The Biomat

The biomat is a naturally occurring tar-like substance that forms on the bottoms and sides of the drainfield trenches. It is made up of living anaerobic (without oxygen) organisms, which feed on organic matter in the wastewater. As the biomat matures it grows thicker slowing down the flow of wastewater to the surrounding soil. As the wastewater passes through the biomat pathogenic organisms and viruses are removed. On the outside of the trench, beyond the biomat where the soil is not saturated, are living colonies of aerobic (with oxygen) bacteria. These aerobic bacteria colonies feed on the biomat and keep it from becoming so thick that wastewater will no longer pass through.

When soil floods these aerobic bacteria colonies will die off and no longer keep the biomat in check. The biomat will grow too thick and drainage will stop. Further, as these colonies die off they leave behind sulfides, which over time will clog soil passages stopping the flow of water. When the biomat grows too thick a waterproof barrier develops and absorption stops.