A new approach for chemical oxygen demand (COD) measurement at high salinity and low organic matter samples

Background, aim, and scope: Chemical oxygen demand (COD) is used as a discharge standard parameter in wastewater treatment plant design, environmental modelling and many other applications. Chloride interference is an important problem of COD measurement for wastewaters containing low organic matter and high chloride concentrations. In case of chloride concentrations up to 2,000 mg/L, mercury sulphate addition at a ratio of 10:1 (HgSO₄:Cl^-) can adequately mask the interference. When chloride concentration exceeds 2,000 mg/L, this ratio becomes ineffective to hinder the interference. At this point, it is proposed to use a greater and constant ratio of mercury sulphate addition. However, this application sometimes results in extra mercury sulphate addition which is not necessary. Even in some cases, greater addition of mercury sulphate alone is not a solution to erroneous measurement results. The purpose of the study is to determine optimum HgSO₄:Cl^- ratios according to the chloride concentrations of the samples and to show the importance of the strength of the digestion solution for the correct determination of the COD parameter. Materials and methods: CODs of the synthetic samples containing varying COD and chloride concentrations were measured by closed reflux colorimetric method using three digestion solutions having different strengths. Results: It is indicated in this study that a constant ratio of mercury sulphate can only prevent chloride interference up to a specific chloride concentration. Conclusions: Achieving high precision results in case of low organic matter and high chloride concentration can only be possible by both decreasing the concentration of oxidant and adding mercury sulphate.