The biological mat (biomat) is a black, jelly-like mat about one to two inches thick, that forms at the gravel-soil interface at the bottom and sidewalls of the drainfield trench. The biomat is composed of microorganisms (and their byproducts) that anchor themselves to soil and rock particles, and whose food is the organic matter in the septic tank effluent. Since the biomat has a low permeability, it serves as a valve to slow down and control the rate of flow out of the trench into the drainfield soil, and also serves as a filter to provide effluent treatment. Also known as a clogging mat.