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• An in-situ pH control of the base solution in BPMC can be achieved by modulating the electric current. • A higher current efficiency for NH 4 + in BPMC can be sustained with pH control compared to the standard constant current method. • For pH 10.5 the energy consumption for NH 4 + removal with pH control was between 35.3 MJ·kgN -1 and 78.1 MJ·kgN -1 during constant current operation. • With pH control, the mass transport of NH 4 + to the base is lower than during constant current operation. Despite growing scientific interest in the past decade, bipolar membrane electrodialysis has seen limited advancement in controlled operation of water dissociation via the bipolar membrane (BPM). For nutrient recovery applications, such as ammonia (NH₃) extraction from anaerobic digestion reject water, implementing in-situ pH control in the base solution could enhance energy efficiency. By controlling the electric current, pH is regulated through OH⁻ generation from the bipolar membrane (BPM). Once the targeted pH is reached, the electric current is applied in pulses and pauses with the purpose to sustain and to not overpass the pH setpoint. A selective electrodialysis reversal (SEDR) combined with a two-compartment bipolar membrane electrodialysis (BPMC) and vacuum membrane stripping (VMS) enabled the recovery and conversion of ammonium ions (NH₄⁺) into volatile ammonia (NH₃). Operating the BPMC with the developed pH control method lowered energy consumption ( E N H 4 + ) and improved current efficiency for NH₄⁺ removal compared to constant current (CC) operation. Under pH control, the BPMC maintained the target pH throughout the whole operation, with an E N H 4 + between 12.5 and 35.3 MJ·kgN⁻¹, compared to 12.1 and 78.6 MJ·kgN⁻¹ under CC. The current efficiency was maintained across setpoints with pH control, ranging between 25 % and 29 %. With CC, the current efficiency declined from 27 % to 12 % at higher current densities. Furthermore, pH control applying a pulsed electric current reduced the occurrence of scaling by minimising the transport of divalent cations across the cation exchange membrane and CO 2 formation in the acid compartment. Similar removal efficiencies were attained, applying pH controlled operation and CC; however, both methods performed a declining removal efficiency during 30 h operation. The developed pH control method can provide distinct improvement in scale-up applications, where energy reduction by preventing excessive water dissociation by the BPM is of interest. In addition, external caustic dosing can be substituted by pH control with a BPMC layout of the stack, reducing the residual impurities of the chemical dosing.
Kaniadakis et al. (Tue,) studied this question.