Which initial and final dissolved oxygen readings are valid for calculating BOD?

Which initial and final dissolved oxygen readings are valid for calculating BOD?

• A. 8.28 and 0.74
• B. 7.25 and 5.25
• C. 8.25 and 7.74
• D. 7.95 and 6.94

Answer: (B)

Biochemical Oxygen Demand is determined from the drop in dissolved oxygen over the test period, and that drop needs to be in a measurable but not extreme range. A common validity criterion is that the DO depletion (initial minus final) should fall roughly between 2 and 6 mg/L. Looking at the pairs: the first has a drop of 7.54 mg/L (8.28 down to 0.74), which is too high to be considered valid. The second pair drops from 7.25 to 5.25, a 2.00 mg/L decrease, which fits the valid range. The third pair only drops 0.51 mg/L, and the fourth about 1.01 mg/L, both below the minimum. Since the second pair meets the typical 2–6 mg/L depletion criterion, it’s the valid one for calculating BOD.

Biochemical Oxygen Demand is determined from the drop in dissolved oxygen over the test period, and that drop needs to be in a measurable but not extreme range. A common validity criterion is that the DO depletion (initial minus final) should fall roughly between 2 and 6 mg/L. Looking at the pairs: the first has a drop of 7.54 mg/L (8.28 down to 0.74), which is too high to be considered valid. The second pair drops from 7.25 to 5.25, a 2.00 mg/L decrease, which fits the valid range. The third pair only drops 0.51 mg/L, and the fourth about 1.01 mg/L, both below the minimum. Since the second pair meets the typical 2–6 mg/L depletion criterion, it’s the valid one for calculating BOD.