Question

The domestic sewage in large cities

• A. has high BOD as it contains both aerobic and anaerobic bacteria
• B. is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants (STPs)
• C. when treated in STPs does not really require the aeration step as the sewage contains adequate oxygen
• D. has very high amounts of suspended solids and dissolved salts

Answer 1

The correct option is B is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants (STPs)

In the sewage treatment plant (STPs) domestic sewage in large cities first undergo -

(i) Primary treatment where physical removal of particles takes place.
(ii) Then it undergoes secondary treatment, in which the effluent is passed into the aeration tank in which air is pumped in, which allows growth of aerobic microbes into flocs.
(iii) Next, BOD of the effluent is significantly reduced, the effluent is then passed into a settling tank where bacteria flocs are allowed to sediment, called as activated sludge.
(iv) 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 back into large tanks called anaerobic sludge digesters.