Enhancing ammonia recovery through pH polarization in bipolar membrane electrodialysis†

Ammonia recovery from food waste (including its liquid digestate) is highly emphasized in wastewater treatment and management. Among various membrane-based separation technologies, bipolar membrane electrodialysis (BMED) without anion exchange membranes (AEMs) is an attractive candidate for selective ammonia separation with reduced scaling problems. In this study, a bench-scale BMED stack was built using 5 pairs of cation exchange membranes (CEMs) and bipolar membranes (BPMs). A simulated food liquid digestate was treated using a lab-scale BMED stack to examine the ammonia separation with 3 different intermembrane distances (0.82, 1.64, and 2.46 mm). The highest electric current and ammonia separation were obtained for the intermembrane distance of 1.64 mm, while the BMED stack with 3 spacer gaskets (2.46 mm) still showed comparable separation performance without significant decreases in electric current or ammonia recovery. The residual Ca²⁺ and Mg²⁺ in the cleaning-in-place (CIP) solutions indicated that there were no noticeable scaling problems during the BMED operation. Finally, the pH polarization between the base and feed cells was found to minimize the ammonia back-diffusion even with the highly accumulated ammonia concentration (>11 000 mg_N L⁻¹) in the base cell. With the relatively low energy requirement (1.24–6.78 kW h kg_N⁻¹), BMED lacking AEMs with wide intermembrane distances was confirmed to be a sustainable candidate for ammonia recovery from wastewater with high levels of ammonia.