A septic tank is a device for treating feces and filtering out sediment. Its principle is that solid matter decomposes at the bottom of the tank, while the water-soluble substances rise to the top and flow out through pipes, preventing pipe blockages and allowing sufficient time for the solid matter (faeces and other waste) to hydrolyze. A septic tank refers to a small treatment structure that separates and sedimentates domestic wastewater and undergoes anaerobic digestion of sludge.

A septic tank is a fundamental sludge treatment facility and also serves as a pre-treatment facility for domestic wastewater, its function being:

1. Ensure environmental sanitation in residential communities, preventing the spread of domestic wastewater and pollutants in living environments.

2. Eliminate mosquito and fly eggs in the anaerobic digestion environment of septic tanks.

3. Temporary storage of sludge, anaerobic digestion for organic sludge, and matured organic sludge can be used as agricultural fertilizer.

4. Preliminary treatment of domestic wastewater, sedimentation of impurities, and hydrolysis of large molecular organic matter into smaller molecules such as acids and alcohols, improving the subsequent wastewater treatment.