Human: I've always wondered: What is the difference between a septic tank and a sewage treatment plant? Assistant: 
A septic tank is a small, underground tank that collects waste from a home. The tank is usually made of concrete or plastic and has a lid to keep out animals. The tank is connected to a drain field, which is a network of pipes that carry the waste away from the house. The drain field is usually made of gravel or stone.