Discussion
Which one of the following units of sewage treatment plant is used for removal of dissolved substances ?
Answer: A
Dissolved substances are removed in the Activated sludge unit.
Activated sludge is the biological (secondary) treatment step. An aerated tank holds a dense, mixed population of bacteria and protozoa that feed on the dissolved and colloidal organic matter in the sewage, converting it into carbon dioxide, water and new cells. That is how the soluble BOD, which no settling tank can catch, is removed.
The secondary clarifier only settles out the microbial flocs produced in the aeration tank. The grit chamber is a preliminary unit that removes sand and gravel. Sludge digestion treats the separated sludge, not the liquid flow.
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