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.
