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.
| BOD₅ | COD | TOC | |
|---|---|---|---|
| What it measures | oxygen used by microorganisms | oxygen needed for chemical oxidation | the mass of organic carbon |
| How | incubation 5 days at 20 °C, in the dark | dichromate oxidation at 148 °C, ~2 h | combustion/oxidation and CO₂ measurement |
| Unit | mg O₂/l | mg O₂/l | mg C/l |
| Covers | only biodegradable matter | biodegradable and refractory matter | all organic carbon |
| Waiting time | 5 days | ~2-3 hours | minutes |
| Method standard | ISO 5815-1:2019 | ISO 6060:1989 / ISO 15705:2002 | EN 1484:1997 |
| Typical relation | the lowest value | in surface waters usually 2-3× BOD₅, less in domestic sewage | different unit - do not compare directly |
Three methods, three different questions - which is why they appear together in a report, not interchangeably.
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
The most informative figure is the COD/BOD₅ ratio - how much of the organic matter is available to microorganisms at all:
| COD / BOD₅ | What it means |
|---|---|
| < 2 | readily biodegradable matter - typical of domestic sewage |
| 2-4 | a mixture; partial biodegradability |
| > 4 | a predominance of refractory compounds - a lead towards industrial effluent, landfill leachate or natural humic substances |
Besides that: high BOD₅ → a lot of readily degradable matter → a real risk of an oxygen drop, and all three indicators rise with the organic load of the catchment. A fall in BOD₅ downstream is often the work of self-purification rather than the disappearance of the source - which is why the result is read along the watercourse, not at a single point.
⚠️ The ratio is a lead, not proof. Humic waters (peatlands, brown waters) have high COD and low BOD₅ for entirely natural reasons - the number then looks like industrial effluent while it is ordinary catchment geochemistry.
Sources
- Determination methods (editions verified 18 August 2026): BOD - ISO 5815-1:2019 (dilution and seeding, 5-day incubation at 20 °C); COD - ISO 6060:1989 (dichromate) and ISO 15705:2002 (sealed-tube test); TOC - EN 1484:1997. ⚠️ For TOC, “ISO 8245” and “EN 1484” are not interchangeable: the European standard covers ISO 8245:1987 as not equivalent (NEQ), so a slash between them overstates the match.
- Requirements for urban waste water (the origin of the limits in permits): Directive 91/271/EEC concerning urban waste-water treatment; national limit values are set by implementing regulations. ⚠️ The legal position is changing: Directive 91/271/EEC applies until 1 August 2027, when it is replaced by Directive (EU) 2024/3019 (recast - among other things, the agglomeration threshold drops from 2 000 to 1 000 p.e.). (Checked 17 August 2026.)
- Classification of surface waters (BOD₅ and TOC are physico-chemical elements of the assessment): see water quality classes.
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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