Introduction –
- COD & BOD are the two main characteristic properties water that indicate the degree of pollution of water.
- Both the parameters test the oxygen-demanding strength of the wastewater via different methods and properties.
- The basic features of COD (Chemical Oxygen Demand) & BOD (Biochemical Oxygen Demand) and their basic differences are explained in this article.
| Parameters | COD | BOD |
| Definition | COD measures the oxygen demand for the decomposition of both inorganic and organic matter present in wastewater | BOD measures the oxygen demand for decomposition of organic material by microorganisms present in wastewater |
| Oxidation | By chemical process | By biological process |
| Oxidant used | Chemical Agent – Potassium Dichromate | Aerobic microorganisms |
| Conditions maintained for test | Acidic conditions | Aerobic conditions |
| Matter considered for decomposition | Both Organic and Inorganic matter | Only organic matter |
| Required Time | Takes about 2 to 3 hours, is quick method | Takes 5 days(for standard BOD test), is a slow method |
| Required Temperature | At high temperature of 150°C | At low temperature of 20°C |
| Values | COD values are higher than BOD | BOD values are lower than COD |
| Permissible Limits | 250 – 500 ppm | 30 mg/l |
| Preferable waste treated | Domestic and industrial waste. | Mainly sewage and wastewater from different water bodies. |
| Limitation of the test | Can’t oxidize compounds like Acetate, Toluene, Benzene, etc. by the oxidizing agent. | Only measures biodegradable matter, can’t be preferred for industrial waste. |
What is COD/BOD Ratio?
- It is generally said that COD & BOD shares a theoretical relationship.
- The COD/BOD ratio is an indicator for the presence of organic and inorganic matter present in the wastewater, compost and other natural and man-made material .
- COD is always greater than BOD, hence the ratio will be always greater than 1.
- To find COD and BOD ratio, perform COD and BOD protocol for given water sample.
- Divide the COD obtained value by BOD value for given water samples.
Note –
- COD/BOD ratios can be highly variable and depends on source of water.
- If high COD/BOD indicates that the water contains greater percentage of slowly biodegradable and non-biodegradable material in the sample.
- The ratio of BOD/COD (reverse) would change the value of sample water sample.
Standard Ratio –
BOD/COD Ratio is used in –
- In domestic waste the ratio between BOD/COD is 1:2.
- For untreated wastewater the BOD/COD ratio is between 0.3 to 0.8.
COD/BOD Ratio is used in –
- The food processing wastewater will have the COD/BOD ratio of 2:1.
- The textiles wastewater may contain dyes will have higher ratio of COD/BOD about 5:1.
Takeaways –
- If BOD/COD is 0.92 to 1, waste water is fully biodegradable.
- If BOD/COD > 0.6 , biological treatment is acceptable.
- If BOD/COD is between 0.3 to 0.6, then some adjustment is required.
- If BOD/COD < 0.3, then the biological treatment is not possible.
References –
https://extension.uga.edu/publications/detail.html
https://slideplayer.com/amp/10219336/
https://cgi.tu-harburg.de/~awwweb/wbt/emwater/lessons/lesson_a1/lm_pg_1068.html
