Microbes in sewage treatment
Municipal waste-water which contains large amounts of organic matter is called sewage.
Before disposal, hence, sewage is treated in sewage treatment plants (STPs) by the heterotrophic microbes to make it less polluting.
Sewage treatment is carried out in two stages.
- Primary treatment –
- These treatment steps basically involve physical removal of large and small particles.
- Initially, floating debris is removed by sequential filtration and then the grit are removed by sedimentation.
- All solids that settle form the primary sludge, and the supernatant forms the effluent.
- The effluent from the primary settling tank is taken for secondary treatment.
- Secondary treatment or biological treatment-
- The primary effluent is passed into large aeration tanks where it is constantly agitated which allows vigorous growth of useful aerobic microbes into flocs.
- Flocs are the masses of bacteria associated with fungal filaments to form mesh like structures.
- While growing, the microbes significantly reduces the BOD (biochemical oxygen demand) which is the amount of oxygen required to oxidize total organic matter in the effluent.
- The BOD test measures the rate of uptake of oxygen by micro-organisms, the greater the BOD of waste water, more is its polluting potential.
- The effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment and the sediment is called activated sludge .
- A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
- The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters where other kinds of bacteria grow anaerobically which digest the bacteria and the fungi in the sludge.
- During digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide which form biogas .
- The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
Fig. sewage plant