
The Details of Septic Designs: A more extensive list of detailed septic planning and design articles available online is provided after the "Basic Advice for Conventional Septic System Design" just below.
Difficult septic system installation sites which are rocky, steep, wet, having high groundwater or shallow bedrock, or sites limited in size are likely to require an alternative septic design using special methods and products. For those conditions see the "DESIGN ALTERNATIVES" links at the left of these pages.
Basic Advice for Conventional Septic System Design
Septic Tank Sizing: Most jurisdictions want to see a minimum of 1000 gallons. But the right septic tank size depends on the anticipated usage not an arbitrary number. To determine the required septic tank size you would need to look at the "Tank Size" article listed below to see the required tank size based on daily water volume usage and perhaps the "Water Usage" article which discusses water usage requirements to confirm that you're estimating the water volume reasonably.
Septic Tank Location:: Articles titled "Clearances for Septic Components" and :Clearances for Wells" tell you where you can put the tank relative to other site features. Usually the tank is located close to the building for shorter waste line runs and because that's where there is often plenty of backfill to bury a septic tank during construction. Special tank designs are available for problem sites.
Septic Tank Retention Time if you just wanted to know our opinion about tank size as a function of effluent retention time, see "Retention Time in Septic Tanks".
Septic leach field or drainfield size and layout planning: see our article "Absorption Field Design" which sizes the field based on its percolation rate and our article "Soil Percolation Tests" which describes performing a perc test, with perhaps a look at "Biomat Formation" to understand biomats that form below a drainfield and why a leach field in a wet area won't treat the effluent.
Leach Field Trench Specifications: Standard leach field trench specifications: the typical numbers you'll see for trench design in typical soils with acceptable percolation rate and excluding special and problem sites will look something like this:
Leach line 4" perforated PVC pipe will slope at 1/4" per ten feet of length
A good trench is 18" to 36" wide and 18 " to 30" deep (no deeper), and the pipe is put with holes down, and runs in a 12" fill (in the trench) of clean gravel 1 1/2-3" diameter stones.
In the gravel in the trench, the perforated leach line pipe is placed on top of 6" of gravel and then is covered with additional gravel until the gravel is 2" over the pipe. With a 4" pvc pipe this is how I got the 12" fill of gravel.
Remember to direct surface or subsurface runoff away from the septic system components. Otherwise you may flood the system in wet weather.
"Fields: How Big Should the Leach Field Be?" has notes on calculating the number of linear feet of leach line trench for a conventional septic system leach field.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Dosing Systems, Pressure - Pressure dosing, using manifold systems, rigid pipe systems, and drip irrigation systems to control movement of septic effluent to the absorption system
Drainfield Design size and specifications for septic drain fields, septic leaching beds or leach fields, absorption beds, gravelless systems, seepage pits
Drawings and sketches of general septic system designs and septic system types - septic plans
Raised Bed Septic Systems raised bed effluent disposal system as a component of alternative septic systems for difficult sites
Retention Time in Septic Tanks Septic Tank Pumping Guide discusses retention time and "net free area" in a tank (allowing for sludge and scum layers)
Sand Bed Septic Systems for effluent disposal as a component of alternative septic system design for difficult sites
Seepage Pits for onsite wastewater disposal - design specifications. For more detail see DRYWELL DESIGN & USES design specifications
Septic Tank Types: characteristics and inspection guide for steel, concrete, plastic, fiberglass, and home made septic tanks
Seepage Pit Collapse or Failure - stages in the life of a drywell, seepage pit, cesspool, and what to do if the pit is damaged
Sewage Contaminants which may be released from sewage spills from residential septic systems and wastewater disposal systems
Steep Slope Septic Designs - how to install sewer lines from building to septic tank; how to install septic drainfields, leach field systems at steep or sloped sites.
Sewage Treatment Systems for onsite wastewater treatment & disposal, small residential and community system designs
Tank Size: septic tank size and capacity vs. usage determine the required septic tank size, calculate size of an existing septic tank - 2005 Ed.
Tank Tees & Baffles: septic tank baffles and tees, design to prevent system clogging
Septic Tank Soil Absorption Systems - Decentralized Systems Technology Fact Sheet, US EPA, EPA 932-F-99-075, September 1999 - original citation www.epa.gov
Also see these Septic System Inspection & Testing articles
Dye Amounts, Water Volume: how much septic dye and how much water to use to perform a septic dye test
Dye Tests: how to perform a Septic Loading and Dye Test - the complete procedure for septic loading & dye testing, a septic function test
Failure Causes - Septic Systems Basic Septic Inspection Procedures: for septic tanks, septic drainfields, cesspools, drywells, distribution piping
Failure Causes - Septic Tanks - How to Inspect Septic Tanks and evaluate the septic tank condition, baffles, sludge levels, damage, evidence of septic failure, etc.
