A Biomat is a layer of partially decomposed organic waste often found on the bottom and sides of an absorption area’s trenches. This anaerobic condition prevents decomposition of waste.
Biomats - Articles


A biomat is a black, slimy jelly-like, slowly permeable layer of partially decomposed organic waste containing microorganisms and their by-products that seek to feed and grow in an anaerobic environment. Biomats are often found on the bottom and sidewalls of the aggregate-soil interface of an absorption area's trenches or beds.

How and why biomats form?

Biomats form because the infiltrating soil surfaces, designed to receive wastewater and be maintained in an oxygen-rich (aerobic) condition, have become wet and oxygen starved (anaerobic). Biomats form when the wastewater loading to an area is:

  • hydraulically overloaded because it is receiving wastewater continually keeping the area wet, or
  • organically overloaded at a rate that does not permit the immediate environment to decompose the waste in a timely manner.

Hydraulic Overloading

The development of a biomat on the bottom and sidewalls of the aggregate-soil interface within the absorption area is usually caused by hydraulic overloading. When septic tank effluent is distributed to the absorption bed or trenches by gravity, most of the water is applied to the initial few feet of the area.

In other words, all of the effluent falls out of the first two or three holes in the distribution pipe and lands on the bed or trench bottom immediately below these holes. Because the bed or trenches were designed to receive this effluent uniformly, the very small area receiving the excessive volume of effluent becomes wet and eventually saturated.

In this overly wet environment the aerobic microorganisms (aerobes) can no longer live and give way to anaerobic microorganisms (anaerobes) that thrive in these wet, oxygen starved conditions. So, what causes the biomat? When the anaerobes take over because of the wet conditions, two things happen

  1. the rate at which the organic matter (BOD) is broken down is greatly reduced (aerobes convert organic matter to carbon dioxide and water much faster than anaerobic microorganisms), and
  2. the waste products generated by the anaerobes create a slimy, jelly-like, slowly permeable layer we call a biomat.

After the small area under the first few distribution pipe holes has developed a biomat, which will no longer permit the wastewater to infiltrate into the soil it rests on, the excess wastewater will naturally flow over the biomat to adjacent unclogged soil, see Figure 1. In time this once permeable soil will also develop a biomat. This process will be repeated over and over again until the entire aggregate-soil interface has a biomat on it. When this occurs, the wastewater will begin to pond in the beds or trenches and eventually backup into the septic tank and then your home. Note: pumping your septic tank will NOT correct this problem. The liquid ponded in the aggregate must also be pumped.

Figure 1. Biomat formation schematic

Organic Overload

Independent of the hydraulic loading, if the wastewater delivered to the absorption area is high in organic matter the soil environment may not be able to decompose the organic matter to carbon dioxide and water as fast as it is delivered to the site. When this occurs, the area will, again, become anaerobic and a biomat may develop. These slimy, jelly-like, slowly permeable layers are nearly impermeable and can cause the wastewater to begin ponding and backing up into the system.

Is a biomat helpful?

Some have argued that a biomat is necessary and helpful to the processes that treat the wastewater coming from a treatment tank. Not true. There is no treatment process that is enhanced by a biomat. Some have said that the biomat is needed to slow the intake of water into the soil so the water flow regime within the soil will be aerobic. Not true. The design loading rates prescribed to size an absorption area ensure aerobic conditions. No special help is needed. Some have argued that effluent ponding in an absorption area is a normal, expected occurrence. Not true. If there is water ponded in your bed or trenches for more than a few minutes following large water uses in the home, your system is in the process of failing.

How can the biomat be removed?

The key to removing a biomat is to change the wastewater loading to the biomat so the biomat can be exposed to oxygen (an aerobic environment). With the oxygen restored, the aerobic microorganisms begin to reestablish themselves and will quickly break down the biomat returning the area to its original permeable condition. Suggestion: One remedy that has been shown to be effective in controlling or removing a biomat from an absorption area is to have your septic tank pumped just before you and your family leaves for a one- to two-week summer vacation. The period of non-use during your vacation restores an aerobic environment within the absorption area, thus resulting in the biodegradation of any biomats that may have formed. You will most likely return to a functioning system.

How to prevent the development of a biomat

Make sure the biomat susceptible areas are kept aerobic. There are a growing number of wastewater treatment units available to homeowners that are designed to create and maintain aeration in the effluent leaving the treatment tank. Some of these units are experimental, others are proven technologies. They can make a difference.

For additional assistance contact:

your local Sewage Enforcement Officer or County Extension Agent

Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle pike #268
Mechanicsburg, PA 17050
Telephone: 717-761-8648

Pennsylvania Septage Management Association (PSMA)
Box 144
Bethlehem, PA 18016
Phone: 717-763-PSMA