Human: What is the difference between a septic tank and a sewage treatment plant? Assistant: 
A septic tank is a small, underground holding tank used to treat wastewater from a building. It is designed to separate solid waste from liquid waste in order to reduce the amount of water that needs to be treated by a sewage treatment plant. A sewage treatment plant is a large facility that treats wastewater from multiple buildings before releasing it into a river or other body of water.