Water quality and wastewater measurement

What is the difference between COD and BOD?

By the AQUAIOT engineering team. Published 11 August 2026.

• COD is chemical, BOD is biological. One oxidises nearly everything, the other only what bacteria can digest.
• COD takes hours, BOD takes five days. That is the practical difference on a works.
• COD is always higher. It captures material that bacteria cannot break down.
• The gap between them is informative. A wide ratio suggests material biological treatment will not remove.

What each test actually does

Both are oxygen demand measurements, which is to say both answer the question of how much oxygen this water will remove from a river. They answer it by different routes.

The COD test uses a strong chemical oxidising agent, conventionally potassium dichromate under acidic conditions, to oxidise essentially all the oxidisable material in the sample. It is aggressive and indiscriminate, and it completes in a matter of hours.

The BOD test does the opposite. It measures the oxygen actually consumed by microorganisms breaking the sample down under controlled conditions, conventionally over five days at 20 degrees Celsius, which is where the notation BOD5 comes from. The sample is diluted with a nutrient rich buffer, incubated in the dark, and the fall in dissolved oxygen is measured.

So COD asks what could in principle be oxidised. BOD asks what bacteria will actually oxidise, in a defined window.

Why COD is always the larger number

This follows directly from the definitions and is a useful sanity check on any dataset. A reported BOD higher than the COD from the same sample indicates an analytical problem.

Chemical oxidation reaches material that biological oxidation does not. Detergents, solvents, hydrocarbons and various synthetic compounds are oxidisable by dichromate but are resistant to biological breakdown, at least within five days. All of that contributes to COD and little or none of it to BOD.

The difference between the two is therefore meaningful in itself. A sample where COD and BOD are reasonably close is largely biodegradable, and a conventional biological treatment process should handle it. A sample with a COD far in excess of its BOD contains a substantial fraction that biological treatment will not remove, which points either to a trade effluent input or to a need for a different treatment stage.

Which to use, and where continuous measurement fits

The choice is usually driven by time rather than preference.

BOD is the parameter most directly relevant to sizing and assessing biological treatment, because it describes the load the bacteria will actually see. It is also commonly the parameter written into a discharge permit. Its weakness is fatal for operational control: a five day result describes a works as it was last week. By the time a BOD excursion is confirmed, the discharge has long since reached the river.

COD is used operationally precisely because it is fast. Many works establish a site specific ratio between COD and BOD from paired samples, then use COD as a same day proxy for process control while BOD remains the compliance measure.

Optical sensing has extended this further. Full spectrum ultraviolet and visible absorbance correlates with organic load and can be read continuously without reagents, so it is normally used as a continuously available surrogate, calibrated against laboratory COD and BOD for the specific effluent, rather than as a replacement for either reference method.