For example, when I determine COD/BOD =2.1 for initial sample. Then I am treating the waste water . Can I use the calculated COD values of the treated waste water to determine BOD value by using the ratio I initially calculated?
COD=2000mg/l
2000/BOD=2.1
so BOD=952.38mg/l
Dear Santosh i just endorse the answer given by P Brignoli
Concur with first two. If you had substantial data to show a long term relationship, you might be OK with the assumption as long as conditions do not change,. With substantial data, you might even be able to model the relationship if that is important to you to address any variances with temperature, inflow and other variables. Even then if you develop a reliable relationship, I would continue to check it regularly. If this is a legally permitted system, stick with the required protocols unless due to the stability of the waste substances, you get a variance from the permitting agency that would allow use of COD as the standard. That would then be preferable than having a BOD standard and monitoring requirement in the permit, and trying to calculate BOD from COD.
I think that the permanganate test reflects the BOD better than the COD, and it is faster. In any case, you must consult with the agency you are reporting on.
It is better to analyse BOD and COD periodically, in particular for better result. With this variation in BOD, COD can be monitored in better way.
Dear Santosh Murugesan
In my opinion and experience from working on wastewater treatment research, BOD changes rapidly with time . so you cannot use the calculated ratio for determining BOD from COD. if analyzing BOD is not easy for you and you dont want to measure it every time its better to store your samples in freezing conditions (-18 °C). then you can neglect BOD changes.
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