Sewage treatment includes following steps: 

(1) Preliminary Treatment:

• Screening: The larger suspended or floating objects are filtered and removed in screening chambers by passing the sewage through screens or net in the chambers.
• Grit Chamber : Filtered sewage is passed into series of grit chambers which contain large stones (pebbles) and brick-ballast. Coarse particles which settle down by gravity are removed.

(2) Primary treatment (physical treatment):

• The sewage water is pumped into the primary sedimentation tank where 50- 70% of the suspended solid or organic matter get sedimented and about 30- 40% (in number) of coliform organisms are removed. 
• The organic matter which is settled down is called primary sludge. 
• Primary sludge is removed by mechanically operated devices. 
• Dissolved organic matter and microorganisms in the supernatant (effluent) are then removed by the secondary treatment.

(3) Secondary treatment (biological treatment):

• The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. 
• The mesh like masses of aerobic bacteria, slime and fungal hyphae, known as floes are formed. 
• Aerobic microbes consume most of the organic matter and this reduces BOD (Biochemical Oxygen Demand) of the effluent.

(4) Tertiary treatment:

• Once the BOD is sufficiently reduced, waste water is passed into a settling tank where the floes are allowed to sediment.
• The sediment is called activated sludge. 
• Small part of activated sludge is transferred to aeration tank and the major part is pumped in to large anaerobic sludge digesters. 
• In these tanks, anaerobic bacteria grow and digest the bacteria and fungi in the sludge and gases like methane, hydrogen sulphide, CO₂ , etc. are released. 
• Effluents from these digesters are released in natural water bodies like rivers and streams after chlorination which kills pathogenic bacteria. 
• Digested sludge is then disposed.