Septik Tank

For sanitary waste water.
It is the underground sedimentation tank inside which sewage is retained for a short period while it is decomposed and purified by bacterial action. The organic matter in the sewage settles to the bottom of the tank; a film forms excluding atmospheric oxygen and anaerobic bacteria attack the solid matter, causing it to disintegrate, liquefy, and give off gases. The gases are discharged via a vent and the liquids overflow through an outlet and into a disposal field where they can reach the sewer pipe.