Sewage treatment is carried out in two stages: 

(i) Primary treatment or physical treatment

• It is the physical removal of large and small particles from sewage.
• First, the floating debris is removed by sequential filtration by passing through wire mesh screens.
• Then, the grit (soil and small pebbles) are removed by sedimentation in settling tanks. The sediment is called primary sludge and the supernatant is the effluent.
• The effluent is taken for secondary treatment. 

(ii) Secondary treatment or biological treatment 

• Primary effluent is passed into large aeration tanks with constant mechanical agitation and air supply.
• Useful aerobic microbes grow rapidly and form flocs. 
• The growing microbes consume organic matter and thus reduce the biochemical oxygen demand (BOD). 
• When BOD of sewage has reduced, the effluent is passed into settling tank.
• Here, the bacterial flocs settle and the sediment is called activated sludge.
• A small part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.
• In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as methane, hydrogen sulphide and CO₂ which form the biogas. 
• The effluent can now be released into the water body.