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• Septic System Drainfield Absorption System Biomat Formation
• What causes septic drain field clogging and expensive drainfield repairs

This page 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.

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 document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems.

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WHAT's A "BIOMAT" - What is a Septic Drainfield "Biomat" Layer?

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 the 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.

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.

BIOMAT FORMATION PATTERN IN SOIL - 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.)

DRAINFIELD CLOGGING PROCESS - 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 Loading & Dye Test Procedure

EXTENDING DRAINFIELD 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 filtersprevent 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 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.

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Additives & Chemicals: Septic System Maintenance - Are they even needed? Are they even legal?
Don't Flush these things into a septic system
How Big Should the Septic Leach Field Be
Septic Tank Pumping Guide
Water Softeners and septic systems.

More Information on Building Diagnostic Inspections and Repairs

• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Septic System Information Website home page for this topic
• National Environmental Services Center, Pipeline magazine, 304-293-4191 - original source for this article.

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