What it is
Chemical oxygen demand (COD) uses a strong oxidising agent to break down almost all the organic matter in a sample, in a couple of hours rather than five days. It captures material that bacteria cannot easily digest, so it is always higher than BOD. The ratio between the two tells a treatment specialist how biodegradable your effluent is.
Where it comes from
The same organic sources as BOD, plus harder-to-degrade material: oils, solvents, surfactants, dyes, cleaning chemicals, and process chemicals from manufacturing. A COD far higher than BOD often points to chemical rather than food-based contamination.
Why it matters
COD is an environmental and consent measure. Many water companies set a COD limit in the consent and use COD rather than BOD to calculate charges, because it is quicker to measure. A sudden rise in COD with little change in BOD usually means a chemical has entered the drain and is worth tracing straight away.
What can be done about it
Trace the source first: a COD spike almost always has a specific cause such as a cleaning cycle, a product change or a spill. Then treat at source where possible, by segregating chemical waste for separate disposal and reviewing cleaning chemicals. Physical and chemical treatment such as coagulation and dissolved-air flotation lowers COD where biological treatment alone will not.
Information last reviewed 6 October 2026 by Pozzani Pure Water.