What do BOD and COD measure in water quality assessment, and what does a higher value indicate?

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Both BOD and COD are measures of how much oxygen is needed to break down the organic material in water. BOD (biological/biochemical oxygen demand) estimates the amount of oxygen microorganisms will consume to biologically decompose biodegradable organic matter over a set period (usually five days). COD (chemical oxygen demand) estimates the amount of oxygen that would be needed to chemically oxidize organic pollutants using a strong oxidant, giving a faster, broader measure that includes both biodegradable and some non-biodegradable organics. A higher value indicates more organic pollution and a greater potential for oxygen depletion in the water because more oxygen would be consumed during decomposition or chemical oxidation. Typically COD is higher than BOD because chemical oxidation reaches a wider range of compounds than microbial degradation.