Energy-efficient seawater softening and power generation using a microbial electrolysis cell-reverse electrodialysis hybrid system

Here we describe the development and testing of a hybrid system that combines microbial electrolysis and reverse electrodialysis (RED) to benefit energy production from seawater. A tubular, continuous-flow, microbial electrolysis cell (MEC) was used prior to RED to remove multivalent ions (Ca2+ and Mg2+) that are known to decrease RED power generation due to high electric membrane resistance. When a membrane electrode assembly was applied to reduce external resistance in the MEC, Ca2+ and Mg2+ were effectively removed with efficiencies of 84 ± 5% and 99 ± 5% (current density of 4.0 ± 0.2 A/m2), respectively, and H2 was simultaneously generated. A high H2 (purity > 99.5%) production rate (2.00 ± 0.09 m3/m3·d) at an applied voltage of 1.5 V, and maximum electrical energy efficiency (169 ± 4%) was accomplished at an applied voltage of 0.9 V and with an anode hydraulic retention time of 6 h. Effluent (47 mS/cm) from the MEC was fed to the RED stack as a high-concentration solution and the conductivity of treated seawater was lower than untreated seawater (53.7 mS/cm) due to Ca2+ and Mg2+ elimination from the seawater. Despite its lower conductivity, treated seawater produced higher power (0.29 W/m2, 26% increase) compared to untreated seawater due to the removal of Ca2+ and Mg2+. Therefore, the MEC is superior to other energy-consuming seawater pretreatment system as it produces energy during seawater pretreatment and successfully integrates with RED to enhance power generation.