Biomat Basics

Watch for waterlogged trench conditions that will hinder proper onsite sewage system performance

Question:

I believe that there are two mats in a sewage system; one is a clogging mat and there is an underlying biomat. A saturated clog mat cannot provide treatment as it has no oxygen. A clog mat will result in a more evenly spread effluent through your trench and, therefore, a more even flow through your actual biomat. In sandy soils there will be sufficient air movement to prevent the clog mat from plugging. This won’t happen in finer textured soils. Also, I don’t think that the Long Term Acceptance Rate is a treatment rate. I also think that the LTAR will become lower and lower and the system will fail.

Answer:

I have observed excavated soil treatment systems and the biological mat. There is a single mat layer, which forms on the soil surface of a trench. I cannot accept your idea that there is a clogging mat and an “underlying biomat” because there simply are not two mats.

According to research, the biomat (biological mat) develops because the soil filters the fine suspended particles from sewage tank effluent. I am not aware of any research indicating the biomat is two separate layers.

As effluent flows into a trench, it moves rapidly downward through the distribution media such as trench rock. Where the distribution media meets the soil, the soil filters out the suspended solids and the mat begins to form.

As the mat thickens near the head end of the trench, the flow of liquid through the mat slows down. The effluent then moves farther along the trench and the mat layer expands until the entire trench bottom has a biological mat. Effluent begins to pond in the entire length of the trench. As the effluent level in the trench rises, the biomat also forms along the trench sidewalls as effluent percolates outward into the soil.

ESTABLISHING THE LTAR

As the biomat develops and liquid begins to pond on the biomat, it is always oxygen deficient or anaerobic on the trench side. With proper soil texture and no internal water-logged conditions in the soil, the mat will be aerobic on the soil side. But the soil must have adequate porosity and large enough pore spaces, or voids, so that oxygen is available in the soil air.

The oxygen in the soil pores decomposes the organic matter on the bottom of the mat to decrease its thickness. The organic solids in the system effluent accumulate on the trench side of the mat to increase the thickness of the anaerobic mat.

The biomat on one side is increasing in thickness and on the other side it is being decomposed. Eventually, this increase and decrease in biomat thickness comes into equilibrium. The LTAR is thus established for that soil.

Liquid continues to flow through the biomat at this constant rate, which is well below the infiltration rate of the original soil. This LTAR used to size the soil treatment system. I agree, the LTAR is not a treatment rate, but it is fundamental to the removal of pathogens from the sewage tank effluent.

You stated you believe that “the Long Term Acceptance Rate will become less and less and the system will fail.” Yes, this indeed may happen if the biomat becomes thicker. And why would the biomat thicken? Because the solids built up faster on the trench side than being decomposed on the soil side. And why were the solids building up faster? The septic tank was not being serviced and cleaned as needed. For the LTAR to remain constant, the effluent quality has to be relatively constant. A major portion of onsite system “failures” can be traced to sewage tank failures sending excess amounts of sewage solids into the soil treatment system.

The biomat will be thicker on fine textured soils than on coarse textured soils. If the biomat is thicker, the LTAR will be smaller. What is the reason for this? In a fine textured soil, the pores are smaller, and the soil has less ability to exchange oxygen from the air. Less oxygen is available to decompose the biomat on the soil side. A fine-textured soil must not be saturated (with water) and the soil pores must contain soil air with oxygen.

DON’T FORGET PATHOGEN REMOVAL

So far I have written about only the LTAR and how it is established. A separate function of the biomat is pathogen removal. I agree the “saturated clog mat” you refer to cannot remove pathogens. Pathogen removal does not take place in the trench, but in the soil outside the trench.

Pathogen removal takes place in the soil because the biomat restricts the rate of flow of liquid into the soil. This causes partially saturated flow of liquid through the soil profile. Because of their greater capillary attraction, only the fine soil pores contain liquid. The larger soil pores contain soil air with oxygen. The liquid moves slowly downward in the capillary pores of the soil. Soil bacteria are active in the aerobic soil pores and destroy or inactivate the pathogens. The pathogens are not filtered out by the biomat, but are destroyed in the soil profile when partially saturated flow takes place.

It is important to recognize partially saturated flow in the soil is necessary for pathogen removal. It is also important to realize oxygen is necessary for the soil bacteria to be active. A soil with little or no oxygen will soon become waterlogged and anaerobic conditions will prevail. The soil will be gray in color. Such soils are not suitable for the installation of a trench or bed system. An alternative sewage treatment system, such as a mound, is needed for such a soil profile.

Fine textured soils have smaller void spaces than coarser textured soils. The oxygen transfer is slower in fine textured soils. This is the reason there should be a limit on the width of seepage beds and a limit on the width of sewage treatment mounds. The soil under the middle of a very wide seepage bed in a fine textured soil has little or no oxygen exchange and soon becomes anaerobic. When this happens, the biomat no longer breaks down adequately on the soil side. The biomat thickens, restricting flow, and the system fails.



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