COD represents both organic and inorganic content that can be chemically oxidized by strong Oxidizing agent whereas BOD represents Organic contents alone that can be oxidized by microorganism . For example, a sample have COD-1600mg/l and BOD 1000mg/l. Now if an organic compound can be oxidized by both strong oxidizing agent and micro organism means, how would they be represented? And in this condition, would BOD and COD be dependent on each other?
They provide different information. COD represents the carbon that can be oxidized chemically. BOD represents that portion that is available to the microbes that were present int he seed. Both are useful information, but there is not a constant relationship between the two. I have seen recirculating systems where the BOD stated constant and the COD increased steadily. You can take the time to calibrate a specific system that is at steady state and use COD as an operational variable, but you will have to continually check the BOD to make sure things are not drifting on you.
Dear Santosh,
The difference between COD and BOD is due to substances that can not be broken down biologically. The ratio of BOD and COD is an indicative measure of the "degradability" biochemical compounds present in the water. The ratio COD / BOD evolves around 2.5 (recently discharged waste water) to 10-20 after total decomposition. In the latter case, we speak of a well-mineralized water. However, when toxic compounds are present, the biological activity is slowed down and, therefore, the amount of oxygen consumed after 5 days is less. This also results in a high COD / BOD ratio.
With my best regards
Prof. Bachir ACHOUR
Not exactly, but continuous addition of pollutants and discharges may effect the values.
COD is higher than BOD
As the other have repplied COD and BOD provide differente information, however there is a ratio BOD/COD for wastewater that can used to choose biological treatment or physicochemical treatment for wastewaters. I can´t remember exactly but if the BOD/COD (it´s value it´s always less than 1.0) ratio is about 0.5-0.7 a biological treatment is suggested (UASB, Activated sludge, SBR, etc) if it´s about 0.3 you may use other technologies for WWT.
Cheers, hope my answer has helped you Santosh.
I found the answer. little BOD is a part of COD.so they are normally represent BOD and COD in ratio and not by percentage . Organic compounds sometimes can be oxidized chemically and also by using Microorganism, that is why change in BOD affects COD value.
Since BOD/COD ratio is not identical for all the wastewatrs, if you have only one wastewater, then (from my experience) it is easy to have an approximately typical BOD/COD ratio. So in the beginning, you can just indicate your COD and BOD values from different sample of the wastewater and determine your ratio. If there are always variables in your system, then you can check it again, if the ratio remains stable or not. Also, in the literature, there are many reports for standard BOD/COD ratios, you can get an idea in what range your samples may be in. After having this basic values, you can accordingly design or alter the appropriate treatment.
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