Media Filter Septic Systems types of alternative septic systems using sand, peat, textile, foam cube or other filter media, single pass vs. recirculating media filters, how to identify them.
Septic Sludge & Scum Levels in Septic Tanks - Measuring the Level of Accumulated Solids, Sludge and Floating Scum in Treatment Tanks
Septic Tank Inspection Procedure - How to Inspect Septic Tanks and evaluate the septic tank condition, baffles, sludge levels, damage, evidence of septic failure, etc.
Septic Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
SPOTTING SEPTIC BREAKOUTS - Spotting Dyed Septic Effluent Breakout Outside - Where Septic Dye is Likely to Show Up During a Septic Dye
Septic System References & Books
"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
"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.
Manual of Septic Tank Practice, US Public Health Service's 1959
Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
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.
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.)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389
(2006), Advanced Onsite Wastewater Systems Technologies.
Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going
for homeowners but is a text we recommend for professionals--DF.
AEROBIC SEPTIC SYSTEMS
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.
Design Manuals for Septic Systems
US EPA Onsite Wastewater Treatment Systems Manua [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)
"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
"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.
Manual of Septic Tank Practice, US Public Health Service's 1959.
Onsite Wastewater Disposal Books
Onsite Wastewater Disposal, R. J. Perkins;
Quoting from Amazon: This practical book, co-published with the National Environmental Health Association,
describes the step-by-step procedures needed to avoid common pitfalls in septic system technology.
Valuable in matching the septic system to the site-specific conditions, this useful book will help you install a reliable system in
both suitable and difficult environments. Septic tank installers, planners, state and local regulators, civil and sanitary engineers,
consulting engineers, architects, homeowners, academics, and land developers will find this publication valuable.
Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and a weak index. (DF volunteers to serve as indexer if Burks/Minnis re-publish this very useful volume.)While it contains some material more technical than needed by homeowners, Burks/Minnis book on onsite wastewater treatment systems a very useful reference
for both property owners and septic system designers. We refer to it often.
While Minnis says the best place to buy this book is at Amazon (our link at left), you can also see this book at Minnis' website at http://web page .pace.edu/MMinnisbook
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
Planting Over Septic System Component", Daniel Friedman (author/editor, InspectAPedia.com), The Innovator, Winter/Spring 2008, BCOSSA, British Columbia OnSite Sewage Association, 201-3542 Blansard St., Victoria BC V8X 1W3 Canada
Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Both new and used (low priced copies are available, and we think the authors are working on an updated edition--DF.
Quoting from one of several Amazon reviews: The basics of septic systems, from underground systems and failures to what the owner can do to promote and maintain a healthy system, is revealed in an excellent guide essential for any who reside on a septic system. Rural residents receive a primer on not only the basics; but how to conduct period inspections and what to do when things go wrong. History also figures into the fine coverage.
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
Grass is Always Greener Over the Septic Tank, Bombeck, Erma: $ 5.99; FAWCETT; MM;
This septic system classic whose title helps avoid intimidating readers new to septic systems, is available new or used at very low prices.
It's more entertainment than a serious "how to" book on septic systems design, maintenance, or repair. Not recommended -- DF.
US EPA Onsite Wastewater Treatment Systems Manual 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
Water Wells and Septic Systems Handbook, R. Dodge Woodson. This book is in the upper price range, but is worth the cost for serious septic installers and designers.
Quoting Amazon: Each year, thousands upon thousands of Americans install water wells and septic systems on their properties. But with a maze of codes governing their use along with a host of design requirements that ensure their functionality where can someone turn for comprehensive, one-stop guidance? Enter the Water Wells and Septic Systems Handbook from McGraw-Hill. Written in language any property owner can understand yet detailed enough for professionals and technical students this easy-to-use volume delivers the latest techniques and code requirements for designing, building, rehabilitating, and maintaining private water wells and septic systems. Bolstered by a wealth of informative charts, tables, and illustrations, this book delivers: * Current construction, maintenance, and repair methods
* New International Private Sewage Disposal Code
* Up-to-date standards from the American Water Works Association
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.
The NSFC Products List has an excellent list of design manuals/modules available from their website or by telephone 800-624-8301
Submissions welcome. send us a suggested document link or request an exchange of website links
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