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Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
- What is a septic biomat? - what is the septic drainfield "biomat" layer?
- How the biomat forms: how does the Biomat Form in a septic system effluent absorption system? Septic biomat constituents: what is the biomat made from? Septic biomat formation patterns in soil trenches How septic biomat formation, though necessary, clogs & ultimately can cause septic drain field clogging and expensive drainfield repairs Septic drainfield clogging process: how the soakaway bed or drainage trench fails: the biomat clogging process How to Extend the Life of A Septic Drainfield by Protecting the Biomat
- SEPTIC BIOMATS
- Questions & Answers about the septic biomat: what is it, how does it form, what does it do, how long does the biomat last?
- References
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
The septic system biomat: this article explains the formation of the biomat below septic drainfield trenches and around cesspools and some drywells and explains
what the biomat does. The biomat, a bacterial slime layer in the soil below the leachfield and around other wastewater disposal systems, is
a critical component of private septic systems - it is responsible for treatment and reduction of biological solids and pathogens in
septic wastewater effluent which is discharged into the soil from a septic tank. The most common and most expensive failure of private
systems occurs as soil clogging and failure of the absorption system to continue to accept water.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
What is a septic biomat? - what is the septic drainfield "biomat" layer?
Here we'll describe how the biomat
forms, how the soil eventually becomes clogged by a too-thick and too-extensive biomat, and how you can extend the life of the soil
absorption system by protecting the formation of the biomat. The origin of this text is the Winter 2005 issue of Pipeline, a publication of
the National Environmental Services Center, used with permission. [Edits and additions to the text are by the website author.]
This article is part of our series: Inspecting, Testing, & Maintaining Residential Septic Systems.
The biomat is a bacteria layer which forms in soil below and around drainfield trenches where septic effluent or wastewater
is discharged. This layer is critical in the processing of fine biological solids and pathogens which are in the effluent, and
without it the septic system would not be adequately treating the effluent. Inadequately-treated effluent released into the ground
risks contamination of nearby ponds, wells, streams, etc. A similar layer also forms around drywells used to accept graywater from
buildings.
Septic effluent, (or onsite wastewater), is discharged into a soil absorption system (or drainfield, seepage pit, or cesspool) from
the septic tank which should, if it's working properly, have retained all large solids. The job of the soil absorption system, or "SAS", is
to further treat the effluent to reduce the level of biological solids and pathogens to a level acceptable for further movement of
the liquid into remaining soils. Inadequate treatment of effluent would mean that sewage and pathogens would be discharged into
and contaminate nearby ground water.
How the biomat forms: how does the Biomat Form in a septic system effluent absorption system?
As the effluent is discharged into the SAS, bacterial growth develops beneath the distribution lines where they meet the gravel or
soil. This layer is known as the clogging mat, clogging zone, biocrust, and bioformat.
It's also referred to by some as the "slime layer" and it's
easily visible as a usually-gray slimy layer in the soil displayed if one excavates a cross-section of an absorption system trench.
This biomat (biological mat) is a black, jelly-like layer that forms along the bottom and sidewalls of the drainfield trench.
This clogging zone [eventually] reduces infiltration of wastewater into the [surrounding] soils.
Septic biomat constituents: what is the biomat made from?
The biomat is composed of anaerobic microorganisms (and their by-products) that anchor themselves to soil and rock particles.
"Anaerobic" refers to microorganisms which do not require high levels of oxygen, as opposed to "aerobic" organisms which do. Septic
waste and wastewater treatment involves both aerobic and anaerobic organisms. "Aerobic" bacteria require oxygen.
In either case but digesting different pathogens, the bacteria's food is the organic matter in the septic tank
effluent.
Less than one centimeter to several centimeters thick, the biomat acts as the actual site for effluent treatment.
Septic biomat formation patterns in soil trenches: the Biomat formation pattern in leach field trenches
The biomat forms first along the trench bottom near the perforations [in the drainfield piping which delivered the effluent along
the trench] where the effluent is discharged, and then up along the trench walls.
[As the bottom becomes clogged effluent rises in the
trench and seeps into the soil along the trench sides.]
The biomat-coated soil is less permeable than fresh soil, so incoming effluent
will move across the biomat and trickle along the trench bottom to an area where there is little or no biomat growth. (See growth pattern
in the diagram on this page.)
Septic drainfield clogging process: how the soakaway bed or drainage trench fails: the biomat clogging process
Biomats tend to restrict the flow of effluent through the drainfield, but are crucial because they filter out viruses and pathogens.
As the biomat develops, the soil infiltration rate decreases. Once the hydraulic loading rate exceeds the soil infiltration rate, ponding
starts. At some point wastewater will either back up into the home or break out onto the soil surface.
Septic loading and dye tests look for this "breakout" of effluent on the soil surface - a condition which will occur
when the biomat has become so thick that septic effluent no longer percolates through it to the soils below.
If you dig a neat cross section of a traditional leach field trench, and if it was properly constructed,
you'll see the perforated effluent pipe surrounded first by gravel, and then the sides of the trench as it was originally
cut through the soil. You will also see a 1cm (about 1/2") to 5cm (about 2") thick gray band around the perimeter of the trench - this is where the
soil clogging has occurred.
