How is CBOD measured to exclude nitrogenous demand?

• A. The nitrifying organisms are inhibited from using oxygen by adding a nitrification inhibitor to the samples.
• B. Nitrification is inhibited by lowering the pH to extreme levels.
• C. The samples are aerated to promote nitrification.
• D. Nitrification is not addressed in CBOD testing.

Answer: (A)

In CBOD testing, the goal is to measure the oxygen demand from carbon oxidation while excluding the oxygen used for nitrification of nitrogen compounds. Nitrifying bacteria would otherwise consume oxygen to convert ammonia to nitrite and nitrate, adding to the observed BOD. To keep the measurement focused on carbonaceous demand, a nitrification inhibitor is added to the samples, blocking the activity of nitrifying organisms. This prevents the nitrogenous portion of the oxygen demand from contributing to the reading, so the results reflect mostly the carbon-based BOD.

Lowering the pH to extreme levels is not the standard method and can disrupt many microbial processes, making results unreliable. Aerating the samples would actually promote nitrification, not inhibit it, which would increase nitrogenous demand rather than exclude it. Saying nitrification is not addressed would ignore the need to separate carbon- and nitrogen-related oxygen demands.