Well,
Relationship is not so easy to calculate, as a rare case applies to another case, and here it is preferable to measure the two indicators(BOD and cod) to determine the percent.
Regards
The greater the readily biodegradable organic-C (e.g. volatile suspended solids) in the wastewater, higher the BOD reduction which will also result in corresponding reduction in COD. There is no universal BOD: COD ratio that can be applied to wastewater since each wastewater must be treated on a case by case basis.
Thank you for your answers, Mr. Hussien and Mr. Selvarajah.
@Mr. Selvarajah: As an example, if we say that the COD is 300ppm, the initial BOD is 100ppm and I manage to reduce the BOD to about 40ppm, then how do I quantify the reduction in COD?
COD is always higher than BOD and the ratio is typically 1/1.25 = 0.8 for raw wastewater. However, the ratio can be as low as 0.1, depending on your wastewater quality. For example, untreated wastewater can have BOD/COD = 0.3 to about 0.8, after primary sedimentation the ratio is around 0.3 to 0.6, and after aeration/final sedimentation it can be as low as 0.1 to 0.3. You can see the trend; as the quality is improved, the ratio is smaller.
So, your initial ratio of 100 BOD/300 COD = 0.33 will change as you remove BOD from 100 to 40 mg/L. You can anticipate the final (after BOD reduction) ratio to be smaller than 0.33. An educated guess would be 200 mg/L COD which gives you a ratio of 0.2, that is 40/200.
There is no exact relationship between BOD and COD however it has of particular wastewater treatment plant. As Dr. Vit said, generally, the BOD is reduced COD is reduced too.
Dear Divya
In your example of 100 BOD being reduced to 40, certainly there will be reduction in COD, however, it is not easy to predict the COD reduction stoichiometrically from that of BOD unless a substrate established ratio for COD and BOD is known. I am reluctant to apply your 1:3 ratio, for the following reasons:
To establish such a relationship one must use total carbonaceous BOD to avoid interference from biological oxidation of ammonical-N. BOD test performed without the use of nitrification inhibitors account for ammoniacal-N oxidation in addition to the carbonaceous oxidation. Despite using carbonaceous BOD, the reproducibility of BOD results may still be poor given the inoculating bacterial population and composition can vary from time to time unless a known seeding is used in the laboratory. In contrast, given COD is a simple chemical method, as a method it has high reproducibility provided the sample is representative and homogenised well.
Often researchers are anxious to predict BOD from COD values (not vice versa), since COD method is instant and inexpensive. For the above reason, I don't see any practical application in predicting COD from BOD values. In your case, and in the absence your substrate characteristics, I suggest the best practical way forward to is to monitor for COD. However, if you have already completed your trial and attempt to predict COD retrospectively, you may not be able to obtain a predicted COD value that will be closer to actual value by just using 1:3 ratio.
Kind regards
Selva
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