If BOD is on your permit, COD almost certainly is too. The two show up side by side on every lab report, they both measure oxygen demand, and operators routinely treat them as two names for the same thing. They are not the same thing — and the difference between them is often the difference between a clean inspection and a surcharge letter.
Here is what COD actually measures, what your COD:BOD ratio is telling you, and what to do when the number climbs.
What Is COD in Wastewater?
COD, or chemical oxygen demand, is a measure of the total oxygen required to chemically oxidize every organic and inorganic compound in a water sample. Unlike BOD, it captures compounds that are chemically oxidizable but biologically resistant — the material a treatment plant’s microbes cannot break down. That word total is what makes COD matter.
Biological treatment at your POTW handles biodegradable organic material well. But not everything in industrial wastewater is biodegradable. Solvents, complexing agents, certain surfactants, and process byproducts can pass through biological treatment chemically unchanged. COD sees them. BOD does not.
The COD test works fast. A strong chemical oxidant — usually potassium dichromate — is added to the sample under heat and acidic conditions. Everything oxidizable is oxidized, and the oxygen equivalence is measured. Per Standard Methods 5220, a COD result is available in roughly 2–3 hours, against the five-day minimum for a BOD5 test.
