BOD stands for
Answer: A
Biochemical oxygen demand measures the oxygen that microorganisms consume while degrading the organic matter in a sample, so it is an indirect measure of how much biodegradable material the water carries. It is the central number in judging both sewage strength and treatment performance.
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The standard BOD test is conducted for
Answer: B
Dissolved oxygen is measured before and after five days of incubation in the dark at 20 degrees, and the difference is reported as BOD5. Five days captures most but not all of the demand, which is why the figure is a convention rather than a complete measurement.
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A high BOD in an effluent is harmful to a receiving stream because it
Answer: C
Microorganisms degrading the organic load consume dissolved oxygen, and if the demand outruns reaeration the water goes anoxic and fish and invertebrates die. Limiting BOD in the discharge is therefore the primary purpose of secondary treatment.
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Chemical oxygen demand differs from biochemical oxygen demand in that it measures
Answer: D
COD uses a strong chemical oxidant and takes hours rather than days, and it includes material that microorganisms cannot degrade. It is therefore usually higher than the BOD of the same sample and is used where a quick result matters.
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Dissolved oxygen in a river falls immediately downstream of a sewage outfall and then recovers. This is
Answer: A
Microbial degradation of the organic load draws oxygen down, and as the load is consumed reaeration and photosynthesis restore it, producing the characteristic sag and recovery. The length of river affected is a measure of the discharge's strength.
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Preliminary treatment of sewage consists mainly of
Answer: B
Bar screens take out rags and large debris and grit channels drop out sand and gravel, protecting the pumps and later stages from damage and abrasion. No appreciable BOD is removed at this point; it is purely protective.
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Primary treatment of sewage is essentially a
Answer: C
Sewage is held in a tank long enough for settleable solids to fall out as sludge and grease to be skimmed from the surface. No microbial degradation is intended, which is why the stage is described as physical rather than biological.
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Primary sedimentation typically removes suspended solids in the range
Answer: D
Correct answer: 50 to 70 per cent
Primary sedimentation is simple settlement, so it removes what will settle in a couple of hours - about two thirds of the suspended solids and a third of the biochemical oxygen demand. The dissolved and colloidal fraction passes through, which is what the biological stage exists to treat.
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Secondary treatment of sewage is almost always
Answer: A
The point of the stage is to have microorganisms consume the dissolved and colloidal organic matter that settling cannot remove, whether in suspension as activated sludge or as a biofilm on media. Both routes rely on microbial metabolism.
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United States secondary treatment standards limit BOD5 and suspended solids to a monthly average of
Answer: B
The standard is 30 milligrams per litre as a monthly average and 45 as a weekly average, together with at least 85 per cent removal. Expressing it both as a concentration and as a percentage prevents a plant from meeting the limit merely by dilution.
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