The domestic sewage in large cities
(a) Has a high BOD as it contains both aerobic and 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 creation step as the sewage contains adequate oxygen.
(d) Has very high amounts of suspended solids and dissolved salts

Hint: Domestic sewage has two main fluxes: one is grey water which is from kitchen sinks, wash basins, laundry washing, showers, baths etc., and the second one is black water which is from toilets and urinals.

Complete step by step answer:
Let’s discuss the options and find out the correct answer
- BOD can be defined as a measure of the amount of oxygen needed to remove waste organic matter from water in the process of decomposition by aerobic bacteria (those bacteria that live only in an environment containing oxygen.
So (a) is an incorrect option.
- Sewage is contaminated water having food residue, animals and human excreta, detergents, discharges from commercial and industrial establishments. The domestic sewage is handled first by aerobic and then by anaerobic bacteria in Secondary Treatment Plants (STPs).
So, the correct answer is (b).
- When treated in STPs, it does not really require the aeration step as the sewage contains adequate oxygen and has very high amounts of suspended solids and dissolved salts. So (c) is an incorrect option.
- The biological procedure is then followed by additional settling tanks to withdraw more of the suspended solids. In this method, waste water is first treated with microbes aerobically and anaerobically in anaerobic digester tank.
So (d) is an incorrect option.

Note: Sewer system consists of a network of pipelines to carry sewage. Sewage from our houses leaves through the drain pipes and reaches the interconnected network of sewer mains of the city. The sewage is then passed onto the wastewater treatment plants through these large underground pipes. In order for the waste to flow easily, the sewer system has been given a slope. As the sewer pipes are underground, manholes are arranged at desired places so that persons can enter the sewers through the manhole and fix the blocked or leaky pipes. Large cities have centralized sewage systems, complete with underground pipes, pumping stations, and treatment plants.