Understand the question answered by each test
COD measures chemical oxidation demand under the conditions of the analytical method. BOD₅ measures oxygen consumption over five days of incubation. These are different mechanisms: a compound may respond to chemical oxidation without being consumed biologically within that period. Neither test identifies the individual substances present in the sample.
Ask whether the laboratory reports total BOD₅ or carbonaceous BOD with nitrification inhibition. Nitrogenous oxygen consumption can increase the result without representing additional organic carbon. Meaningful comparisons also require information about filtration, sample preservation and the analytical basis used for each reported fraction.
Calculate the ratio on a consistent basis
Divide BOD₅ by COD when both are expressed as mg O₂/L and relate to the same sample. Do not combine Monday’s BOD result with the COD of Wednesday’s cleaning discharge. When comparing total and soluble values, specify the separation procedure: the filter used defines that operational fraction.
Illustrative calculation: BOD₅ of 300 mg/L and COD of 1,000 mg/L give a ratio of 0.30. This does not prove that the remaining 70% is impossible to biodegrade. It may include slowly degradable material, particles, method effects and inhibited biological activity. The ratio is not a guaranteed removal percentage.
Investigate low or inconsistent results
Review dilutions, seed, pH, residual oxidants, salinity and potential toxicants with the laboratory. An unadapted population can respond differently from established plant biomass. For COD, check the handling of relevant interferences, including chloride. Resolve unexplained analytical discrepancies before using them to justify equipment expenditure.
A series covering production families is more useful than a simple good-or-poor classification. Compare raw and treated water, and investigate oxygen-consumption profiles where slow degradation is suspected. Respirometry, inhibition tests and trials with adapted biomass help distinguish questions that the ratio cannot answer by itself.
Connect the diagnosis to process selection
Where particulate loading dominates, assess separation and solids management. Where soluble biodegradable loading remains, assess biological capacity and oxygen supply. UF can retain biomass and suspended matter in an MBR, but does not make all soluble COD retainable. RO transfers components into a concentrate stream whose disposal or recovery route must be practical.
Common questions
Is there a universal ratio for declaring wastewater biodegradable?
No. Indicative ranges depend on the wastewater and the decision being made. Retention time, biomass condition and inhibitors affect actual treatment performance.
Does low BOD₅ prove that water is non-toxic?
No. Toxic compounds or inhibition of the test itself may coexist with low BOD. Toxicity requires a specific assessment rather than inference from a low oxygen-demand result.