1. Primary Treatment. It includes physical processing, such as sedimentation, floatation, shredding (fragmenting and filtering). These processes remove most of the large debris. The primary sludge obtained when all the solids settle down is sent for secondary treatment.

2. Secondary Treatment. It is a biological method in which micro-organisms are used.

Activated Sludge Method. Sewage, after primary treatment, is pumped into an aeration tank or oxidation pond where it is mixed with air and sludge containing bacteria. Bacteria forms flocs along with fungus . The flocs sediment and form activated sludge. A part of activated sludge is used as an inoculum and pumped back. The remaining sludge is pumped into anaerobic sludge digesters where bacteria decomposes the organic matter and releases a mixture of gases that forms biogas.