Answer:
Septic tanks use gravity to separate solid waste from water, and then use bacterial activity to break down the solids at the bottom (called sludge). When you flush a toilet or take a shower, the waste water flows into the septic tank. Once in the tank, the solids settle to the bottom, and the water (when it's relatively free of particulates) enters and outlet tube where it flows into a drain field. The sludge at the bottom of the tank decomposes as bacterial colonies break down the matter into simpler components until the tank is cleaned (usually once every 3 years).