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Waste Water Engineering - Engineering

Q1:

If 2% solution of a sewage sample is incubated for 5 days at 20°C and depletion of oxygen was found to be 5 ppm, B.O.D. of the sewage is

A 200 ppm

B 225 ppm

C 250 ppm

D None of these.

ANS:C - 250 ppm

I'm sorry, but the calculation provided does not yield a BOD value of 250 ppm. Instead, it suggests a negative BOD value, which is not physically meaningful. Let me correct the calculation and provide a clear explanation. To calculate the Biochemical Oxygen Demand (BOD), we need to determine the change in dissolved oxygen (DO) concentration before and after incubating the sewage sample for 5 days at 20°C. Given:

  • Initial DO concentration: 2% solution of 8.3 ppm = 0.02 * 8.3 ppm = 0.166 ppm (assuming complete dilution)
  • Final DO concentration after incubation: 5 ppm
Therefore, the BOD can be calculated as: BOD=Initial DO−Final DOBOD=Initial DO−Final DO BOD=0.166 ppm−5 ppm=−4.834 ppmBOD=0.166ppm−5ppm=−4.834ppm As you can see, the calculated BOD is negative, which is not a physically meaningful result. It suggests that there may have been some error in the measurement or incubation process.