BOD and COD are two of the most important measures of organic pollution in water. The ratio between them (the BOD/COD ratio) tells you whether the pollution can be biologically treated, and what kind of intervention is needed. Here is what you need to know.
What Are BOD and COD?
BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen consumed by aerobic bacteria and microorganisms to decompose organic matter in a water sample over a specific time and temperature. It measures only the biodegradable fraction of organic matter which is the part that living microorganisms can actually break down.
COD (Chemical Oxygen Demand) measures the oxygen required to chemically oxidise all organic matter in a sample (biodegradable and non-biodegradable). It uses potassium dichromate and returns results in a few hours.
COD is almost always higher than BOD for the same sample. The gap between the two represents the proportion of organic matter that microorganisms cannot break down.
What Is the BOD/COD Ratio and How Do You Calculate It?
The BOD/COD ratio divides the BOD value by the COD value to produce a number between 0 and 1, known as the Biodegradability Index. The closer it is to 1, the more biodegradable the organic matter.
Formula: BOD/COD Ratio = BOD (mg/L) ÷ COD (mg/L)
EXAMPLE: A water sample from a food processing facility returns:
BOD = 300 mg/L | COD = 500 mg/L
BOD/COD ratio = 300 ÷ 500 = 0.6
Result: Readily biodegradable – suitable for standard biological treatment.
How to Interpret Your BOD/COD Ratio
Use the table below to interpret your results and identify what treatment approach is appropriate.
Why the BOD/COD Ratio Matters
Both BOD and COD are core parameters monitored under EPA discharge licences in Ireland. Understanding the ratio between them is important for several reasons:
- Regulatory compliance: EPA licences specify BOD and COD thresholds for discharge into watercourses. The ratio helps you understand whether your effluent is treatable and compliant.
- Treatment selection: A low ratio signals that standard biological treatment will not be effective. Chemical or advanced oxidation methods may be needed instead.
- Pollution source identification: A sudden drop in the ratio can indicate a new industrial input or toxic discharge inhibiting biological activity in your system.
- Environmental protection: High BOD and COD in rivers and lakes depletes dissolved oxygen, harming aquatic life. This is especially relevant for Ireland’s agricultural and rural watercourses.
BOD/COD testing is relevant to: Wastewater treatment plant operators, agri-food and dairy processors, pharmaceutical manufacturers, food and beverage producers, agricultural businesses near watercourses, and any facility holding an EPA discharge licence.
BOD and COD Testing with Southern Scientific
Southern Scientific Ireland provides INAB ISO 17025-accredited water monitoring services including BOD and COD testing for environmental, industrial, and agricultural clients across Ireland. Results are returned within 10 working days, with clear guidance on interpretation included.
With over 30 years of experience and more than 240,000 samples analysed each year, our team is on hand to advise on what your results mean and what steps to take next.
