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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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Basic Septic System Design: Choose 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.
For more in-depth information about septic system design see our list of detailed septic design articles
at SEPTIC SYSTEM DESIGN.
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 at More Reading (at end of this article) under SEPTIC SYSTEM DESIGN BASICS will allow you to answer basic septic system design questions.
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.
Usually the septic 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
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.
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
These questions & answers about how to determine the required size or dimensions of a septic tank were posted originally at this article: SEPTIC SYSTEM DESIGN BASICS
On 2017-08-23 by (mod) - size a septic tank using influent and effluent BOD loadings
Robby, Please see your question and our detailed reply now found at SEPTIC TANK SIZE
On 2017-08-23 by Robby
I am working on a project that will utilize septic tanks (several in parallel and series). The WW flows are 20,000 GPD, and influent WW BOD strength is about 450 mg/L and it will be permitted with an effluent strength of 100 mg/L.
The effluent will be disposed of via surface irrigation. I have not been able to find any literature on how to size a septic tank using influent and effluent BOD loadings. All literature I have found is based on number of bedrooms. This OSSF system will serve a Kid's Summer Camp.
Thanks and I look forward to your response. Robby
On 2017-04-18 by (mod) - how to seal septic tank concrete lid or lid on the D-box
If your concrete septic tank lid-to-tank mating surfaces have a pre-cast tongue and groove shape then you'll want to use a gasket such as the products sold by ConSeal (http://conseal.com/ Tip City Ohio USA, 800 332 7325 ) (InspectApedia has no relationship with companies / products discussed here and we do not sell anything - Ed.)
If you are sealing a concrete septic tank lid that does not use an integral gasket, you'll want to use butyl sealant/caulk as it has good adhesion properties with concrete. But take care: the concrete surfaces must be clean and dry, otherwise the sealant will not bond nor seal properly.
On 2017-04-18 1 by mikie
what type of sealant do i use when reinstalling concrete lid on my distrabution box on my septic system
On 2016-11-13 by (mod) - definition of "step" in drainfield or soakbed piping
A for-sure answer would depend on the context in which the word "step" or "stepped" draining piping is used, but typically stepped piping refers to how the wastewater distribution pipes are placed in soil along a slope: runs parallel to the fall line have to step down at the end of each run, across a required separation space, then begin the next run across the fall line going back in the opposite direction.
On 2017-01-22 20:41:59.112863 by charlie
What doe's the word "step" mean when speaking of septic drainage pipe to drain field?
On 2016-11-13 by (mod) - who is allowed to design septic tanks?
A septic engineer or perhaps a civil engineer trained in onsite waste disposal can design septic tanks in most countries. Beware that home-made "septic tanks" can be dangerous, risking fatal collapse.
On 2016-11-13 by Abdul Sayed
who can we design septic tanks i want information by email email@example.com tanks
who can we design a septic tank by formula
On 2016-08-01 by josh
Thanks for the response
Will discuss floating material position, scum and sludge depths and visual inspection results in depth with a septic guy next time I pump.
On 2016-07-31 by (mod) - does it matter if a septic is turned the wrong way with incoming sewage entering at the tank outlet end?
Well indeed backwards piping of a septic tank can cause trouble, as the relative elevation of inlet is usually intended to be higher than outlet - we are avoiding flooding and backflow and clogging.
The normal level of sewage in a tank is just at the bottom of the outlet pipe. Search InspectApedia for SEPTIC TANK LEVELS to see details.
Search InspectApedia for SEPTIC TANK PUMPING FREQUENCY to see a guide on how often to pump your tank.
On 2016-07-31 by Josh
My septic service pump man says the top half of my 1000 gallon concrete tank was installed 180 degrees wrong. In other words, the inlet opening is where the outlet opening should be. It is also not level - it is pitched a bit toward the outlet. This system has been working fine for 46 years, however.
In it's first eleven years there were six people in the house, then two for a few years when my wife and I moved in, then four as my family grew, and now two again. I used to have it pumped every five years. My septic guy said something about toilet paper a few years ago after pumping it and suggested it should be pumped every two years.
