Optimization of electrolytic system type for industrial reverse osmosis concentrate treatment to achieve effluent quality and energy savings

Abstract

This study demonstrated the remarkable potential of electrolytic system type for industrial reverse osmosis concentrate (ROC) treatment on effluent quality and energy requirements. Following their individual system optimization, the required current density decreased from 20 mA cm−2 for the batch system during 4 h of electrolysis, to 5 mA cm−2 for a continuous-flow system (CFS) without effluent recirculation (CFS-A), and 3 mA cm−2 for a CFS system with a rear (CFS-B) or front (CFS-C) buffer tank for effluent recirculation (R = 1). The final or effluent chemical oxygen demand (COD) and NH4+−N levels were 31 and 0.4 mg L−1 (Batch), 24 and 0.56 mg L−1 (CFS-A) and 5.7–8.1 and 0.24–0.34 mg L−1 (CFS-B or -C), respectively. Besides the highest pollutant removal, the optimized CFS-B and -C systems also achieved the highest treatment capacity and energy efficiency, and the lowest effluent toxic organic byproduct levels due to the homogeneous diffusion characteristic of CFS systems, and the reutilization of long-lived active chlorine species (ACS) oxidants in CFS-B/C electrolytic effluent. As compared with the optimized CFS-B system, the optimized CFS-C system at an identical recirculation ratio further achieved the lowest effluent ACS level due to the ACS consumption in the front buffer tank (rather than direct discharge in the CFS-B system) and lower flow rate into the electrochemical cell (than that in the CFS-B system), and was finally recommended for future engineering implementation.