The organic carbon in water and wastewater is composed of a variety of organic compounds in various oxidation states.
Some of these carbon compounds can be oxidized further by biological or chemical processes, and the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) may be used to characterise these fractions.
The presence of organic carbon that does not respond to either the BOD or COD test makes them unsuitable for the measurement of total organic carbon.
Total organic carbon (TOC) is a more convenient and direct expression of total organic content than either BOD or COD, but does not provide the same kind of information.
If a repeatable empirical relationship is established between TOC and BOD or COD, then TOC can be used to estimate the accompanying BOD or COD. This relationship must be established independently for each set of matrix conditions, such as various points in a treatment process.
Unlike BOD or COD, TOC is independent of the oxidation state of the organic matter and does not measure other organically bound elements, such as nitrogen and hydrogen, and inorganics that can contribute to the oxygen demand measured by BOD and COD. TOC measurement does not replace BOD and COD testing.
The organic and inorganic carbon present in the sample are being converted to CO2 either by thermal treatment(Combustion) or by chemical treatment (UV/persulphate digestion).