The effluent level is just at the bottom of the outlet. I can't see how the sludge level can be higher than it used to be. What questions should I ask to understand this?
On 2015-07-28 by (mod) - how to find out what kind of septic system we have
Lauren it sounds as if you are describing either a mound system or a pre-cast concrete seepage pit or cesspool system. Because there are life safety hazards (falling into a septic tank is fatal) you would be best served by having an on-site inspection and if necessary excavation to be sure that you not only know the type, but the condition of the equipment and to be sure that there are no missing or unsafe covers.
Beware: don't let a madman with a backhoe tear up the whole property: doing so is unnecessarily expensive and destructive. An experienced contractor knows how to spot probably equipment locations and to dig by hand or with a gentle touch.
On 2015-07-27 by Lauren
Would you be able to tell me how to find out what kind of septic system we have and exactly where leaching field is? We live in MA and were told it is a beehive. There are 2 tanks and they are made of concrete. Many thanks!
On 2015-04-13 by (mod) - assumptions when debating acceptable septic design with the code official?
With all due respect, things are getting a bit out of hand here. We are piling assumptions atop of assumptions and waving our arms wildly. This is just too much speculation for me to have confidence in any conclusions we might try to draw. Having seen septic systems installed, done a few, made lots of measurements, dug lots of holes, I am not likely to bet the farm (or your farm) on so much speculation.
If there is a problem with system function it needs to be explored and diagnosed.
If there is a compelling need to know more accurately how the drainfield trenches were constructed, let's take a post hole digger or a spade and dig some representative cross-sections up to see what's there.
On 2015-04-13 by Deputy B
Thanks for your response regarding my question on the measurements. I have a follow-up septic system/math question. :-) The inspector indicated to me that he did use the as-built plan to determine the 5 feet 8 inch measurement for the bottom of the soil absorption system.
For argument's sake, let's assume the as-built plan is accurate. Based on my "knows enough to be dangerous" research, it seems like the inspector used the top of foundation as a benchmark and determined the level of the lowest distribution box by subtracting 95.3 from 100.0. This gives you 4.7 feet or 4 feet 8 inches.
If you then add 12 inches of stone, you get 5.8 inches, which is the amount the inspector indicated the soil absorption system was below grade. However, my house is a split-level with windows in the basement so the top of my foundation is about 2 feet 8 inches above the ground. I'm assuming that the measurement should have taken that into account.
If my assumption is accurate, the distribution box with the 95.3 measurement would be about 2 feet below ground (which seems to be a reasonable number compared to 4 feet 8 inches) and the stones a foot below that would put lowest point of the soil absorption system at 3 feet. In addition, the last inspection of the system in 2002 indicated the ground water level was at 5 feet and the system passed.
If the lowest part of the soil absorption system was 5 feet 8 inches, it should have failed. I've owned the house since then and there have been no changes to the septic system. Let me know your thoughts.
Question: hole in a septic piping elbow?
(Feb 19, 2014) Anonymous said:
I have a separate tank for my waste water (grey water). my solids run into my first tank and then into the waste water tank. The waste water is pumped uphill from my waste water tank. the pipe ,which is 2" , makes an upside down 'u'. where the 2" pipe turns horizontally and heads up hill, there is a 3/8" hole in the bottom of the elbow.
Is this hole supposed to be there? when I turned on the pump after unthawing the tank I noticed the hole. I replaced the elbow and hooked everything back up. I guess I am just looking to make sure that hole is supposed to be there or not. I didn't put a hole in the new elbow. I am hoping that this is correct.
Anon I'm not sure I have a clear understanding of your system and piping but I'm guessing that the "upside down U or elbow" you describe may have included an air inlet to allow air back into the line so that when the pump stops and wastewater wants to drain back down into the downhill-loated wastewater tank we don't back-siphon water out of the drainfield.
