BIOMAT EXPLAINED

Biomat on bottom & sides of leaching bed trench

Biomat found in soil sample from a soil auger

Biomat is a naturally occurring tar-like substance that forms on the bottoms and sides of the leaching bed trenches.  This layer is critical in the processing of fine biological solids and pathogens which are in the effluent, and without it the septic system would not be adequately treating the septic system effluent. Inadequately-treated effluent released into the ground risks contamination of nearby ponds, wells, streams, etc. A similar layer also forms around drywells used to accept greywater from buildings.

It is made up of living anerobic (without oxygen) organisms, which feed on organic matter in septic system wastewater.  As the biomat ages it grows thicker slowing down the flow of wastewater to the surround 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 colonies will die off and no  longer keep the biomat in check.  The biomat will grow too thick and drainage will stop.  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.