Question

The domestic sewage in large cities
A. Is processed by aerobic and then anaerobic bacteria in the secondary treatment in sewage treatment plants (STPs).
B. When treated in STPs does not really require the aeration step as the sewage contains adequate oxygen.
C. Has very high amounts of suspended solids and dissolved salts.
D. Has a high BOD as it contains both aerobic and anaerobic bacteria.
E. Is processed by anaerobic bacteria in the primary treatment and by aerobic bacteria in the secondary treatment in sewage treatment plants (STPs).

Answer 1

Hint: Domestic or municipal wastewater is known as sewage which is produced by a community of people. According to the rate of flow, physical condition, toxic and chemical constituents and the status of bacteria, the sewage is classified.

Complete answer:
The city sewage is a wastewater which includes food residue, human and animal excreta and waste matter from the household and industrial establishments. The sewage of large cities is processed by aerobic and anaerobic bacteria in the secondary treatment of sewage in sewage treatment plants. The treatment of sewage water is done in order to reuse it for various purposes. The treatment of water includes:
Primary treatment – The treatment utilizes the settling tanks in which the sewage water is stored on upward and horizontal tanks. The retention time of 15-45 minute is taken in water flow to suspend 70% of solids either settle to the bottom of the tank or flow as a scum on the top of the tank.
Secondary treatment- In this phase of the treatment the digestion of the organic waste dissolved in waste water is done by aerobic or anaerobic processes.
Tertiary treatment: In this phase the suspended matter is removed and reduces BOD further. This process is used as physical or chemical.
Sludge treatment: Sludge is the settled organic solid waste of water recovered during the whole treatment process. It is treated an-aerobically to make it safe before disposal of the wastes.

Hence, the correct option is A. Is processed by aerobic and then anaerobic bacteria in the secondary treatment in sewage treatment plants (STPs).

Note: Raw sewage is also disposed into the rivers, streams or other water bodies which causes serious pollution by receiving such water. These dumps of sewage lead to pollution of water bodies and also lead to death of organisms in these water bodies. There should be treatment of wastewater before dumping it in the water bodies.