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The domestic sewage in large cities
A
Has very high amounts of suspended solids and dissolved salts
B
Has a high BOD as it contains both aerobic and anaerobic bacteria
C
Is processed by aerobic and then anaerobic bacteria in the secondary treatment in Sewage Treatment Plants (STPs)
D
When treated in STPs does not really require the aeration step as the sewage contains adequate oxygen
Detailed Solution
In secondary (biological) treatment, the primary effluent is passed into large aeration tanks where it is constantly agitated and air is pumped in; aerobic microbes grow into flocs and consume the organic matter, reducing the BOD.
The effluent then goes to a settling tank where the flocs sediment as activated sludge.
The major part of this sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge and produce biogas ($CH_4$, $H_2S$ and $CO_2$).
So the sewage is processed first by aerobic and then by anaerobic bacteria.
Domestic sewage is 99.9% water with only about 0.1% impurities, so it does not have very high amounts of suspended solids and salts.
Its high BOD is due to biodegradable organic matter, not because it contains both types of bacteria, and aeration is essential because sewage is poor in dissolved oxygen.
Hence the correct statement is that it is processed by aerobic and then anaerobic bacteria in the secondary treatment in STPs.
The effluent then goes to a settling tank where the flocs sediment as activated sludge.
The major part of this sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge and produce biogas ($CH_4$, $H_2S$ and $CO_2$).
So the sewage is processed first by aerobic and then by anaerobic bacteria.
Domestic sewage is 99.9% water with only about 0.1% impurities, so it does not have very high amounts of suspended solids and salts.
Its high BOD is due to biodegradable organic matter, not because it contains both types of bacteria, and aeration is essential because sewage is poor in dissolved oxygen.
Hence the correct statement is that it is processed by aerobic and then anaerobic bacteria in the secondary treatment in STPs.