Go through the process of treating waste water

-sewage is screened by passing thru a mesh removing solids and grit -sewage is allowed to settle in large sedimentation tanks which produces a liquid effluent and a sludge that sinks to the bottom -the sludge gets digested by anaerobic bacteria -the liquid effluent contains a large amount of organic materials and harmful microorganisms so air is bubbled thru it to allow aerobic bacteria to multiply and digest the harmful matter