Biomat is the black, slimy dirt found over a septic system. (See the photo above.) It is a byproduct of anaerobic bacteria and it is the culprit that gives off that pungent septic system smell. According to inspectapedia.com, bio-mat "is a bacteria layer which forms in soil below and around drain field trenches where septic effluent or wastewater is discharged. This layer is critical in the processing of fine biological [...]