Sewage treatment is undertaken principally to
Answer: A
Correct answer: protect public health and the receiving water
Everything else is secondary. Treatment exists to prevent disease transmission and to stop the receiving water being deoxygenated by the organic load. Gas, fertiliser and reclaimed water are useful by-products, and none of them would justify the works on its own.
Please log in or register to report this question.
Aerobic treatment processes require
Answer: B
Correct answer: a continuous supply of oxygen
The organisms oxidise the organic matter and need oxygen to do it, so aeration must continue whenever load is present. That is why aeration dominates the works’ energy consumption, and why a failure of the blowers turns an aerobic tank septic within hours.
Please log in or register to report this question.
Which one of the following is an anaerobic process?
Answer: C
Correct answer: Sludge digestion
Digestion is carried out in a sealed tank by anaerobic organisms, ending in methanogenesis. The other three all depend on oxygen: a trickling filter draws air through its media, a rotating contactor lifts its biofilm into the air, and activated sludge is aerated deliberately.
Please log in or register to report this question.
The final effluent from a well-run treatment works is
Answer: D
Correct answer: clear but not sterile
A well-run works produces an effluent low in solids and oxygen demand, but treatment reduces rather than eliminates the organisms - typically by two or three orders of magnitude. Where the receiving water is used for bathing or shellfish, a disinfection stage is added for that reason.
Please log in or register to report this question.
Grit is removed early in treatment chiefly to
Answer: A
Correct answer: protect pumps and pipework from abrasion
Grit is sand and similar dense inorganic matter, and it wears pumps, valves and pipe bends rapidly and accumulates in tanks. Removing it early is a mechanical protection rather than a treatment step - it contributes almost nothing to the oxygen demand and carries few pathogens.
Please log in or register to report this question.
Sludge is the term for
Answer: B
Correct answer: the settled solids separated during treatment
Sludge is what settles out, at whatever stage: primary sludge from the settlement tanks and secondary or surplus activated sludge from the biological process. Its treatment and disposal commonly account for half the cost of running a works, which is out of all proportion to its volume.
Please log in or register to report this question.
