What distinguishes COD from BOD?

• A. COD measures oxygen consumed by microbes decomposing biodegradable organics
• B. BOD measures oxygen required to oxidize both biodegradable and nonbiodegradable organics
• C. COD measures oxygen required to oxidize both biodegradable and nonbiodegradable organics; BOD measures oxygen consumed by microbes decomposing organic matter
• D. BOD is typically faster to measure than COD

Answer: (C)

The key idea is how each test reflects what’s in the water and how it oxidizes it. COD (Chemical Oxygen Demand) measures the amount of oxygen required to chemically oxidize all oxidizable organics in the sample, including both biodegradable and nonbiodegradable substances, using a strong chemical oxidant. BOD (Biochemical Oxygen Demand) measures the amount of oxygen consumed by microorganisms as they biologically oxidize the organic matter over a defined period (usually 5 days at a set temperature). Because COD uses chemical oxidation, it captures the total oxidizable organic content quickly, while BOD depends on microbial activity and reflects only the readily biodegradable portion over several days. Consequently, COD values are generally higher and provide a broader measure of pollution potential than BOD. The statement that COD measures oxygen required to oxidize both biodegradable and nonbiodegradable organics and BOD measures oxygen consumed by microbes decomposing organic matter correctly captures the distinction. The other descriptions misstate the oxidizing method, the scope of what BOD captures, or the relative time required to complete the tests.