The biomat (biological mat) is a black, jelly-like mat that forms along the bottom and sidewalls of the drainfield trench. It is composed of anaerobic microorganisms (and their by-products) that anchor themselves to soil and rock particles. Their food is the organic matter in the septic tank effluent. Since the biomat has a low permeability, it slows down the rate of flow out of the trench into the drainfield soil and also serves as a filter to provide effluent treatment. With a well-developed biomat, wastewater maybe temporarily ponded in the drainfield trench, yet the soil a few inches outside the trench will be unsaturated.
The biomat forms first along the trench bottom and then up along trench walls. It has less permeability than fresh soil, so incoming effluent will pond over the biomat and trickle along the trench bottom to an area where there is little or no biomat; eventually the biomat will line the bottom of the trench and form up along the walls as well.
The Biomat Is a Living Filter
Through filtration and biological activity, the biomat is very effective at removing viruses and, in fact, filters out pathogenic bacteria and parasites.
Biomat in Equilibrium
In the trench, in a saturated environment, the anaerobic organisms in the biomat feed on the organic material in the wastewater; this causes the biomat to grow thicker and decreases permeability. On the soil side of the biomat, in an unsaturated environment, aerobic soil bacteria feed on and break down the biomat. In ideal conditions, these two processes go on at the same time, so the thickness and permeability of the biomat stay fairly consistent. Beautiful!
Since the biomat is alive, its equilibrium can be upset. Failure to regularly pump out the septic tank can result in an excess of organic material (food) to the biomat organisms, causing excessive growth and, therefore, reduced permeability In saturated soils, aerobic conditions no longer exist, and controlled breakdown of the biomat by aerobic soil bacteria will not occur.
If the septic system is poorly maintained, too much wastewater is flushed down the drain, or the drainfield soil remains saturated, the biomat eventually will grow too thick and dense, and the effluent sent to the drainfield will exceed the amount that can filter through the biomat. This can result in excessive ponding in the trenches, backflow into the septic tank (and possibly also into the house), or surfacing of effluent above ground over the drainfield—in other words “failure.”