The domestic sewage in large cities:
Has a high BOD as it contains both aerobic and bacteria
Is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants (STPs)
When treated in STPs does not really require the aeration step as the sewage contains adequate oxygen
Has very high amounts of suspended solids and dissolved salts

Secondary wastewater treatment processes use microorganisms to biologically remove contaminants from water. This can be aerobic or anaerobic, each process utilising a different type of bacterial community. Aerobic biological processes are common in municipal waste water treatment. In the aerobic system, the organic contaminants are converted to carbon dioxide, water, additional microorganisms, and other end products. Anaerobic processes convert organic contaminants to a biofuel gas comprising carbon dioxide, methane and other end products. It is used to treat high strength waste water where it is impractical to utilise aerobic processes or where producing a biofuel is desired. These processes are done by using the aerobic and anaerobic bacteria. So, the correct answer is option B.