Sewage treatment

BOD is the amount of oxygen needed by microorganisms to break down the organic matter in water.
A high BOD means a large amount of organic matter, i.e. the water is highly polluted.

In anaerobic sludge digesters bacteria digest the sludge and produce a mixture of gases: methane, hydrogen sulphide and carbon dioxide (biogas).

During anaerobic digestion of sludge, bacteria produce a mixture of methane, hydrogen sulphide and carbon dioxide (biogas).

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.

BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
It measures the organic matter present in sewage water: the greater the BOD, the greater the polluting potential.

Primary treatment is a physical process: floating debris is removed by filtration and grit by sedimentation.
In secondary treatment the primary effluent is passed into aeration tanks, where aerobic microbes grow vigorously into flocs (masses of bacteria associated with fungal filaments).
These microbes consume the major part of the organic matter in the effluent, which greatly reduces its BOD.
The sludge is then digested by anaerobic bacteria in anaerobic sludge digesters, producing biogas.
Since the work is done by microorganisms, secondary treatment is called biological treatment.
Hence secondary sewage treatment is mainly a biological process.