Bipolar Membrane Electrodialysis for Nutrients Separation and Recovery from Wastewater

Abstract

Bipolar membrane electrodialysis (BMED) is an electro-membrane separation technology used for over 20 years to treat liquid wastes, including wastewater from various industrial processes, municipal wastewater, and animal farms. This technology has been developed for its ability to dissociate H₂O into H⁺ and OH⁻ ions and to convert cations and anions in the feed solution into bases and acids, respectively, by applying an electrical potential. In comparison with alternative treatment technologies, BMED has some advantages, such as no need for chemical reagents, high degrees of efficiency, and straightforward operational procedures. However, BMED has been limited by a high level of preparation expense for BMs, membrane deposition, and ion loss during operation. This review mainly describes the principles and designs of BMED, their compositions along with their advantages and disadvantages, the membrane fouling and cleaning, and also the trend of their application in wastewater treatment. Finally, the results on nutrient recovery by BMED show that the performance of BMED depends on the nature of the ion exchange membranes used, the composition of the sample solution, the process conditions, and the configuration of the membrane stack.

1. Introduction

In recent years, electro-membrane technologies have become more interesting in the field of environmental protection [1]. Among them, bipolar membrane electrodialysis (BMED) is a process in which, under an electrical potential, a selective separation of ions (cations and anions) and their recoveries in the form of base and acid without adding any chemicals is achieved through the use of cation-exchange membranes (CEMs), anion-exchange membranes (AEMs) and bipolar membranes (BMs) [2,3,4]. The method described above is a water treatment technology that can save the health of the environment by avoiding its contamination. It can be applied to the treatment of municipal wastewater, wastewater from industry, brackish water, in drug and food industries, chemical processes, and heavy metals removal due to its ability to separate and recover ionic compounds in acidic and alkaline solutions from salt solution under the effect of electrical potential [3,5,6].

Due to the increasing amount of nitrogen and phosphorous in wastewater, some researchers have made efforts to recover these nutrient elements from waste liquids. According to Chirag et al. [7], nutrients extraction from wastewater can occur via different technologies such as chemical precipitation or crystallization, gas-permeable membranes, and liquid–gas stripping. These treatment technologies use different chemicals for the pre- and post-treatment of water, which have a substantial negative impact on the environment, while the BMED process produces both concentrated and dilute streams without adding chemicals [8,9]. Furthermore, BMED can be used to separate and recover phosphates and ammonium from liquid waste in their acidic and basic forms [10], respectively. This technology can also accomplish a separation of monovalent ions against multivalent ions by using monovalent selective ion exchange membranes. Wu et al. [11] reported that BMED, which is an electro-membrane process, is an efficient method for separating and recovering both NH₄⁺, PO₄³⁻, and NO³⁻ from wastewater. Despite its demonstrated potential, the practical application of the BMED process remains constrained by high operational costs, membrane fouling, ion losses, and limited understanding of how membrane properties, operating conditions, and stack configuration influence its nutrient recovery performance.

The aim of this review is to provide a comprehensive analysis of BMED for sustainable wastewater treatment, focusing on its principles, fouling and scaling mechanisms, operational and clean mitigation strategies, and recent advances in simultaneous separation and recovery of nitrogen (NH₄⁺ and NO³⁻) and phosphorus (PO₄³⁻).

2. Principles of BMED

The development of the electro-membrane process has been limited by the higher H₂ and O₂ generation. In 1956, Frilette [12] proposed the bipolar membrane (BPM), which has low energy consumption [13]. During BMED, water dissociation within the bipolar membranes produces H⁺ and OH⁻ whithout generating H₂ and O₂ gases or damaging the bulk cell components. Additionally, the amount of energy consumed by BMED for 1 mol of water is 79.9 KJ/mol, while that of ED is 198.5 KJ/mol [4]. When compared with other electrochemical processes, these characteristics offer BMED greater application opportunities [2,4]. BMED is a membrane process distinguished by the application of BPM between electrodes [4]. This process is a rather new electro-membrane technique that uses an applied voltage to dissociate water into protons (H⁺) and hydroxide ions (OH⁻) [3,14]. This innovative technique features three fundamental configurations (two-chamber BMED containing base or acid; three-chamber BMED) per one cell pair according to its application and three types of ion exchange membranes (BM, CEM, and AEM) in specific arrangements [4,15]. Each unit of the three-chamber BMED has CEM, AEM, and BM and can be beneficial for the separation and recovery of salts as acids and bases [16].

