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Biological Oxygen Demand (BOD) & Chemical Oxygen Demand (COD)

BOD & COD

BOD 

Biological oxygen demand measures the amount of oxygen required by microorganisms like Bacteria to oxidize biodegradable wastes in waters. 

SI unit 

It is measured in mg/l as milligram per liter. In other words, mass of oxygen consumed per liter of solution. 

Explanation 

Small amount of oxygen is present in water is called as dissolved oxygen which is consumed by water bodies. Microorganisms like Bacteria uses this DO (dissolved oxygen) to oxidize the wastes in water. If large amount of wastes present, number of microorganisms will increase. Hence, higher BOD value, higher oxidation takes place and then amount of DO will begin to decline. 



Similarly, if there is no waste body present in bacteria, there will be no bacteria existence in water. Hence the value of BOD will be lower and amount of Dissolved oxygen (DO) would be higher. 

Diagrammatically, 

Waste-bodies ⬆️~Bacteria⬆️~BOD⬆️~DO ⬇️

Method 

Take a water sample containing waste bodies along with bacteria in a cylinder. Now leave it for 5 days at 20°C. After 5 days, you will measure the amount of DO and value of BOD in that water. 

Organic wastes 🔀 micrograms 🔀 BOD

COD 

Chemical Oxygen Demand measures the amount of oxygen required to oxidize the organic and inorganic wastes by using  chemicals like Potassium dichromate along with sulphuric acids. 

SI unit 

It is measured in mg/l as milligram per liter. In other words, mass of oxygen consumed per liter of solution. 

Explanation

Chemical Oxidants like Potassium dichromate, Potassium iodate and Potassium permanganate etc oxidizes organic wastes utilizing Dissolved oxygen. Higher oxidizable organic matters, higher COD value but DO will be reduced.

Dramatically, 

Waste bodies ⬆️~COD⬆️~DO⬇️

Method 

A sample of water containing waster matters put in a container. Now, add chemical Oxidant like Potassium dichromate along with sulphuric acid. Leave it for few days. When oxidation will complete. Concentration of organics can be measured by calculating the amount of chemical oxidant left in water, usually done by titration. 

Organic matter 🔀 Chemical Oxidant 


Difference between BOD & COD 

1. Process

BOD is biological oxidation process.

COD is chemical oxidation process. 

2. Source

BOD uses microorganisms to oxidize waste bodies in water 

COD uses chemicals to oxidize organic & inorganic wastes in water.

3. Efficiency

BOD measures amount of DO within 5 days at 20°C. 

COD measure amount of DO within few days at specific temperature.

4.  Values

BOD is lower than COD. 

COD is higher than BOD.






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