Complex waste streams, such as sludge reject water from anaerobic digestion, often contain a multitude of cations at varying concentrations, with the ammonium ion (NH₄⁺) typically being the most abundant. Recently, electrodialysis (ED) has been developed as a technology for the removal and recovery of NH₄⁺ from reject water. However, the further development of ED for targeting NH₄⁺ recovery faces lack of reliable performance prediction and standardized operating strategies due to cation competition. It is postulated that selective rejection of divalent cations can be achieved through the use of monovalent-selective cation-exchange membranes (mCEMs). Due to their higher electrical resistance, questions remain regarding the cationic compositions under which mCEMs provide a substantial performance benefit. In the present study, three feed solutions simulating different molar cation compositions of reject water were subjected to ED using both mCEMs and conventional cation-exchange membranes (CEMs). The feed solutions were categorized based on the molar fraction of NH₄⁺ relative to the total cation content. Results showed that the perm-selectivity of NH₄⁺ over Mg²⁺ and Ca²⁺ was enhanced when using mCEMs. Moreover, mCEMs resulted in a higher overall NH₄⁺ removal efficiency compared to standard CEMs. At a relatively low NH₄⁺ molar fraction (0.32), mCEMs outperformed CEMs in terms of current efficiency. Notably, at an intermediate molar fraction (0.58), energy consumption was lower for mCEMs compared to CEMs, but only up to a critical degree of desalination (CDD). At a high molar fraction (0.89), the contribution of mCEMs was insignificant compared to that of CEMs. The CDD was identified as a pragmatic operational parameter beyond which further desalination leads to disproportionate energy penalties. For on-site-process-control of NH₄⁺ removal with ED, the CDD demonstrated that membrane selection and operating thresholds are strongly dependent on reject water composition.

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_NH4⁺) 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_NH4⁺ 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 CO₂ 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.

The removal of ammonium and ammonia, represented as total ammoniacal nitrogen (TAN), from reject water through electro-dialysis (ED) and bipolar membrane electrodialysis (BPMED) encounters challenges such as organic fouling, NH₃ back-diffusion, and high energy consumption. The efficacy of electrodialysis reversal (EDR) combined with bipolar membrane electrodialysis using cation-exchange membranes (BPC) was assessed as a more practical configuration (EDR + BPC). Additionally, a novel configuration involving monovalent selective cation-exchange membranes (MSCEMs) in an EDR + BPC setup (SEDR + BPC) was investigated. Comparisons were made among BPMED, EDR + BPC, and SEDR + BPC under three load ratios (L_N) of 0.8, 1, and 1.3 during continuous operation. The innovative SEDR + BPC configuration, with an L_N of 0.8, exhibited the lowest energy consumption for transported TAN (E_TAN) at 4.4 MJ·kgN⁻¹ removal and achieved the highest TAN removal efficiency of 78 % with an L_N of 1.3. In contrast to conventional BPMED, SEDR + BPC allowed for the recovery of potentially back-diffused NH₃ into the acid chamber, minimizing transport losses. Furthermore, scaling in the base chamber was reduced due to the contribution of MSCEMs when applying an L_N of 0.8. The MSCEMs increased the molar ratio of TAN over (Mg²⁺ + Ca²⁺) in the concentrate and decreased it in the diluate. EDR + BPC and SEDR + BPC configurations exhibited stable and lower cell resistance throughout the operation compared to BPMED, attributed to their ability to generate higher concentration gradients. The results clearly demonstrated the feasibility of low-energy TAN removal from real reject water from sludge anaerobic digestion using the SEDR + BPC setup.