What is the difference between COD and BOD and when is COD measured?

What is the difference between COD and BOD and when is COD measured?

• A. BOD measures biodegradable organics; COD measures both biodegradable and non-biodegradable organics and is faster to run, often used when BOD is impractical.
• B. BOD measures inorganic nutrients; COD measures dissolved solids.
• C. Both measure same; COD is slower.
• D. COD measures only biodegradable.

Answer: (A)

The key idea is how these tests relate to what they quantify and how quickly you can get results. BOD measures the oxygen microorganisms would consume to biologically degrade the biodegradable organic matter in a sample over a set period (usually 5 days at about 20°C). It specifically reflects biodegradable organics. COD, on the other hand, measures the oxygen needed to chemically oxidize both biodegradable and non-biodegradable (or nonbiodegradable) organic matter in the sample, using a strong oxidant under acidic conditions. Because it's a chemical, not biological, reaction, COD runs much faster—typically a few hours. So COD is used when you need a rapid estimate of the total oxidizable organics and when a 5-day BOD test isn’t practical (for example, for fast process control or samples that can’t be kept for 5 days). This is why the statement that COD measures both biodegradable and non-biodegradable organics and is faster to run is correct. The other ideas aren’t accurate: BOD doesn’t measure inorganic nutrients or dissolved solids; COD isn’t limited to biodegradable material; and COD is not slower than BOD.

The key idea is how these tests relate to what they quantify and how quickly you can get results. BOD measures the oxygen microorganisms would consume to biologically degrade the biodegradable organic matter in a sample over a set period (usually 5 days at about 20°C). It specifically reflects biodegradable organics.

COD, on the other hand, measures the oxygen needed to chemically oxidize both biodegradable and non-biodegradable (or nonbiodegradable) organic matter in the sample, using a strong oxidant under acidic conditions. Because it's a chemical, not biological, reaction, COD runs much faster—typically a few hours.

So COD is used when you need a rapid estimate of the total oxidizable organics and when a 5-day BOD test isn’t practical (for example, for fast process control or samples that can’t be kept for 5 days). This is why the statement that COD measures both biodegradable and non-biodegradable organics and is faster to run is correct.

The other ideas aren’t accurate: BOD doesn’t measure inorganic nutrients or dissolved solids; COD isn’t limited to biodegradable material; and COD is not slower than BOD.