Chemical Oxygen Demand (COD)

One problem with the BOD test is that it takes 5 days to run. If the organic compounds were oxidized chemically instead of biologically, the test could be shortened considerably. Such oxidation can be accomplished with the chemical oxygen demand (COD) test. Because nearly all organic compounds are oxidized in the COD test, while only some are decomposed during the BOD test, COD results are always higher than BOD results. The common compounds which cause COD to be higher than BOD include sulphides, sulphites, thiosulfates and chlorides. One example of this is wood pulping waste, in which compounds such as cellulose are easily oxidized chemically (high COD) but are very slow to decompose biologically (low BOD).

This test is carried out on the sewage to determine the extent of readily oxidizable organic matter, which is of two types.

1. Organic matter which can be biologically oxidized is called biologically active.
2. Organic matter which cannot be oxidized biologically is called biologically inactive.

COD gives the oxygen required for the complete oxidation of both biodegradable and non-biodegradable matter. COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. It is an indirect method to measure the amount of organic compounds in water. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution.

The standard COD test uses a mixture of potassium dichromate and sulphuric acid to oxidize the organic matter (HCOH), with silver (Ag+) added as a catalyst. A simplified example of this reaction is illustrated below, using dichromate (Cr₂0₇^2-) and hydrogen ions (H+). A sample is refluxed in strongly acidic solution with a known excess of potassium dichromate (K₂Cr₂O₇) for 2-3 hours. After digestion, the remaining unreduced K₂Cr₂O₇ is titrated with ferrous ammonium sulphate to determine the amount of K₂Cr₂O₇ consumed. Then, the oxidizable matter is calculated in terms of oxygen equivalent. This procedure is applicable to COD values between 40 and 400 mg/L.

A known amount of a solution of K₂Cr₂O₇ in moderately concentrated sulphuric acid is added to a measured amount of sample and the mixture is boiled in air. In this reaction, the oxidizing agent, hexavalent chromium (Cr^VI), is reduced to trivalent chromium (Cr^III). After boiling, the remaining Cr^VI is titrated against a reducing agent, usually ferrous ammonium sulphate. The difference between the initial amount of Cr^VI added to the sample and the Cr^VI remaining after the organic matter has been oxidized is proportional to the chemical oxygen demand.

Total Organic Carbon

Since the ultimate oxidation of organic carbon is to CO₂, the total combustion of a sample yields some information about the potential oxygen demand in an effluent sample. A common application of total organic carbon testing is to assess the potential for creating disinfection by-products. Disinfection by-products are the result of halogens (e.g., bromine, chlorine) or ozone interacting with naturally occurring organic carbon compounds during the drinking water disinfection process. For example, trihalomethane, a carcinogen, is created when halogens displace three hydrogen ions on methane. Water that is high in total organic carbon has a greater potential to develop disinfection by-products. Some of the organics can be removed by adding levels of treatment specific for organic carbon absorption. It is usually not economically feasible to remove all naturally occurring organics from finished drinking water.

Total organic carbon is measured by oxidizing the organic carbon to CO₂ and H₂0 and measuring the CO₂ gas using an infrared carbon analyser. The oxidation is done by direct injection of the sample into a high-temperature (680-950°C) combustion chamber or by placing a sample into a vial containing an oxidizing agent such as potassium persulfate, sealing and heating the sample to complete the oxidation, then measuring the C02 using the carbon analyser.