The following data pertain to a sewage sample at a point source. Initial dissolved oxygen (DO) = 12 mg/l; Final DO = 4 mg/l; Dilution of sewage was done to 2%. The Biochemical oxygen demand (BOD) of the given sample sewage is :

The following data pertain to a sewage sample at a point source. Initial dissolved oxygen (DO) = 12 mg/l; Final DO = 4 mg/l; Dilution of sewage was done to 2%. The Biochemical oxygen demand (BOD) of the given sample sewage is :

Right Answer is:

400 mg/l

SOLUTION

We know

BOD = (Initial DO – Final DO) × Dilution Factor

Where

Dilution factor = (volume of diluted sample)/(volume of undiluted sewage sample)

Initial DO = 12 mg/l

Final DO = 2 mg/l

2% diluted sample means: 2 ml of sewage is diluted to make 100 ml of test sample; and hence dilution factor would be 100, as 2ml has been diluted by 100 times to make 100 ml. Hence

volume of diluted sample = 100 ml

volume of undiluted sewage sample = 2

BOD = (12 − 4) × 100/2

= 400 mg/l

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