Anaerobic digestion of sludge produces a gas consisting largely of
Answer: A
Sludge is stabilised in a sealed heated tank where fermentative bacteria and methanogenic archaea convert the organic solids to biogas, chiefly methane with carbon dioxide. The gas is commonly burned to heat the digester and generate power.
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The organisms that produce methane in a sludge digester are
Answer: B
Methanogens belong to the Archaea, are strictly anaerobic, and complete the digestion by converting acetate, hydrogen and carbon dioxide to methane. Their sensitivity to oxygen and to pH is why digesters must be sealed and carefully controlled.
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Anaerobic digestion is chosen for sludge rather than aerobic treatment mainly because it
Answer: C
Correct answer: needs no aeration and yields usable gas
Aeration is the largest energy cost at a works, and anaerobic digestion avoids it entirely while producing methane that can run the plant. The trade is that digestion is slow - twenty to thirty days - and needs heating and mixing, and it reduces rather than eliminates the pathogens.
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A septic tank treats domestic sewage by
Answer: D
Solids settle and are slowly digested without oxygen in the sealed tank, while the clarified liquid passes to a soakaway or drainage field where aerobic soil organisms complete the treatment. Accumulated sludge must be pumped out periodically.
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Water is tested for coliforms rather than for pathogens directly because coliforms are
Answer: A
Pathogens are shed intermittently, are often present in small numbers and each needs its own method, so testing for all of them is impractical. An indicator that is abundant whenever faecal matter is present and easy to culture answers the safety question far more reliably.
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The maximum contaminant level goal for total coliforms in drinking water is
Answer: B
The goal is complete absence in a 100 millilitre sample, and any coliform-positive sample must then be examined for faecal coliforms or E. coli. Setting the goal at zero reflects that no level of faecal contamination is regarded as acceptable in drinking water.
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Escherichia coli is regarded as the ideal faecal indicator because
Answer: C
Because it lives in the gut and dies off quickly elsewhere, finding it means recent faecal contamination rather than growth in the environment. Other coliforms occur in soil, on vegetation and as biofilms in pipes, so they are less specific.
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Total coliforms are a less specific indicator than E. coli because they also occur in
Answer: D
Members of the group are widespread in nature, so a positive total coliform result may reflect soil washings or regrowth in the mains rather than sewage. It remains the standard screening test, with positives followed up specifically for faecal organisms.
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The multiple-tube fermentation procedure gives a result expressed as the
Answer: A
Replicate tubes are inoculated with measured volumes, the pattern of positive tubes is recorded, and statistical tables convert it into the most probable number per 100 millilitres. It is an estimate with confidence limits rather than a direct count.
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The membrane filter technique for water bacteriology involves
Answer: B
Bacteria retained on the membrane grow into countable colonies when the filter is laid on a selective medium, giving a direct count from a known volume. It is quicker and more precise than the tube method, which is why most laboratories prefer it.
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