Human: How does a septic system work? Assistant: 
A septic system is an underground sewage treatment system that uses bacteria to break down waste. The bacteria in the tank produce methane gas, which is then used as fuel for the home. The liquid waste is sent through pipes to the septic tank and then pumped out of the tank into a drain field. The drain field consists of a series of trenches filled with gravel or stone. As the waste flows through the drain field, the bacteria breaks it down further and the solids are filtered out. Finally, the treated water is allowed to percolate into the ground where it is absorbed by the soil.