Off course, BOD OR COD will increase over time.
COD tests only take a few hours rather than days (Biological Oxygen Demand, or BOD, levels take around five days to measure).
COD increases as the concentration of organic material increases. It also increases if inorganic compounds susceptible to oxidation by the oxidant (typically dichromate) are present.
All organic material will be increased over time, because of oxygen depletion and the anaerobic reactions that occur.
Thank all of you for your answers. But sir I am asking about same sample which I have collected and analyzed at 1st day. In that sample I am not adding any other sample, beside that COD and BOD will increase with time?
It depends on which wastewater it is - influent/effluent/MLSS? The higher the organic material and biomass content there is from the beginning, the higher the oxidizability over time.
How was the sample stored? Was it frozen/cooled? Was it stored under light exclusion? The storage conditions are crucial to reduce microbial activity, which reduces the increase of BOD5 over time.
I think that the above answers are presented according to misunderstanding of the question.
If you save the sample in a freeze temp or around 4oC then the organic content will stop from degradation, then the COD will be almost the same over the time. If you save the sample inappropriately then the COD will be less due to the degradation of the organic compounds and other processes (such as settling, oxidation and others). Please read articles on how to preserve the samples during the sampling and storage to keep them representing the real situation.
I hope this helps.
Since COD involves the amount of oxygen consumed or needed to oxidize organic materials (soluble or insoluble) in a wastewater by a strong oxidant(e.g Potassium dichromate). The COD process is influenced by PH, temperature, kind of microbes, organic and inorganic materials. Since there would be some microbes still present in your wastewater degrading the organic materials, COD would decrease with time. To add a little to what has been said careful storage of samples is key.
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