What it means
Biochemical oxygen demand measures oxygen used by microorganisms over time, commonly five days for BOD5. Chemical oxygen demand uses a chemical oxidant to estimate oxidizable material. Both help describe organic loading, but they are not identical tests.
What operators watch
- Influent and effluent BOD help show treatment efficiency.
- High organic loading can affect aeration demand and process stability.
- COD results are often faster but not a direct replacement for BOD compliance where BOD is required.
Common exam traps
- Treating BOD and COD as the same number.
- Forgetting units are usually mg/L.
- Ignoring flow when converting concentration to loading.
Field example
If influent BOD rises sharply, operators may see increased oxygen demand and changes in sludge activity.
