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The domestic sewage in large cities:

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

Correct answer: is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants (STPs).

Solution

In secondary treatment, the aeration tank promotes aerobic microbial growth that breaks down organic matter. The settled sludge is then further decomposed by anaerobic bacteria in the digester, so the process involves both types of microbes in sequence.

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