Compared with other electrochemical process, BMED consumes low energy and has high economic effectiveness [4]. The voltage (0.828 V) needed for water dissociation in BMs is significantly less than that of water electrolysis on electrodes (2.057 V) [4,17]. The following reactions show the potentials required on electrodes and in BMs comparatively.

3H₂O → 1/2O₂ + H₂ + 2H⁺ + 2OH⁻, 2.057 V

(1)

Cathodic reaction: 2H₂O + 2e → H₂ + 2OH⁻, 0.828 V

(2)

Anodic reaction: 2H₂O → O₂ + 4H⁺ + 4e, 1.229 V

(3)

$$2H_{2}O {}_{\underset{K_{-2}}{\leftrightarrow }}{}^{\stackrel{K_{+2}}{\leftrightarrow }} 2{\mathit{OH}}^{-}+H_3{O}^{+}$$

(4)

The low energy consumption of BMED helps to save power and the necessity of cooling. Another advantage of bipolar membrane water splitting is the absence of gas production, such as H₂ and O₂, during catalytic water dissociation within the membrane stack. Consequently, acids and bases are produced inside the membranes, away from the electrodes, significantly reducing electrode degradation and corrosion [17].

2.1. BMs

In the past decade, BMs have been one of the various kinds of ion exchange membranes (IEMs) that have drawn more attention. These membranes consist of two layers: an anion exchange layer (AEL) and a negatively charged cation exchange layer (CEL), as well as an interface between the two layers known as the “bipolar junction” or “hydrophilic interface layer” (IL) [16,18]. In contrast to IEMs used for the separation of ions, BMs are made to split water into hydroxyl and hydrogen ions at the bipolar intersection after an electrical current is applied. With the electrical current application, hydrogen and hydroxyl ions are produced by water electro-dissociation at the intermediate layer of the BPMs and move toward the cathode and anode compartments through CEL and AEL, respectively [19]. The arrangement of BMs to the electrodes depends on the target: to ensure the water splitting into OH⁻ and H⁺, the CEL must be positioned towards the cathode and the AEL towards the anode. This arrangement is a so-called “reverse-bias” condition. In fact, the protons (H⁺) pass through CEL, while OH⁻ ions flow through AEL, creating an acid and a base on the membrane’s opposing sides (Figure 1a). If the electrical field’s orientation across the BPM is inverted, the CEL will then face to the anode and the AEL to the cathode. This position is called the “forward-bias” condition (Figure 1b). Therefore, H⁺ and OH⁻ ions move out of the external solutions towards the hydrophilic interface layer to form water from the acid–base neutralization reaction. Figure 1 shows the principle of BM under reverse-bias and forward-bias modes.

Figure 1. Principle of a BM: (a) BM under reverse bias, where water dissociates into H⁺ and OH⁻; (b) BM under forward bias, where ions are transported into the membrane and form water at the bipolar junction.

In BM, two reactions of protonation and deprotonation can be used to explain the mechanism of water dissociation [16,20].

$$\mathbf{C}+{\mathbf{H}}_{\mathbf{2}}\mathit{O} {}_{\underset{{\mathbf{K}}_{-\mathbf{2}}}{\leftrightarrow }}{}^{\stackrel{{\mathbf{K}}_{+\mathbf{1}}}{\leftrightarrow }} 2{\mathbf{CH}}^{+}+{\mathbf{OH}}^{-}$$

(5)

$${\mathbf{CH}}^{+}+{\mathbf{H}}_{\mathbf{2}}\mathbf{O} {}_{\underset{{\mathbf{K}}_{-\mathbf{2}}}{\leftrightarrow }}{}^{\stackrel{{\mathbf{K}}_{+\mathbf{2}}}{\leftrightarrow }} \mathbf{C}+{\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{+}$$

(6)

CH⁺: fixed charged groups of AEL; K_+1;+2: forward rate constants; and K_−1;−2: backward rate constants.

