Enhanced electrodialysis by electric double-layer capacitors to minimize membrane use in digestate ammonia recovery: mechanisms and effects

Anaerobic digestion technology effectively recovers energy from livestock manure but discharges digestate with high ammonia nitrogen (NH 4 + -N). NH 4 + -N in digestate can be effectively recovered by electrodialysis (ED) technology. However, the implementation of ED technology is limited by membrane and energy costs. Here, we present a capacitor-circuit-switching electrodialysis (CCSE) strategy to enhance ED efficiency by integrating an electric double-layer capacitor (EDLC). We reveal that the energy storage mechanism of EDLC improves the power density while the intermittent operation mitigates the concentration polarization effect. In the practice of the real digestate, CCSE elevated the pH to 9.4, and 98.8 % of the NH 4 + ions migrated to the cathode compartment after 3.5 h. The specific energy consumption and the current efficiency are 15 kWh/kg NH 4 + -N (N) and 64.1 %, respectively. High pH contributes to carbon reduction in the stripping process, and subsequent alkaline contamination can be controlled by neutralization of the anode effluent. Moreover, the CCSE-stripping system achieved 95 % NH 4 + -N removal and 93 % stripping recovery efficiency. This work provides valuable insights and solutions for the design and optimization of ED systems.