BOD, COD, TOC - indicators of organic matter in water
In water-quality reports a trio of abbreviations keeps recurring: BOD, COD, TOC. All of them describe organic matter in the water and the related oxygen demand - but in different ways. Understanding the differences makes it easier to read results and assess a water’s loading.
BOD - biochemical oxygen demand
How much oxygen micro-organisms will use up while decomposing organic matter. Most often BOD₅ is determined (after 5 days of incubation). A high BOD is a sign of a large load of easily degradable organic matter - e.g. sewage - and a real risk of oxygen deficiency in the water.
COD - chemical oxygen demand
How much oxygen (or rather oxidant) is needed to oxidise organic matter chemically. COD covers a broader range of compounds than BOD (including ones that are harder to degrade), which is why it is usually higher. It is determined by the dichromate or permanganate method.
TOC - total organic carbon
A direct measure of the amount of organic carbon in the water. It does not speak directly of oxygen demand, but it is a consistent, instrumental measure of organic-matter content.
How to read them together
- high BOD → a lot of easily degradable matter (e.g. sewage) → risk of a fall in oxygen,
- the COD/BOD ratio → a hint of how “degradable” the matter is (a large excess of COD suggests compounds that are harder to biodegrade),
- all three rise with the organic loading of the catchment.
These indicators are linked with oxygen conditions and belong to the physico-chemical elements of status assessment.
🧮 Tool: BOD5 calculator - compute the result from the oxygen at the start and after 5 days of incubation, with an optional dilution and a blank correction.
In practice
BOD, COD and TOC are determined by a laboratory - in the field what counts is correct sampling. It is worth keeping the results together with the rest of the monitoring; in LimnoLog you add them to the right session and see them in the context of oxygen and nutrients.
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