Comparison of Determining Methods of Chemical Oxygen Demand for Conventional and Saline Wastewaters

In the presence of chloride, bromide and iodide the reported values for Chemical Oxygen Demand (COD) are not real and contain error. The purpose of this study was determining the error in calculating the COD based on Water and Wastewater Standard Testing Methods, in presence of the chloride ion, in various concentrations of COD. The experiment was done via two without any removal of the chloride ion in the first method and using the proposed method of removing the chloride ion by Water and Wastewater Standard Testing Methods in the second method. In order to remove the impact of the chloride ion a HgSO4:Cl ratio equal to 10:1 was used in this method. There were 22 samples that were evaluated twice. NaCl and COD ratios in samples were varied between 1.5-5 g/L and 400-1500 mg/L respectively. Samples divided into two groups, one group without NaCl, the spike sample, and other group unspike sample that had various concentrations of NaCl. The error value in the first method was 16% and in second method was 10.2%. In the second methods 63.6% of the values were lower than the real values; however in the first method 36.4% of values were lower than the real values that can be due to oxidation of chloride ion with potassium dichromate (K2Cr2O7). Presence of chloride ion in saline wastewater can lead to an error in determining the COD value. Sometime this error is positive and sometime negative. These errors are greater in high concentration of COD which could be due to chloride ion oxidation by dichromate and remained some non oxidation organic materials.