Water quality Part 2: Physical, chemical and biochemical methods BS 6068-2.34:1988

The chemical oxygen demand, COD, of water as determined by this dichromate method can be considered as an approximate measure of the theoretical oxygen demand, i.e. the amount of oxygen consumed in total chemical oxidation of the organic constituents to inorganic end products (see also clause 9). The degree to which the test results approach the theoretical value depends primarily on how complete the oxidation is. A great number of organic compounds are oxidized to an extent of between 90 % and 100 %, and for waters where these compounds predominate, such as municipal effluents, the COD value is a realistic measure of the theoretical oxygen demand. For other waters which contain large quantities of certain substances that are difficult to oxidize under the conditions of the test (see clause 9), the COD value is a poor measure of the theoretical oxygen demand. This may be the case for some industrial effluents. The significance of a COD value thus depends on the composition of the water studied. This should be borne in mind when judging results obtained by the method described in this Section of BS 6068.