CAN/CGSB-9.2 No. 3-94

1.1 This method determines the chemical oxygen demand (COD) of water and wastewater by reaction with a measured excess of potassium dichromate under reflux conditions. The COD is defined as the milligrams of oxygen consumed by one litre of sample under the conditions of the test. 1.2 This method is not to be considered as a replacement for the biochemical oxygen demand (BOD) test, CAN/CGSB-9.2 No. 2. The conditions of oxidation used in the COD test procedure are much more severe, with the result that COD test results will almost invariably be several times higher than BOD test results obtained on the same sample. 1.3 In general, BOD test results will more closely parallel the conditions prevailing for natural degradation of dissolved materials in water and wastewater. However, in the case of waters containing materials which are toxic towards the micro-organisms essential to the BOD test procedure, the COD method as described here may provide the only procedure available for evaluating oxidizable matter in an effluent. 1.4 In general, this method will be used on unfiltered samples. In the case of these samples, suspended organic solids present will be largely or completely oxidized during the test and will therefore contribute to the COD result obtained on the test specimen. it must be recognized that the presence of suspended solids may impose a significant problem in obtaining representative aliquots. Where it is desired to measure the COD of the dissolved solids only, samples must be filtered through a medium retentive filter paper (Note 1). 1.5 This method is used to determine COD levels up to 500 mg/L directly on undiluted test specimens. This range can be extended upwards by appropriate dilution of the test specimen. 1.6 It may be desirable or necessary to filter samples which contain gross heterogeneities which are obviously not representative of the water or wastewater stream under examination. If the sample has been filtered, this must be stated in the report.