Alternating bed septic systems with parallel or interspersed drainfields
What is an alternating bed septic system?
How Far Apart Are the Alternating Septic Bed Drainfield Trenches and Leach Lines?
How is Effluent Dispersal Switched Between Alternating Septic System Beds
How Often is Effluent Distribution Switched Between Alternating Septic Beds
Questions & Answers about septic dosing systems and alternating bed septic system designs
This article defines, illustrates, and explains the operation of alternate bed (or alternating bed) septic systems, a variation on dosing systems
for septic system effluent final treatment and disposal. We explain how alternating bed septic drainfields work, how the fields are switched, and how often they are alternated or switched.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.
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.
While alternating bed septic systems are not complicated and are discussed here as a septic design alternative, the concept - manually or automatically switching effluent disposal between two or more collections of drainfield trenches - is similar to septic dosing systems in concept. Readers of this article should also see GRAVITY/SIPHON DOSING SYSTEMS and also PRESSURE DOSING SYSTEMS. Page top image shows a sketch of a basic septic system design using gravity dosing, with effluent flowing from a septic tank to a dosing chamber and from
there to a drainfield. Image: Indiana state health department.
What is an alternating bed septic system?
Alternating bed septic system designs are intended to decrease hydraulic overloading of septic drainfield soils by switching effluent distribution between two or more sets of septic drainfield trenches.
The alternating drainfield areas may be uniformly interspersed (alternating individual septic drainfield lines) or they may be parallel in the same area, or they may even be in completely different physical areas on a property. The layout choice for alternating septic bed systems depends on the space available as well as soil characteristics.
In the U.S., some states such as Massachusetts, define alternating bed systems as a design intended to provide a backup septic capacity: "
Alternating Bed Systems - Also known as an alternating leachfield. An absorption system designed with a backup absorption field for use while the primary absorption field rests."
How Far Apart Are the Alternating Septic Bed Drainfield Trenches and Leach Lines?
The answer is, it depends: on
The amount of site space available
The percolation rate and other characteristics of the drainfield soils
State and local building and health codes
Common local practices: often traditional drainfield trenches were separated by 6' ( 7' in some areas) and septic drainfield trenches are usually limited to 100' in length. Some installers place alternative septic trenches between the original drainfield lines. particularly where an alternating bed septic design is being considered, this seems to us quite reasonable: excavation of a slow or failing septic drainfield trench and examination of it in cross section usually discloses that the soil clogging that is occurring is in the first few inches of soil around the drainfield trench perimeter.
How is Effluent Dispersal Switched Between Alternating Septic System Beds
Alternating drainfield designs are used most often on flat sites and in combination with a programmable or timed automatic distributing valve, but they may be operated manually.
Our friend and associate Victor Faggella reports a long-standing and traditional alternating septic bed design at which the property owner simply opened the distribution box for his drainfield and periodically switched a plug from one set of drainfield pipes to the other. (See SEPTIC D-BOX INSPECTION)
An example of a floating outlet design for dosing systems and that can be adapted to alternating bed septic system designs to handle dosing type septic system effluent disposal can be seen in animation at Rissy Plastics FLOUT floating outlet for septic effluent dispersal.
How Often is Effluent Distribution Switched Between Alternating Septic Beds
Sometimes installed as a retrofit where an existing septic drainfield is failing, the contractor may make a backup absorption field, adding the ability to route septic tank effluent to either field.
The "backup" septic drainfield is used while the primary field is rested and allowed to recover through biological activity. In this design septic drainfields or leaching beds are often alternated every 6 months. We recommend that the alternation dates be shifted so as to avoid always using the same field during the wettest months of the year.
Frequently-Asked Questions (FAQs) about septic dosing systems and alternating bed septic system designs
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.
Septic Tank Soil Absorption Systems - Decentralized Sysetms Technology Fact Sheet, US EPA, EPA 932-F-99-075, September 1999 - original citation www.epa.gov
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com
Victor Faggella, is a senior home inspector in New York and can be reached at Centurion Home Inspections, Inc. Mahopac, NY 10541. 845-628-0941 vjf@centurion-inspections.com The company has offices in Mahopac, NY, Woodbury CT., and Mansfield Center, CT.
Hankey and Brown home inspectors, Eden Prairie, MN, technical review by Roger Hankey, prior chairman, Standards Committee, American Society of Home Inspectors - ASHI. 952 829-0044 - hankeyandbrown.com
Rissy Plastics, 350 Cedar Lane, Torrington, CT 06790 USA, Tel: 877-221-4426, Email: tanxrus@aol.com, for information about the the Flout™ floating outlet valve dosing system control for septic systems and onsite wastewater disposal.
Construction Guidelines for Gravity and Flood-Dose Trench Onsite (Septic) Systems, Indiana state health department
Maintenance of Low Pressure Distribution Septic Systems, Vermont Cooperative Extension
Dosing Gravity Drainfield Systems, Recommended Standards and Guidance for Performance, Application, Design, and Operation & Maintenance, Washington
State Department of Health, July 1, 2007
Septic Effluent Dosing System Designs, Products and Suppliers
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.
Rissy Plastics, 350 Cedar Lane, Torrington, CT 06790 USA, Tel: 877-221-4426, Email: tanxrus@aol.com, for information about the the Flout™ floating outlet valve dosing system control for septic systems and onsite wastewater disposal.
Some basic information about handling septic effluent follows.
How and When Septic Effluent is Moved Through a Septic System
Septic effluent is distributed to a system final treatment and disposal using either gravity
methods (which depend on terrain slope) or pressure methods (which use a pump
to move effluent to its destination treatment and disposal area).
Methods For Septic Effluent Distribution Using Gravity Systems
Single Effluent Line: A 4" perforated PVC pipe receives effluent by gravity from the septic tank. The pipe is buried in a gravel trench and may be run in a straight line or a loop.
Distibution Box/Network of Lines: A distribution box receives effluent by gravity from the septic tank and routes it to a network of perforated pipes.
The network is made of multiple independent trenches which maybe on a flat or sloped site.
Serial relief line: multiple, serially connected trenches are built on a sloping site and used serially.
Drop box: multiple independent trenches are built on a sloping site, connected from drop boxes.
Gravity Dosing, Bell Siphon Dosing, Float Dosing (discussed in this document): 4" perforated pipe, with or without a
distribution box, are installed all at a single elevation.
A hinged "bucket" chamber receives effluent and periodically, as it fills, the bucket tips to spill effluent into
the piping system (A "dipping" or "tipping" system).
Bell siphon dosing systems (a bell and siphon
method of moving effluent to the drainfield) or float-controlled (a floating valve opens or closes) septic effluent dosing system designs are also
available and are discussed in this document. Gravity dosing systems distribute effluent periodically rather than continuously to the absorption field,
letting the field rest between doses and extending its life and capacity. However because the effluent dose is "poured"
suddenly into the drainfield, local spot or point overloading may still occur.
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). 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.
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.
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
Mobile View
