Content Writer-SME
The primary difference between chemical oxygen demand and biochemical oxygen demand depends upon the utilization of oxygen.
- The amount of oxygen required to break the organic material in the absence of microbes following oxidation is called chemical oxygen demand or COD.
- The amount of oxygen required by microorganisms to break the desired organic matter is called biochemical oxygen demand or BOD.
Here, in this article, we have included more grounds of comparison to understand the difference between BOD and COD more accurately.
| Table of Contents |
What is Chemical Oxygen Demand?
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The principle of chemical oxygen demand (COD) states that, “Under acidic conditions, the complete oxidation of all organic compounds into carbon dioxide can be done easily by the use of strong oxidizing agents.”
- The high concentration of COD represents the presence of organic materials in wastewater in large quantities.
- This can pollute the local environment if not treated.
What is Biochemical Oxygen Demand?
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The amount of dissolved oxygen consumed by aerobic bacteria during the process of decomposition of organic matter is estimated by an analytical parameter called biochemical oxygen demand (BOD).
- High BOD in marine environments causes suffocation and death of aquatic organisms.
- Low BOD value is the indication of purity of water and that water is also considered as safe for drinking.
- The permissible limit of BOD for drinking water is 1-3 mg/L.
- However, the presence of non-biodegradable materials in water bodies can not be estimated by the BOD test.
Differences between BOD and COD
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BOD and COD are differentiated from each other on different grounds which are mentioned below:
| Basis of Comparison | BOD | COD |
|---|---|---|
| Definition | The estimation of oxygen quantity which is required for breakdown of organic matter is called BOD. | A method of estimation of oxygen quantity which is required for breakdown of organic and inorganic materials is called COD. |
| Process | The oxidation process in the case of BOD estimation involves microorganisms. Thus, it is considered a biological oxidation process. | In this case, oxidation takes place through chemical reactions. |
| Duration | The estimation of BOD takes 5 days. | It takes less time than BOD estimation, nearly 2-3 days. |
| Values | Lesser than COD | The value of COD is always more than that of BOD values. |
| Permissible Limit for Water Bodies | 30 mg/L | 250-550 ppm (parts per million) |
| Importance | It is only useful in assessment of biodegradable wastes present in the water bodies. | Both biodegradable as well as non-biodegradable wastes are easily through COD test. |
Things to Remember
- BOD and COD are important parameters that help in effective treatment of wastewater.
- The fastest way to reduce COD is by the addition of hydrogen peroxide in the wastewater.
- Oxidation of organic materials can be more efficiently done through COD estimation.
- Low BOD value indicates purity of water.
- Both BOD and COD tests are conducted to indicate pollution in water bodies.
Sample Questions
Ques. What is the fixed limit of biochemical and chemical oxygen demands for waste water? (2 marks)
Ans. The fixed limit of biochemical (BOD) and chemical (COD) oxygen demands for wastewater are approximately 80 ppm and 200 ppm respectively.
Ques. How can we calculate the BOD of waste water? (2 marks)
Ans. The calculation of BOD is useful in measuring the dissolved oxygen intake by microorganisms in water ecosystems. Mathematically, the formula for BOD is given as:
| BOD = (D.O1 –D.O2) x Dilution Factor / Volume of Sample |
Here, B.O.D = Biochemical oxygen demand concentration in mg/L
D.O1 = Initial dissolved oxygen concentration in mg/L
D.O2 = Final dissolved oxygen concentration in mg/L
Ques. Why is the value of COD always higher than BOD? (2 marks)
Ans. In both BOD and COD estimation, oxidation of organic matter takes place and the breakdown of organic compounds, chemical reactions is more favored than microbial action.
Ques. What are the ways to reduce COD? (2 marks)
Ans. There are two ways to reduce COD which are as follows:
- Wastewater Treatment through Coagulation and Flocculation.
- Removal of COD by the action of microorganisms.
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