The following is quoted from W. Wesley Eckenfelder, Jr. in his excellent textbook “Industrial Water Pollution Control”, Third Edition, 2000.
Waste Characteristics—Estimating the Organic Content
Although the interpretation of most of the waste characteristics is straightforward and definitive, special consideration must be given to the organic content. The organic content of the waste can be estimated by the BOD, COD (chemical oxygen demand), TOC (total organic carbon), or TOD (total oxygen demand). Considerable caution should be exercised in interpreting these results.
1. The BOD5 test measures the biodegradable organic carbon and, under certain conditions, the oxidizable nitrogen present in the waste. Usual practice today suppresses nitrification so that only carbonaceous oxidation is recorded as CBOD5.
2. The COD test measures the total organic carbon with the exception of certain aromatics, such as benzene, which are not completely oxidized in the reaction. The COD test in an oxidation-reduction, so other reduced substances, such as sulfides, sulfites, and ferrous iron, will also be oxidized and reported as COD. NH3-N will not be oxidized in the COD test.
3. The TOC test measures all carbon as CO2, and hence the inorganic carbon (CO2, HCO3-, and so on) present in the wastewater must be removed prior to the analysis or corrected for in the calculation.
4. The TOD test measures organic carbon and unoxidized nitrogen and sulfur.
Remember to exercise considerable caution in interpreting the test results and in correlating the results of one test with another. Correlations between BOD and COD or TOC should usually be made of filtered samples (soluble organics) to avoid the disproportionate relationship of volatile suspended solids in the respective tests.