Tertiary or advanced treatment is used to remove
Answer: C
Beyond secondary treatment, further steps take out nitrogen, phosphorus, residual solids and specific chemicals, using filtration, chemical precipitation or biological nutrient removal. It is applied where the receiving water is particularly sensitive.
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Disinfection of the final effluent is carried out to
Answer: D
Secondary treatment lowers the organic load but leaves substantial numbers of organisms, so chlorine, ultraviolet light or ozone is applied before discharge or reuse. It addresses the health risk rather than the oxygen demand.
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In the activated sludge process, wastewater is mixed with
Answer: A
The suspended community of bacteria and protozoa is kept aerated and is returned continuously from the secondary clarifier, so a dense active population meets each new volume of sewage. The recycling is what makes the process work at the speed it does.
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Aeration in the activated sludge process is provided in order to
Answer: B
The organisms oxidising the organic matter are aerobic, so air or pure oxygen must be blown or stirred in continuously. Aeration also keeps the sludge in suspension so that it stays in contact with the waste it is consuming.
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Return activated sludge is recycled from the
Answer: C
Sludge settling in the secondary clarifier is the active biomass, and returning it maintains the population in the aeration tank rather than washing it out with the effluent. Excess growth is drawn off as waste activated sludge for digestion.
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A trickling filter treats sewage by
Answer: D
Sewage is distributed over stones or plastic media carrying a slime layer of bacteria, fungi and protozoa, and organic matter is absorbed and oxidised as the liquid trickles down. Air moving through the bed supplies the oxygen, so it is an aerobic fixed-film process.
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A trickling filter can be expected to reduce BOD by about
Answer: A
At conventional loading rates the biofilm removes most of the organic load, which places the process firmly in the secondary category alongside activated sludge. The exact figure depends on the loading rate and the depth of the bed.
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The slime layer on trickling filter media is properly called a
Answer: B
A biofilm is a community of organisms attached to a surface within a matrix they secrete, and here it is the working part of the process. Floc is the analogous structure in activated sludge, where the community is suspended rather than attached.
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A rotating biological contactor works by
Answer: C
Closely spaced discs on a horizontal shaft dip through the sewage and then into the air as they turn, so the attached film is alternately fed and aerated. Like the trickling filter it is a fixed-film process needing little energy.
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An oxidation pond treats wastewater largely through the combined action of
Answer: D
Algae photosynthesise near the surface and release the oxygen the aerobic bacteria need, while those bacteria degrade the organic load and return carbon dioxide to the algae. It is a slow, land-hungry, very cheap process suited to warm climates.
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