COD-Cr is a parameter that represents most of the organic content in wastewater, as it uses a powerful chemical oxidant. So it is quite rare that it should increase after treatment, unless there are undissolved organic particles in the original mixture. If so, sonication etc. can break up these particles to some degree, and increase the dissolved COD content in the bulk liquid. In this case you might want to measure the COD values of filtered samples, instead of the whole mixed samples. Another possibility is that, some aromatic rings containing N molecules might be indestructible by COD tests, which after fragmentation can become measurable. This possibility, though slim, is there. So in this case TOC is a more suitable measurement for the samples, as it measures all the organic content.

Usually when bigger and more complicated molecules break into smaller ones, the biodegradability of the mixture improves. So if you are able to measure BOD5, you might find that after fragmentation, BOD5/COD ratio increases. That would be a clear indication of the occurrence of fragmentation, whether the measured COD increases or decreases. But if you can't measure BOD5 but only COD, maybe you can try COD-Mn? It uses a less powerful oxidant, and can only measure the simpler organic compounds in water, so it's usually lower than COD-Cr. But this is not a standard procedure, as COD-Mn is only used in drinking water treatment.

COD analysis can be done.

 Thank you Prof. Yi Zhang for your clear explanations about this phenomenon and it will be very helpful to carry out the studies.

Thank you Prof. R S Singh for your response to this query.