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Waste Water Engineering

Q1: If the depletion of oxygen is found to be 2.5 mg/litre after incubating 2.5 ml of sewage diluted to 250 ml for 5 days at 20°C, B.O.D. of the sewage is

A 50 mg/l

B 100 mg/l

C 150 mg/l

D 200 mg/l

E 250 mg/l

ANS:E - 250 mg/l

The Biochemical Oxygen Demand (BOD) is determined by the amount of oxygen consumed by microorganisms while decomposing organic matter in a water sample over a specified incubation period. The formula to calculate BOD is: BOD=Depletion of oxygen×Dilution factorVolume of sampleBOD=Volume of sampleDepletion of oxygen×Dilution factor​ Given:

  • Depletion of oxygen = 2.5 mg/l (the amount of oxygen consumed)
  • Dilution factor = (Volume of diluted sample / Volume of original sample) = (250 ml / 2.5 ml) = 100 (the sample was diluted 100 times)
  • Volume of sample = 2.5 ml
Now, we can calculate the BOD: BOD=2.5 mg/l×1002.5 ml=250 mg/l2.5=100 mg/lBOD=2.5ml2.5mg/l×100​=2.5250mg/l​=100mg/l Therefore, the correct BOD of the sewage sample is 100 mg/l.



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