5.3 MICROBES IN SEWAGE TREATMENT:

Large quantity of waste water is generated every day in cities and towns.

This waste water is also called sewage.

Sewage water usually contains high levels of organic matter, human excreta and domestic wastes and microbes.

Microbes can be pathogenic. It is essential to remove organic matter from the waste water before it is made available for human use.

It cannot be discharged into natural water bodies like river directly.

It must be treated in sewage treatment plants (STPs).

Sewage treatment includes three basic processes namely,

- primary treatment,

- secondary treatment

- tertiary treatment.

Primary Treatment

The primary treatment is a physical process.

It is removal of large pieces of floating debris, oily substances, etc. through filtration and sedimentation.

Secondary Treatment

The primary effluent is passed into the large aeration tanks.

Here it is constantly agitated mechanically and air is pumped into it.

Due to this vigourous growth of useful aerobic microbes into floes takes place.

Floes are the masses of bacteria associated with fungal filaments to form mesh like structures.

These microbes consume the major part of the organic matter in the effluent as they grow.

Due to this BOD (Biological Oxygen Demand) of the effluent is significantly reduced.

Tertiary Treatment

Once the BOD of waste water is reduced, it is passed into a settling tank.

Here the bacterial floes are allowed to sediment. The sediment is called activated sludge.

Small part of this is passed back into aeration tank and the major part is pumped into large tanks called anaerobic sludge digesters.

In these tanks anaerobic bacteria grow and digest the bacteria and fungi in the sludge.

During this digestion gases such as methane, hydrogen sulphide, CO₂ are produced.

Effluents from these plants are released in natural water bodies like rivers and streams.