Human: How does a septic system work? Assistant: 
A septic tank is an underground storage unit for wastewater. It is usually made of concrete and has a lid that can be opened to access the interior. The wastewater is allowed to sit in the tank, allowing solids to settle at the bottom. This process creates a sludge layer which needs to be periodically removed from the tank. The liquid portion of the wastewater then flows into a drain field, where it is further treated by soil bacteria before being released back into the environment.