This illustration shows the two basic working parts of a septic system.

Part 1. Septic Tank

Waste water flows from the house into the septic tank. Heavy solids (sludge) settle to the bottom of the tank. Light solids and grease (scum) float to the top of the tank, where they are held between two baffles that keep the inlet and outlet from clogging. An access cover at the top of the tank allows access from above. An observation pipe is positioned at the top of the tank above the outlet. The outlet allows partially treated waste water to move out of the tank to the leach field.

Part 2. Leach Field

The leach field is a system of perforated pipes laid out on a crushed stone bed, usually larger in size than the septic tank. Partially treated waste water is discharged from the tank through perforated pipes into an absorption field, where the water is further purified by filtration and decomposition by microorganisms in the soil.