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 :
BOD = (Initial DO - Final DO) × Dilution Factor
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 2 ml 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