GATE Environmental Science and Engineering Practice Quiz

Question 1

In the standard 5-day BOD test at 20°C, a diluted wastewater sample has an initial dissolved oxygen (DO) of 8.8 mg/L and a final DO after 5 days of 3.2 mg/L. If the decimal fraction of sample in the BOD bottle is 0.25 and seed correction is negligible, the BOD5 of the wastewater is:

Accepted Answer

22.4 mg/L

Question 2

Which statement best describes the fundamental difference between BOD and COD measurements for wastewater?

Accepted Answer

BOD measures biologically oxidizable matter, whereas COD measures chemically oxidizable matter

Question 3

A wastewater sample has measured BOD5 = 220 mg/L and COD = 180 mg/L. Which conclusion is most appropriate?

Accepted Answer

The data suggest analytical inconsistency or interference because COD should generally not be lower than BOD5

Question 4

In the standard BOD test, nitrification is suppressed when the objective is to measure carbonaceous BOD (CBOD5). The main reason is that nitrifying bacteria:

Accepted Answer

Consume oxygen, causing BOD to include nitrogenous oxygen demand

Question 5

A seeded dilution water blank shows DO depletion of 0.4 mg/L. A seeded diluted sample bottle containing 6% wastewater shows DO depletion of 3.4 mg/L. If the seed volume fraction is the same in both bottles, the corrected BOD5 of the wastewater is:

Accepted Answer

50 mg/L

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