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Waste Water Engineering - Engineering

Q1:

3.0 ml of raw sewage is diluted to 300 ml. The D.O. concentration of the diluted sample at the beginning of the test was 8 mg/l. After 5 day-incubation at 20°C, the DO. concentration was 5 mg/l. The BOD of raw sewerage is

A 100 mg/l

B 200 mg/l

C 300 mg/l

D 400 mg/l

ANS:C - 300 mg/l

ChatGPT ChatGPT To calculate the Biochemical Oxygen Demand (BOD) of the raw sewage, we can use the formula: BOD=Initial DO concentration−Final DO concentration /Dilution factor Given:

  • Initial DO concentration (DOᵢ) = 8 mg/l
  • Final DO concentration (DOₑ) = 5 mg/l
  • Dilution factor =Volume of diluted sample /  Volume of raw sewage
  • =300 ml/3 ml
  • =100
Now, let's plug these values into the formula: BOD=8−5 /100​ BOD=3 /100​ BOD=0.03 mg/l  However, BOD is usually expressed in terms of milligrams of oxygen consumed per liter of sample, so we need to multiply this value by 1000: BOD=0.03×1000=30 mg/l      So, the Biochemical Oxygen Demand (BOD) of the raw sewage is 30 mg/l. Therefore, none of the provided options (100 mg/l, 200 mg/l, 300 mg/l, or 400 mg/l) are correct.