BOD vs COD — What They Measure and Why Wastewater Engineers Need Both

Two numbers show up on almost every water quality report in a wastewater course or a treatment plant log: BOD and COD. Both are described as measuring "how polluted" a water sample is, both are reported in mg/L, and both get thrown around as if they were interchangeable. They're measuring genuinely different things, and knowing which one to reach for — and what the gap between them tells you — matters more than memorising two definitions.

BOD: how much oxygen bacteria actually use

Biochemical Oxygen Demand is the amount of dissolved oxygen consumed by bacteria as they biologically break down the organic matter in a water sample, under standard conditions. The standard test (BOD₅) incubates the sample in the dark at 20°C for 5 days and measures the drop in dissolved oxygen over that period.

Because it's a genuinely biological process, BOD only measures the fraction of organic matter that bacteria can actually break down in that timeframe — the biodegradable portion. Anything present that bacteria can't metabolise (many synthetic organics, for instance) contributes essentially nothing to the BOD number, no matter how much of it is in the sample.

COD: how much oxygen a strong chemical oxidant uses

Chemical Oxygen Demand measures the oxygen equivalent of organic matter that can be oxidised by a strong chemical oxidising agent — typically potassium dichromate under acidic, heated conditions — regardless of whether bacteria could ever break that material down biologically. Because the chemical oxidant is so much more aggressive and indiscriminate than bacteria, it oxidises both biodegradable and non-biodegradable organic matter.

The practical advantage: the COD test takes a few hours, not five days, making it far more useful for routine monitoring and quick process control decisions at a treatment plant.

Why COD is (almost) always the bigger number

Since COD captures everything BOD captures plus the non-biodegradable organic fraction that bacteria can't touch, COD is virtually always equal to or greater than BOD for the same sample. Seeing COD come in lower than BOD on a real report is a red flag for a lab or sampling error, not a genuine result — it would mean bacteria consumed more oxygen than the strongest chemical oxidant available, which doesn't happen.

The ratio that actually matters: BOD/COD

The single most useful thing you can do with these two numbers together is divide one by the other. The BOD/COD ratio tells you what fraction of the total organic pollution is biodegradable — which is exactly the question that decides whether biological treatment (activated sludge, trickling filters, oxidation ponds) will work well on this wastewater at all.

• BOD/COD above roughly 0.5 — a large biodegradable fraction; conventional biological treatment is generally well suited and efficient.
• BOD/COD in the 0.3–0.5 range — biologically treatable, but likely needing a longer detention time or a more robust biological process to get good removal.
• BOD/COD below about 0.3 — a large share of the pollution is non-biodegradable; biological treatment alone will struggle, and physico-chemical or advanced oxidation methods typically need to be part of the treatment train instead of (or ahead of) biological treatment.

Domestic sewage typically sits in a favourable, highly biodegradable range, which is exactly why conventional biological sewage treatment plants work as well as they do. Many industrial effluents — depending on what's actually in them — can sit much lower, which is precisely why "just build an activated sludge plant" isn't automatically the right answer for every industrial wastewater.

A quick way to keep them straight

• Testing method: BOD is biological (bacteria, 5 days); COD is chemical (dichromate oxidation, a few hours).
• What it captures: BOD = biodegradable organics only; COD = biodegradable + non-biodegradable organics.
• Typical value: COD ≥ BOD for the same sample, always.
• What the gap tells you: a wide BOD/COD gap signals a significant non-biodegradable fraction that biological treatment won't remove.