Improved COD Measurements for Organic Content in Flowback Water with High Chloride Concentrations

An improved method was used to determine chemical oxygen demand (COD) as a measure of organic content in water samples containing high chloride content. A contour plot of COD percent error in the Cl(-)-Cl(-):COD domain showed that COD errors increased with Cl(-):COD. Substantial errors (>10%) could occur in low Cl(-):COD regions (<300) for samples with low (<10 g/L) and high chloride concentrations (>25 g/L). Applying the method to flowback water samples resulted in COD concentrations ranging in 130 to 1060 mg/L, which were substantially lower than the previously reported values for flowback water samples from Marcellus Shale (228 to 21 900 mg/L). It is likely that overestimations of COD in the previous studies occurred as result of chloride interferences. Pretreatment with mercuric sulfate, and use of a low-strength digestion solution, and the contour plot to correct COD measurements are feasible steps to significantly improve the accuracy of COD measurements.