Electrochemical oxidation of reverse osmosis concentrate on mixed metal oxide (MMO) titanium coated electrodes

Reverse osmosis (RO) membranes have been successfully applied around the world for wastewater reuse applications. However, RO is a physical separation process, and besides the clean water stream (permeate) a reverse osmosis concentrate (ROC) is produced, usually representing 15–25% of the feed water flow and containing the organic and inorganic contaminants at higher concentrations. In this study, electrochemical oxidation was investigated for the treatment of ROC generated during the reclamation of municipal wastewater effluent. Using laboratory-scale two-compartment electrochemical systems, five electrode materials (i.e. titanium coated with IrO2–Ta2O5, RuO2–IrO2, Pt–IrO2, PbO2, and SnO2–Sb) were tested as anodes in batch mode experiments, using ROC from an advanced water treatment plant. The best oxidation performance was observed for Ti/Pt–IrO2 anodes, followed by the Ti/SnO2–Sb and Ti/PbO2 anodes. The effectiveness of the treatment appears to correlate with the formation of oxidants such as active chlorine (i.e. Cl2/HClO/ClO−). As a result, electro-generated chlorine led to the abundant formation of harmful by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs), particularly at Ti/SnO2–Sb and Ti/Pt–IrO2 anodes. The highest concentration of total HAAs (i.e. 2.7 mg L−1) was measured for the Ti/SnO2–Sb electrode, after 0.55 Ah L−1 of supplied specific electrical charge. Irrespective of the used material, electrochemical oxidation of ROC needs to be complemented by a polishing treatment to alleviate the release of halogenated by-products.

► Five different anodes were evaluated for the electrochemical oxidation of ROC. ► Ti/Pt–IrO2 and Ti/SnO2–Sb outperformed the other electrodes tested in our study. ► The highest formation of THMs and HAAs was observed for Ti/Pt–IrO2 and Ti/SnO2–Sb. ► Indirect oxidation by chlorine was a dominant oxidation mechanism for all anodes.