Septic tanks treat sewage at the place where it is located, rather than transporting the waste through a treatment plant or sewage system. Septic tanks are usually used to treat sewage from an individual building.

• Untreated sewage from a property flows into the septic tank and the solids are separated from the liquid.
• Solid material is separated depending on their density. Heavier particles settle at the bottom of the tank whereas lighter particles, such as soap scum, will form a layer at the top of the tank.
• Biological processes are used to help degrade the solid materials.
• The liquid then flows out of the tank into a land drainage system and the remaining solids are filtered out.