How I will correlate when the amount of TOC was greater than COD after the treatment.?
In my experiment, the ratio of COD:TOC for two sets of results are ranging from 3:1 to 6:1
TOC shows only organic carbon. But COD shows chemical oxygen demand, not only organic carbon, but also metals, which can change their oxidation state. So, COD can be higher than TOC if you have a lot of these metal ions in your water.
If all pollutants are only organic carbon that is degraded in COD reaction, than you can calulate TOC from COD by the formula
TOC=COD*0.375
However
1) some organic matter is hardly degraded in COD reaction (especially hexane and some other hydrocarbons, and in spite of this they are easyly degraded in BOD reaction).
2) some inorganic matter also is involved in COD reaction and consume oxygen from bichromate
3) suspended matter can make some additional troubles when it is unevenly distributed in the water and analysed sample.
4) when the sample is stored still biodegradation can contimue in it changing TOC and COD.
TOC is abbreviation of total carbon conent and COD is the total oxygen demand represents all the O2 required to oxidize organic.. So the TOC/COD ratio is a function of oxidation state of carbon in the organic compound and usually has a value more than 1.
The following discussion is useful:
https://www.researchgate.net/post/What_it_does_really_mean_the_COD_to_TOC_ratio_in_waste_water
https://www.researchgate.net/profile/Ata_Rafiee
"Correlations between TOC and COD are defined in the German wastewater ordinance (AbwV) and can be determined for each substance. Of course, using such measurements, correlation factors for unknown mixtures can also be detected. However, to be able to use such a factor with online measurements, it is required that the water and its mixture of substances do not change in their composition..."
COD will always be than TOC as one carbon (12 mg) during oxidation takes Oxygen (32 mg) as well as other metals too oxidises in to osides formation and their ratio will be different depending on their concentration in the sample.
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