BOD5 vs COD in wastewater: what does each measure, and what does the BOD/COD ratio tell me?

BOD is a biological test. The sample is diluted, seeded with microorganisms if needed, and incubated in the dark at 20 °C; the oxygen consumed in 5 days is BOD5. It measures what bacteria can actually degrade in that time. COD is a chemical test: a strong oxidant (dichromate in hot acid) oxidises nearly all organic matter, biodegradable or not, in about two hours.

COD is therefore higher than BOD5 for two reasons: it includes organics that bacteria cannot use, and even the degradable part is not fully oxidised within 5 days.

The 5-day figure relates to the ultimate demand through first-order kinetics: BOD_t = L0 × (1 - exp(-k × t)), where L0 is the ultimate carbonaceous BOD. With k = 0.23 per day, a value often used for domestic sewage, the 5-day fraction is 1 - exp(-1.15) = 0.68. So BOD5 = 200 mg/L corresponds to L0 of about 200 / 0.68 = 293 mg/L.

k varies with the waste, and it rises with temperature, commonly corrected as k_T = k_20 × 1.047^(T - 20). Determine it from a BOD time series if the design depends on it.

The BOD5/COD ratio is a quick indicator of biodegradability. Your sample is 200 / 380 = 0.53. As a general guide, a ratio of about 0.5 or more means the organic matter is readily treatable biologically; below about 0.3 suggests a large non-biodegradable or inhibitory fraction, where biological treatment alone will leave a high residual COD. Untreated domestic wastewater typically falls between about 0.3 and 0.8.

A food-processing effluent at 0.53 is a normal candidate for biological treatment. Compare the estimated ultimate BOD of 293 mg/L with the COD of 380 mg/L: very roughly 90 mg/L of the COD is not biodegradable and will pass through a biological process.

For design and for day-to-day control, COD has practical advantages: it is fast, repeatable, and allows a mass balance, since COD removed must appear as oxygen consumed plus sludge produced. Modern activated sludge models are written in COD units for that reason, with the COD split into biodegradable and inert fractions.

BOD5 remains the figure used in many discharge permits, so both are usually measured, and the ratio for a given plant is tracked so that COD can stand in for BOD between the slower tests.

Some sources of error worth knowing. In the BOD test, nitrifying bacteria can consume oxygen by oxidising ammonia, which inflates the result, especially for treated effluents; a nitrification inhibitor is added when only the carbonaceous demand is wanted. Toxic or very unusual wastes give falsely low BOD unless the seed is acclimatised. In the COD test, chloride is oxidised and counts as COD unless it is masked, which matters for saline effluents.

The COD test also uses hazardous reagents, so it should be run under the laboratory's safety procedures, with the waste collected for proper disposal.