During the septic loading and dye test, an aggressive volume of water, but not exceeding reasonable septic system
design parameters, is run into the system, forcing effluent in a failed system to the surface (maybe).
The septic dye itself is a harmless but intense dye - it does not "make anything happen, but its
sole purpose is to permit the inspector to distinguish between breakout of (dyed) septic effluent during a septic test and other
site water which might be present, say from a spring or surface runoff.
More Reading:
SEPTIC DRAINFIELD FAILURE DIAGNOSIS
SEPTIC DRAINFIELD INSPECTION & TEST
SEPTIC DRAINFIELD LIFE
Septic Loading & Dye Test Procedure
Extend the biomat life - How to Extend the Life of A Septic Drainfield by Protecting the Biomat
Pump the septic tank: Biomat formation cannot and should not be prevented, but septic tank filters, proper organic loading, and proper maintenance of the
septic tank can slow the rate at which it forms [thus extending the life of the drainfield].
Septic tank filters prevent excess suspended solids
from flowing into the drainfield and can be retrofitted to existing systems.
Washing machine lint filters can reduce the movement of lint into the drainfield. (We've seen promotions for this product but not
any studies supporting the effectiveness of this measure.)
Septic tank outlet baffle filters are sold to reduce movement of fine particulates into the leach field.
Use of these filters will require a suitable access port and regular maintenance at the septic tank.
(We've seen promotions for this product but not
any studies supporting the effectiveness of this measure.)
Drywells to handle graywater can reduce the total liquid load on the leach field - a step often taken at sites with
limited drainage capacity.
Reduce unnecessary water usage, and in particular, be alert for plumbing fixtures that run continuously such as
toilets and leaky faucets.
WATER SOFTENERS & CONDITIONERS should be checked for proper settings for backwash frequency, volume, and salt dose.
Other maintenance that should be performed on the septic system includes having the system inspected and the tank pumped at
regular intervals. Pumping the tank allows it to better settle out solids [by maintaining a larger liquid volume or "net free area" in the
septic tank], also reducing the organic load to the drainfield.
Magic bullets which promise to restore drainfields are either unlikely to produce any lasting effect or
are at risk of contaminating the environment with toxic and perhaps caustic chemicals. These processes are prohibited
in some jurisdictions.
As we explain at SEPTIC DRAINFIELD LIFE
We have seen drain fields still working fine after 25 years and others which failed in the first week of
occupancy of a new home.
In a properly designed septic absorption field the level of usage of the system, site characteristics such
as slope, rock, groundwater level, and soil percolation rate have all been considered.
Barring
foul ups such as we discuss in this document, such a field may
last from 10 to 20 years. USDA sources assert that a properly operated and maintained ST/SAS (septic tank / soil absorption system) should last at least 20 years.
But it's easy to ruin or shorten the life of a drainfield/leaching bed. In fact the same USDA source states that
Studies reported at an Environmental Protection Agency seminar, Orlando FL, November 1979, show that over half [ST/SAS] fail prematurely due to improper operation or lack of adequate maintenance. Generally, these failures occur when the soil-absorption system [drainfield] becomes clogged. Preventable clogging, due to a buildup of solids in the [septic] system, is usually extensive enough to require expensive reconstruction of the system. Failures can also cause nearby ground areas, streams, lakes, and water supply systems to become contaminated. This exposed the public [and USDA, EPA, NPS, FPS, and other government employees] to health threats such as hepatitis, typhoid, diarrhea, and dysentery.
... [ in contrast] The unpreventable failure of the soil-absorption system eventually occurs when growth of the organic material in the wastewater [the biomat (SEPTIC BIOMATS) that forms under and along the sides of a drainfield trench] becomes so large [thick] that they plug up the soil.
Similar studies of advanced wastewater treatment systems such as aerobic systems, sand beds, mound systems similarly found that improper or inadequate operation and maintenance were the primary causes of premature failure of those systems as well.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- National Environmental Services Center, Pipeline magazine, 304-293-4191 - original source for this article.
- CHLORINE IN DRINKING WATER - Does Chlorine in Drinking Water Harm the Septic Tank? - Septic Tank Cleaning Advice
- CHLORINE IN SEPTIC WASTEWATER - Septic Tank Maintenance: Chlorine in Septic Systems - Septic Tank Maintenance Advice
- Percolation Testing Manual, CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf
- Soil Test Pit Preparation, fact sheet, Oregon DEQ Department of Environmental Quality, original source www.deq.state.or.us/wq/pubs/factsheets/onsite/testpitprep.pdf The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or see http://www.oregon.gov/DEQ/
- Thanks to reader Michael Roth for technical link editing 6/29/09.
- The Septic System Information Website home page for this topic
- Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
- The Home Buyer's Guide to Septic Systems
- Septic Tank Pumping Guide: When, Why, How to pump the septic tank
- Home & Outdoor Living Water Requirements
- 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.
- How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
- Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
- Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
- Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
- 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.)
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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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- Inspecting Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.
- 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
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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)
- Eco John® Innovative Toilet Solutions, Global Inventive Industries, Fountain Valley CA, PDF, product brochure
- "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. 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.
- 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 STANDARDS
- SEPTIC MAGAZINES
- 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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