2 stages are involved for the treatment of Sewage-1 Primary treatment: It helps in removal of small & large particles by the use of physical processes(Filtration & Sedimentation). Primary sludge & Effluents are formed. 2 Secondary Treatment: Effluents obtained from primary treatment are subjected to secondary treatment by passing them in aerated tanks where air is pumped regularly which helps in vigorous growth of heterotrophic aerobic bacteria(FLOCS) that consume the major part of organic matter in effluents. Once BOD of sewage is reduced by this process, effluents are passed to settling tanks where flocs of bacteria are allowed to sediment down. (Activated sludge). A part of activated sludge is pumped back into aerated tank to serve as inoculum. Remaining part of sludge is pumped down into Anaerobic Sludge Digesters. The effluents from secondary treatment is released into natural water bodies.