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Septic System Design Basics
Choosing Septic Tank Size, Drain Field Size, Basic Septic Design
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
The ABCs of septic tank and drainfield design: this septic system design article 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
SEPTIC SYSTEM DESIGN BASICS - Septic System Design:Choosing Septic Tank Size, Absorption System Size, Basic Design Notes
How to Use this Septic Design Guide
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.
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 listed at Technical Reviewers & References at the end of this article 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 SYSTEM 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 at HOME BUYERS GUIDE to SEPTIC SYSTEMS.
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 Specifications & Advicve for Conventional Septic Tank & Drainfield System Design
Septic Tank Sizing: Most jurisdictions currently want to see a minimum of 1000 gallons.
Septic tanks may be approved constructed of concrete, fiberglass, or polyethylene. Site-built septic tanks may still be permitted in some jurisdictions. Any of these materials can work fine for a septic tank provided the tank is properly installed. (Lightweight tanks may need to be anchored to prevent floating up out of the ground when empty.)
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 SEPTIC TANK SIZE to see the required tank size based on daily water volume usage and perhaps the WATER QUANTITY USAGE GUIDE article which discusses water usage requirements to confirm that you're estimating the water volume reasonably. Septic tank sizing is explained at SEPTIC TANK SIZE.
Septic Tank Location: Articles titled SEPTIC CLEARANCE DISTANCES and WELL CLEARANCES 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 EFFLUENT RETENTION TIME.
Septic leach field or drainfield size and layout planning: see our article SEPTIC DRAINFIELD SIZE which sizes the field based on its percolation rate and our article EPTIC SOIL & PERC TESTS which describes performing a perc test, with perhaps a look at SEPTIC BIOMATS to understand biomats that form below a drainfield and why a leach field in a wet area won't treat the effluent.
Standard Leach Field Trench Specifications: conventional "two foot" gravel and perforated pipe absorption trench leach field trench specifications are given here. 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. Most septic design authorities refer to a standard or generic two-foot wide trench.
- 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.
- 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.
- SEPTIC DRAINFIELD SIZE includes a "Table of Septic Drainfield Trench Lengths Determined by Soil Percolation Rate and Daily Wastewater Input Flow" that gives assumptions for number of building occupants, average daily gallons of septic wastewater input flow volume in gallons per day, and for conventional gravel trench drainfields, the required trench length in feet for various soil percolation rates.
- SEPTIC SOIL & PERC TESTS explains soil percolation tests that are used to determine the ability of soil at a site to absorb septic effluent and process it to an adequate level of treatment.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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). 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 I recommend for professionals--DF.
- 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.
- 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
- Soil Percolation Tests soil perc testing guide and instructions
- Percolation Testing Manual, CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
- 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
- Save the Septic System - Do Not Flush These Items Down the Toilet, Daniel Friedman, InspectAPedia.com - PDF document, printable
- 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
Septic System Design & Installation Details
- Absorption Field Design size and specifications for leach fields, absorption beds, gravelless systems, seepage pits
- Absorption Field Layout: septic drainfield or leaching bed shape and placement considerations
- Absorption Fields Trench Length has an example of calculating leach line trench length.
- Alternating Bed Septic Systems may provide a retrofit repair for a failing drainfield or a means of extending drainfield life
- Authorities, Septic Design Specifications by Federal and State Regulators
- AEROBIC SEPTIC SYSTEMS: Design Alternatives for Difficult Sites, specifications, photos, sketches, product sources, suppliers, designers
- Books & Building Codes for Septic System Design. Septic System Education Materials
- Baffles & Tank Tees: septic tank baffles and tees, design to prevent system clogging
- Capacity: Determining Needed Septic Capacity - Septic Tank Size vs. Usage: What septic size is required?
- Capacity: Determining Needed Septic Capacity - Based on Water Usage: Typical residential water usage for people, appliances, and activities.
- Cesspools: explained, design specifications, safety, photos, sketches
- Clearances & Distances from septic system components to other site features, property boundaries, waterways, etc.
- Contaminants in sewage which may be released from sewage spills from residential septic systems and wastewater disposal systems
- DRYWELL DESIGN & USES Home Buyer's or Owner's Advice
- Composting Toilets Are They A Septic System Design Alternative for Difficult Sites?
- Design Alternatives for Septic Systems: Advanced & Alternative Septic System Designs for Onsite Wastewater Disposal - design engineers, consultants, products, books for special problem situations, difficult soils, old system repairs. Aerobic septic systems, chemical, composting, incinerating & waterless toilets, Evaporation-Transpiration (ET) Septic Systems, septic media filters, greywater systems, holding tank septic systems, mound septics, raised bed septics, pressure dosing septic systems, sand bed filters, peat beds, constructed wetlands, septic disinfection systems.
- Design Basics for Septic Systems: choosing septic tank size, leach field Size - basic septic system volume and absorption system design guides.
- Disinfection for Aerobic Septic Systems - use of calcium hypochlorite tablets versus swimming pool chorine tablets
- Disinfection for Aerobic Septic Systems - warning about use of swimming pool chorine tablets
- Dosing Systems, Gravity, Siphons, Floats - to control movement of septic effluent to the absorption system
- 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
- DRYWELL DESIGN & USES design specifications for drywells, seepage pits, leaching pits
- Evaporation/Transpiration Septic Systems - septic system design alternatives for difficult sites
- Filters for Septic Tanks & Graywater to protect septic drainfields and reduce drainfield clogging
- Gravelless Septic Systems - design and installation specifications
- Greywater Systems or Graywater systems for disposal of onsite wastewater: designs, products, advice, books.
- Global Warming Effects on Septic Systems and Onsite Wastewater Treatment or Disposal
- Lagoon Systems for human and animal waste and wastewater treatment
- Leach Field Size specifications for leach fields, absorption beds, gravelless systems, seepage pits
- Media Filter Septic Systems as components of alternative septic systems for difficult sites
- Mound Septic Systems as components of alternative septic systems for difficult Sites
- Percolation Tests septic soil tests, deep hole tests
- Pressure Dosing Septic Systems: explanation, design alternatives, photos, sketches, product sources, suppliers, designers
- Products - for Alternative Septic System Designs aerobic system pumps, media filters, gravelless systems, other advanced wastewater treatment products
- Pumps Septic pumps, sewage ejector pumps, grinder pumps, effluent pumps, sump pumps, & septic pumping stations compared; pump alarms. Advice.
- 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
- Toilets, Chemical, Composting, Incinerating: chemical toilets, composting toilets, greywater systems, holding tanks, incinerating toilets
- Wastewater Treatment Basics describes the basic processes that occur in wastewater treatment processes
- Waterless & Low Water Septic Systems, chemical toilets, composting toilets, greywater systems, holding tanks, incinerating toilets
- Wetland Septic Systems
Design Manuals for Septic Systems
- 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)
- "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.
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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