Why is a nitrification inhibitor used in CBOD testing?

• A. To promote nitrification to measure nitrogenous demand.
• B. To inhibit all microbial activity, preventing any oxygen consumption.
• C. To stabilize the sample by adding preservative without affecting nitrification.
• D. To prevent nitrifying bacteria from consuming oxygen, isolating carbonaceous demand.

Answer: (D)

In CBOD testing, the goal is to measure the oxygen demand caused by biodegradable carbon compounds, without including oxygen used to oxidize nitrogen compounds. Nitrifying bacteria consume a lot of oxygen as they convert ammonia to nitrite and nitrate. If nitrification happens during the test, the resulting oxygen uptake includes both carbonaceous and nitrogenous demands, which overestimates the true carbonaceous demand.

A nitrification inhibitor is added to suppress nitrifying bacteria so they don’t consume oxygen during the incubation. This isolates the carbonaceous portion of the oxygen demand, giving a CBOD result that reflects only the oxidation of organic carbon. It’s not about stopping all microbial activity or preserving the sample; it’s about preventing nitrification specifically to avoid obscuring the carbon-related oxygen consumption.