Explain the different steps involved in sewage treatment before it can be released into natural water bodies.

Sewage is treated in sewage treatment plants (STPs) to make it less polluting. It is treated with the help of heterotrophic microbes naturally present in the sewage.

This treatment is carried out in two stages:-

I. Primary Treatment

It includes the physical removal of particles from the sewage through filtration and sedimentation.

These are removed in the following stages –

• Removal of floating debris by sequential filtration

• Soil and small pebbles by sedimentation

• All the solids that settle down form primary sludge and the supernatant forms the effluent.

• The effluent from primary settling tank is taken for secondary treatment.

II. Secondary Treatment

It is the biological treatment of the sewage. It takes place in the following steps –

• The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it which allows vigorous growth of useful aerobic microbes into flocs.

• While growing, the microbes consume major part of the organic matter in the effluent which reduces the BOD.

• The effluent is then passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.

• A small part of activated sludge is pumped back in to the aeration tank where it serves as the inoculums.

• The rest of it is pumped into large tanks called aerobic sludge digesters, where other kinds of aerobic bacteria digest the bacteria and the fungi in sludge and produce gases like methane, hydrogen sulphide and carbon dioxide.

Now the sewage is ready to be released into the water bodies.