Three water samples – river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant – gave BOD values of 20 mg/L, 8 mg/L and 400 mg/L, but their labels were lost. Which sample is most likely the untreated sewage water, and why?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The 400 mg/L sample, because it has the most organic matter for micro-organisms to oxidise
BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. So a sample with more organic matter has a higher BOD, and the greater the BOD, the more polluting the water. Untreated sewage is full of organic matter, the secondary effluent has had most of it removed by microbes, and clean river water has the least. The highest value, 400 mg/L, must therefore be the untreated sewage.
Option (1) The 8 mg/L sample, because micro-organisms in it have already used up most of the oxygen — Incorrect. A low BOD of 8 mg/L means very little organic matter is left for micro-organisms to oxidise. This fits relatively clean river water, not untreated sewage.
Option (2) The 20 mg/L sample, because sewage is partly treated by the microbes naturally present in it — Incorrect. Sewage is treated by microbes only inside a treatment plant; the 20 mg/L value fits the secondary effluent, whose BOD has already been reduced there. Untreated sewage has not been through this step.
Option (3) The 400 mg/L sample, because it contains the most dissolved oxygen for micro-organisms — Incorrect. The number is right but the reason is wrong. BOD does not measure the oxygen dissolved in a sample; it reflects how much oxygen micro-organisms would use up while oxidising the organic matter present.
Option (4) The 400 mg/L sample, because it has the most organic matter for micro-organisms to oxidise — Correct. Untreated sewage carries the most organic matter, so micro-organisms would consume the most oxygen to oxidise it. This gives it the highest BOD, 400 mg/L, and the greatest polluting potential.
NCERT Extract:
Microbes in Sewage Treatment – “… BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.”