You Say ‘Clogging Mat,’ I Say ‘Biomat’

There may seem to be little distinction between the two terms used to explain a failing drainfield system. But the differences are important to understanding proper treatment.

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QUESTION: I have been reading about the clogging mat and how bad that mat is for a septic system. In some of your columns, you’ve referred to the biomat. Is this the same thing? Can you explain its impact on septic systems?

ANSWER: I have written about the biomat before and its necessary function in the treatment of sewage tank effluent by the soil system. Perhaps a little refresher might be in order, since the question has been raised.

As effluent enters the soil treatment system it begins to percolate into the soil on the bottom of a trench or bed system. The soil surface exposed to the effluent filters out the fine organic solids. A layer begins to form and anaerobic bacteria begin to set up housekeeping in the layer. As the layer of sewage solids gets thicker, the flow of sewage into the soil slows.

This condition is extremely important and absolutely necessary for the adequate removal of all suspended solids, as well as bacteria and viruses. For such removal and adequate treatment to take place in the soil, only the finer capillary tubes must contain liquid.

Liquid moves downward through the fine capillary tubes, where the remainder of the fine solids are filtered out, and soil bacteria destroy the pathogens and viruses. The larger soil pores remain open and contain soil air, which has oxygen.

The layer of organic material is anaerobic on the top side and aerobic on the bottom side. The aerobic bacteria feed on the layer on the bottom side, reducing the thickness of the layer. The suspended solids contained in the sewage tank effluent builds up the layer on the top side.



If all goes as planned, the rate of reducing the thickness of the layer becomes the same as the rate of building up the thickness, and biologically the layer is in equilibrium. There will not be enough soil area in the bottom of the trench to treat all the sewage effluent flow when that layer is in equilibrium.

So the depth of the liquid in the trench increases and exposes the soil along the trench sidewalls to the sewage tank effluent. The same process of developing an organic layer at the soil surface takes place along the trench sides as it did in the bottom of the trench.

Onsite systems will have two or more trenches. When the first trench has reached its capacity to treat effluent, the extra effluent will flow through the drop box to the second trench in the system, where the treatment process again takes place.

When effluent flows through the first drop box and into the second trench, this does not mean the first trench is no longer treating effluent at its equilibrium rate. If no sewage flows for a week or two when the family is on vacation, the liquid level in the first trench will drop. Treated effluent is still flowing into the soil. This can be easily verified if each trench has an inspection well, which should have been installed in a proper system.

The soil along the sidewalls of a trench system is extremely important to a successful sewage treatment system. Not only is much more soil exposed to effluent in a trench system compared to a bed system, but the soil along the sidewalls contains much more oxygen than the soil under the bottom of the trench.

The reason is the top of the rock or distribution medium will be within 6 to 12 inches of the surface. The air in the soil pores near the surface contains more oxygen than the air under the bottom of the trench.

There is even less oxygen present in the soil air under the center of a seepage bed. The Onsite Code in Minnesota, and other states, specifies seepage beds shall not be wider than 10 feet. The lack of oxygen in the soil air under the bed is the reason for this requirement.



Field experience has shown wide beds fail because the organic layer builds up on one side, but there are not enough aerobic bacteria to break down the layer on the soil side. The rate of liquid flow into the soil is not as great as the system design allows. Yes, we do now have a clogging mat! But that mat is there because of improper design and installation.

Under the wrong conditions, what I call a biomat does become a clogging mat. However, in a properly designed and installed soil treatment system the biomat continues to serve as a biological layer, providing treatment of the sewage tank effluent as it flows into the surrounding soil.

A properly designed and installed trench soil absorption system should function for a long, long time. Many years ago when I was presenting the idea of drop boxes in my workshops in Minnesota, one contractor told me he had been using them for years and some of his systems were then 30 years old. What a nice testimonial to have at a workshop.

What I call the biomat is called a clogging mat by those who do not understand what is going on in the soil treatment area. Unfortunately, the words “clogging mat” give the implication that the mat will “clog” and stop the flow of liquid into the soil surrounding the soil treatment area.

While the biomat does partially “clog” or slow the flow of effluent into the soil, it does not totally “clog” or stop the flow of effluent into the soil.

An organic mat will develop in all but the very, very coarse soils. There would be no “clogging mat” in such soils, but there also would be no treatment of the sewage tank effluent. That is why Minnesota does not allow other than pressure distribution systems on coarse sands. With a pressure distribution system, the application rate must be low enough to provide adequate treatment of the sewage tank effluent.

Remember, a biological mat is needed for the proper treatment of the sewage tank effluent by the soil. The word “biomat” is obviously a shortening of the term “biological mat.”

My suggestion to onsite sewage professionals is: Get rid of the term “clogging mat,” which implies an onsite system is doomed to failure. This is not true.


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