Where a tank fails: solids that reach the drain field
A septic tank is a settling chamber. Heavy solids sink and build as sludge, grease and light material float and build as scum, and the liquid between them flows on to the drain field. Nothing about that is a problem until the two layers grow thick enough that solids start riding the liquid out of the tank.
That is the failure that costs real money. Sludge and scum that reach the drain field clog the soil the field depends on, and the first sign is usually a slow drain, a gurgle, or a backup in the lowest fixture in the house. Pumping on schedule keeps the solids in the tank, where they belong, instead of in the field.
