First the water is screened by passing through a mesh which removes solids and grit.
The sewage is allowed to settle in large sedimentation tanks which produces heavy semi solid sludge and light liquid effulent.
The sludge is taken away and digested by anaerobic bacteria which produces biogas which can be burned for electricity and the digested sludge can be used as a fertiliser.
The effluent contains large amounts of organic molecules and harmful microorganisms.
Air is bubbled through the effluent so aerobic bacteria multiply and digest organic molecules and microorganism now the efuulent can be safely discharged into rivers.