CLICK for LARGER IMAGE - This illustration shows the stages in soil clogging of a septic
system drain field as the biomat grows from the low-end, up the length and up the sides of the trench. Septic Biomat: defined, properties
Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs

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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.

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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.]

Article Contents:

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

CLICK for LARGER IMAGE - This illustration shows the stages in soil clogging of a septic
system drain field as the biomat grows from the low-end, up the length and up the sides of the trench.

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.)

Page Topics:

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.

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.


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.

Reader Question: What is the effect of epsom salts on the septic tank or the biomat: Counteracting the effect of epsom salts in the septic system

2015/10/22 Catherine Holmes Clark said:

I have a severe medical condition which requires me to use 4-5 cups of Epsom Salts (magnesium sulfate) a day in a bath. Also to do a lot of laundry. I can't get along without these strategies. My new septic system has had an excessive amount of black gunk in the filter the first two years it has been changed. Are my medical strategies causing this? How can I best counteract the effect?

I should have said that after the first year's filter change, I have religiously been using a tie-on filter on my washing machine's exhaust hose. But there is still too much gunk.
I've also been thinking about the fact I use hydrogen peroxide for cleaning surfaces, wondering if this is a problem.

Reply: A diagnostic test & two fixes for rapid sludge formation in the septic tank

First the easy question: a washing machine lint filter won't reduce the level of epsom salts nor affect black goop formation in the septic field; rather it avoids clogging the system with lint.

I'm not sure what the black gunk is, how "excessive" was determined, nor where that gunk was originating. It is normal for septic filters to require more frequent changing than every two years - that may be the root problem. Tell us the septic filter type, manufacturer, and where you live and perhaps we can get an opinion from the filter manufacturer.

Your usage is more than a typical homeowner's bath soak of epsom salts: a typical epsom salt bath soak (recommended by uses two cups of epsom salt in warm water in a normal sized bath tub (about 40 gallons of water), three times a week, soaking for at least 12 minutes. You are using 2-3 times that dose volume.

I agree that your question is very sensible. Epsom salts can include sulfates, calcium and magnesium; the sulfates might encourage growth of a sulphur-loving bacteria. (See my citations below).

I've read an "builder" opinion that unless you dumped 100 times epsom salts into the septic (500 cups a day) as much as you cite (5 cups a day) there'd be no impact on the septic system, but Id did not see any authoritative source supporting that view. Nevertheless it's not likely that normal household use of epsom salts harms a septic system.

A diagnostic test that would be very helpful would cost you very little

I suspect that we'll find that the total salt concentration in your diluted total wastewater daily discharge will be so low as to be harmless. Certainly Cobleigh found no evidence of a concern (Cobleigh 1911).

The following fixes provide a separate combination of actions that both avoid discharging salty bathwater into the septic (even though it's probably ok) and observing the effect of a change in discharge on the septic tank sludge formation rate

  1. A "fix" that might both prove the theory (that epsom salts or the sulphur therein are contributing to slime formation in the septic filter system) would be to route your tub drain to a separate graywater system. Some gardeners even recommend use of epsom salts to soil around rose bushes.
  2. A more evidence-based opinion about the effect of epsom salts on your particular septic tank could be obtained by measuring the sludge and scum layer thickness annually for a few years; if nothing else is changed except a re-routing of the epsom-salt-soak bathtub drainage and if you see a difference in the sludge thickness formation rate that woudl be quite credible.

See REDUCE IMPACT of SOFTENER on SEPTIC - for more about reducing the effect of other salts on the septic system.

Citations on research describing effects of epsom salts on water and on septic systems


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