The domestic sewage in large cities :
- has very high amounts of suspended solids and dissolved salts
- has a high BOD as it contains both aerobic and anaerobic bacteria.
- is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plant (STPs)
- When treated in STPs does not really require the aeration step as the sewage contains adequate oxygen
Answer
(3) is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plant (STPs)
Solution
As per NCERT, in secondary (biological) treatment the primary effluent is passed into aeration tanks, where aerobic microbes form flocs and consume organic matter, reducing BOD.
⇒ The activated sludge is then pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing biogas.
∴ Sewage is processed by aerobic and then anaerobic bacteria in secondary treatment.
(1) Sewage is mostly organic matter, not very high in dissolved salts; (2) high BOD is due to the large amount of organic matter, not because it contains bacteria; (4) the aeration step is essential.
Answer: (3)