How Do Septic Systems Work? Wastewater leaves your home though a pipeline called the mainline, then enters the septic tank (primary receiver). The septic tank holds the waste for primary treatment where solids and liquids are separated by gravity. The heavy digested solids form a layer called "sludge" that accumulates at the bottom of the tank. The lighter materials, (fats, greases, and oils) form a "scum" layer that floats to the top of the tank. Natural bacteria generated by the solid waste partially decomposes the waste in the septic tank and reduces the amount of solid material by as much as 60 percent. Note however that the septic tank is only one part of your septic system. It is designed to remove solids from your wastewater as shown, prior to the wastewater entering the cesspool (drainage area). Solids and sludge should be pumped from the tank every 2 years (as recommended by the county health department) by a licensed septic contractor. This service frequency will prevent solid material overloading the septic tank and more importantly from entering and clogging the cesspool. Homeowners that routinely maintain the septic tank will thus avoid costly repairs to the cesspool drainage area commonly referred to as the overflow.