It all starts with the septic
tank, which serves several functions. Most important, it retains
the solid waste. The heavy solids from the waste settle to the
bottom, forming “sludge” and the oils and light
solids rise to the top, forming “scum” or a “scum
mat.” By separating the solids, the tank allows only the
liquid in between the sludge and scum, known as effluent, to
pass through to the absorption area.