The COD/BOD₅ ratio is COD divided by five-day biochemical oxygen demand, using matched samples and compatible reporting units. It is a screening clue about a wastewater's composition and test response. It cannot by itself identify the treatment process, prove discharge compliance or predict a percentage increase in capital cost.
Calculate the ratio and state which ratio you mean
Write COD/BOD₅ explicitly: BOD₅/COD is its inverse. Both inputs are normally reported as oxygen demand, for example mg O₂/L, so the ratio is dimensionless. Do not pair an inlet COD result with an outlet BOD₅ result, or use samples collected during different production conditions.
| Illustrative matched sample | COD (mg/L) | BOD₅ (mg/L) | COD/BOD₅ | Interpretation boundary |
|---|---|---|---|---|
| A | 600 | 300 | 2.00 | Describes this sample, not a biological-treatment guarantee |
| B | 380 | 30 | 12.67 | Investigate residual composition and test conditions |
| C | 600 | Below quantification | Do not report a precise ratio | Use the laboratory's reporting limit and interpretation |
For sample B, 380 ÷ 30 = 12.6667, rounded to 12.67. These are arithmetic examples, not industry reference ranges. If a result appears inconsistent, check the laboratory data rather than adjusting the numbers to fit an expected ratio.
Check the measurements before interpreting them
University of Georgia Extension's laboratory guide distinguishes COD and BOD tests and discusses using paired sample data. For your analysis, obtain the laboratory methods, sampling dates, preservation conditions, dilutions and quality-control notes. Check whether BOD₅ is carbonaceous BOD₅ and whether nitrification or toxicity affects the comparison.
Do not present an unexplained COD/BOD₅ below one as a normal industry benchmark. Check whether the ratio was inverted, units or samples were mismatched, or different test responses are involved. Where the BOD₅ value is small or uncertain, the quotient can magnify that uncertainty.
Turn a ratio into a useful treatability investigation
- Confirm the load: combine matched concentration results with flow and production records. A similar ratio can occur at very different pollutant loads.
- Separate pollutant forms: compare total and soluble COD, TSS and oil/grease. A large particulate fraction suggests different tests from predominantly dissolved residual COD.
- Investigate biological response: review inhibitory chemicals and use appropriate biodegradability or respirometry testing when biological treatment is being considered.
- Test the candidate process: use coagulation/flotation trials for separable material and suitable trials for dissolved contaminants. Record conditions, residual pollutants, chemicals and sludge.
- Compare the complete outlet duty: use the actual discharge or downstream target, not a ratio threshold.
An MBR may be evaluated for a biological treatment duty; it is not automatically a remedy for refractory COD. DAF should be evaluated for the fraction it can separate under tested conditions, with conditioning equipment specified from trials. Neither process is selected merely because a ratio exceeds a particular number.
What to send for a treatment quotation
Send paired laboratory results, flow and production ranges, chemical inventory, outlet requirements and existing treatment performance. Ask the supplier to identify uncertain assumptions and propose tests before committing to a process and price. A ratio alone cannot justify a 30–50% CAPEX uplift or any fixed operating-cost increase. Discuss the sample data and treatment duty with the enquiry rather than sending only the quotient.
Ratio questions
Is a ratio of two always readily treatable?
No. Check absolute load, nutrients, inhibition, temperature and actual biological response.
Does a rising ratio prove treatment is failing?
No. Removing readily biodegradable material can change the ratio. Compare inlet/outlet loads and required pollutants before judging performance.
Can I compare different factories by ratio?
Only with compatible methods and context. Preserve individual sample conditions and uncertainty rather than turning one result into an industry-wide rule.
