For this reason, some homes require larger tanks than others. The size of the septic tank is determined by the possible occupancy of the house and other factors. A larger tank allows for more extensive decomposition and usually gives better service. Older septic tank installations often were buried completely, even though a clean-out hole with a cover was built into the top. There are many tanks of both designs in use.
Sewage flows through the house sewer to the tank.
The heavier solids settle to the bottom and decompose, forming a sludge. The lighter solids, greases, and fats rise to the surface of the liquid, forming a scum. The center layer of liquid is water containing dissolved and suspended sewage material. It flows from the tank through an outlet pipe to a tile field, deep trench, sand mound, or seepage pit. The opening to the outlet pipe is submerged in the liquid below the scum layer to prevent this matter from entering the pipe.