If that's the case, the opening is needed. But why it would be on the underside instead of on top is a point I don't get either. On the underside I'd think that wastewater would leak out of the vent opening during pumping.
Question: how could the septic inspector have measured distance to ground water?
Deputy B said:
We had a septic system inspection done and the company measured the ground water level at 48 - 50 inches based on the information from an abutting property. They indicated that the soil absorption system was at the proper level but the stone in the trenches was 5 feet 8 inches below grade.
There is no engineering plan on file. The as-built document says that there are 12 inches of stone under the pipe in the trenches, but I'm having a hard time figuring out how they came up with the 5 feet 8 inches measurement.
The only related information in the as-built plan indicates the elevation for the top of the foundation at 100.0 and the various "d" boxes at 96.2, 96.0, 95.6, and 95.3. I'm not sure how that information is used to determine the level of the stone in the trenches.
Can you explain how that measurement was derived with the information in the as-built plan? Assuming the measurements by the inspection company are accurate, is it possible to rectify the situation without putting in an entirely new system? Thanks in advance
Like you I'm impressed that it sounds as if someone used X-ray vision or some other technique to determine the distance between the bottom of the septic system drainfield trenches and the seasonal high level of local ground water: at least that is what one should be measuring.
It is possible that your inspector is reporting data that was on the original or as-filed septic system plans for your home, in which case Kudos to the inspector for taking the trouble to go dig up local septic system records.
At the time that a septic system was originally designed and approved an engineer had test holes dug, percolation rates assessed, and the septic soakbed or drainfield plan would have (or should have) included an assertion that the design will meet your local requiements for the separation distance between bottom of the trenches and top of seasonal groundwater. Typically that's around 24". More in some areas.
Stone levels in trenches might also have been specified by the septic engineer.
Really? However in my OPINIOIN it is misleading (to put it politely) to suggest that those data were actually visually or physically inspected at the time of your septic system inspection and test unless the inspector actually did so.
And it's worth keeping in mind that "as filed" or "as planned" is often not exactly the same as "as built" since the plan can't necessariliy know that the excavator is going to hit a boulder, find higher ground water than records indicate, or that she or he took a short cut and left out most of the gravel (which after all costs something).
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Environment Agency, U.K., "Polution Prevention Guidelines", [PDF] Environment and Heritage Service, Scottish Environment Protection Agency (SEPA), Enviornment Agency, U.K., PPG4 (2006), retrieved 2 September 2015, Environment Agency www.environment-agency.gov.uk HEAD OFFICE Rio House Waterside Drive Aztec West Almondsbury Bristol BS32 4UD Tel: 01454 624 400 Fax: 01454 624 409, Scottish Environment Protection Agency www.sepa.org.uk CORPORATE OFFICE Erskine Court The Castle Business Park Stirling FK9 4TR Tel: 01786 457 700 Fax: 01786 446 885, Environment and Heritage Service www.ehsni.gov.uk HEAD OFFICE 17 Antrim Road Lisburn County Antrim BT28 3AL Tel: 028 9262 3100 Fax: 028 9267 6054, original source: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/290141/pmho0706bjgl-e-e.pdf
Environment Agency, U.K., "Disposal of Sewage Where No Mains Drainage is Available", - retrieved 2 Sept 2015, original source: http://www.doeni.gov.uk/niea/ppg04.pdf These notes are for guidance only. They are produced by the Environment Agency for England and Wales, the Scottish Environment Protection Agency and the Environment and Heritage Service in Northern Ireland, jointly referred to as the Agency or Agencies. Each site will be considered according to local circumstances, and early consultation with your local
Agency Office is advisable. Contact details will be found at the end of these guidelines.
U.K. Code of Practice for the Design of Small Sewage Treatment Works and Cesspools. BS6297:1983: British Standards Institute: Telephone 0181 996 7000
U.K.. Septic Tanks and Small Sewage Treatment Works, A Guide to Current Practice and Common Problems: 1993: CIRIA Technical Note 146: Construction Industry Research and Information Association: Telephone 0171 222 8891
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
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.
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.
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
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