Which statement best explains the difference between BOD and COD tests?

• A. BOD measures inorganic oxygen demand.
• B. BOD measures biological oxygen demand; COD measures total oxidizable organics.
• C. BOD measures chemical oxygen demand; COD measures biological oxygen demand.
• D. They measure pH.

Answer: (C)

BOD and COD both quantify how much oxygen would be consumed by organic matter in water, but they measure different processes and use different time frames. Biochemical Oxygen Demand (BOD) probes how much oxygen microorganisms would use to biologically degrade the biodegradable portion of the organics, typically over five days at 20°C, so it reflects the rate-limiting biodegradable material. Chemical Oxygen Demand (COD) uses a strong chemical oxidant to force oxidation of organics under acidic conditions, usually in a few hours, and represents the oxygen equivalent of all oxidizable organic matter—biodegradable and some non-biodegradable. Because COD oxidizes a broader range of substances, its values are usually higher than BOD.

The statement that BOD measures chemical oxygen demand and COD measures biological oxygen demand swaps the actual meanings. BOD is biological in nature, COD is chemical, and neither test measures pH.