For the weak acid (AH), the reactions are

$$\mathbf{AH}+{\mathbf{H}}_{\mathbf{2}}\mathbf{O} {}_{\underset{{\mathbf{K}}_{-\mathbf{1}}}{\leftrightarrow }}{}^{\stackrel{{\mathbf{K}}_{+\mathbf{1}}}{\leftrightarrow }} {\mathbf{A}}^{-}+{\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{+}$$

(7)

$${\mathbf{A}}^{-}+{\mathbf{H}}_{\mathbf{2}}\mathbf{O} {}_{\underset{{\mathbf{K}}_{-\mathbf{2}}}{\leftrightarrow }}{}^{\stackrel{{\mathbf{K}}_{+\mathbf{2}}}{\leftrightarrow }} \mathbf{AH}+{\mathbf{OH}}^{-}$$

(8)

These protonation–deprotonation reactions of water splitting can be summarized by the following equation:

$$\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O} {}_{\underset{{\mathbf{K}}_{-\mathbf{2}}}{\leftrightarrow }}{}^{\stackrel{{\mathbf{K}}_{+\mathbf{2}}}{\leftrightarrow }} {\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{+}+\mathbf{2}{\mathbf{OH}}^{-}$$

(9)

2.1.1. Cation Exchange Layer (CEL)

In the theoretical modelling of the bipolar membrane, the CEL is conceptualized as an anionic equivalent to a p-type semiconductor [21,22]. In solid-state physics, a p-type semiconductor is created by introducing acceptor impurities (such as boron in silicon) to generate an electron deficit or mobile holes. By analogy, the CEL acts as a p-type region due to its fixed negatively charged functional groups (̶ SO₃⁻) and mobile positive counter-ions (H⁺) [21,23]. When paired with AEL under reverse bias, mobile electrolyte ions are swept away from the interface, forming a localized space-charge depletion zone. The resulting high electric field at the CEL boundary accelerates catalytic water splitting, allowing H⁺ ions to migrate through the CEL matrix to carry electric current without faradaic gas generation [16,21,22,23]. Different acids usually used in CEL are the groups of sulfonic acid and phosphoric acid groups in rare cases [19]. There are also other CELs based on poly (2,6-dimethyl1,4-phenylene oxide) as the organic component and (3-aminopropyl) triethoxysilane as the modified chitosan [20], inorganic hybridizing agent [24], polyetherketone [25], and sodium alginate [26]. Table 1 shows the types of polymers and ion exchange groups given in the literature.

Table 1. CEL types of BMs [15].

Polymer(s)	Ion Exchange Group	Remarks
Poly-styrene-co-divinylbenzene	Sulfonic acid	Heterogeneous (poly-vinylchloride binder)
Poly-styrene-co-divinylbenzene	Phosphoric acid	Poly-ethylene binder
Nafion	Sulfonic acid	Homogeneous
Grafted perfluorinated polymer membranes	Sulfonic acid	-
Poly-butadiene-co-styrene	Sulfonic acid	-
Poly-phenylene oxide or poly-styrene	Sulfonic acid	Homogeneous
Poly-ether sulfone	Sulfonic acid	Homogeneous
Poly-sulfone	Sulfonic acid	Homogeneous

2.1.2. Anion Exchange Layer (AEL)

The AEL is conceptualized as an n-type semiconductor [21,22]. In solid-state device physics, n-type materials are formed by introducing donor impurities (such as phosphorus in silicon) to supply mobile negative charge carriers. Analogously, the AEL functions as an n-type semiconductor due to its fixed positively charged functional groups (̶ NR₃⁺) paired with mobile negative counter-ions (OH⁻), which act as charge carriers [21,23]. When coupled with a cation exchange layer (CEL) under reverse bias, mobile ions are pulled away from the junction interface, creating a space-charge depletion region. The strong electric field generated at the AEL boundary drives catalytic water dissiciation, facilitating the migration of OH⁻ through the AEL matrix while preventing faradic gas production [16,21,22,23]. Quaternary ammonium groups on a polystyrene matrix are present in the majority of AELs of commercial BMs [16]. Tertiary and secondary amines as well as various diamines are examples of anionic groups. Table 2 shows different types of anion exchange membranes reported in the literature [15].

Table 2. AEL types of BMs.

Polymer(s)	Ion Exchange Group	Remarks
Poly-styrene-co-divinylbenzene	Tertiary and quaternary amines	Cross-linked resin, heterogeneous
Poly-sulfone	Di-amines	Cross-linked homogeneous
Poly-sulfone	Quaternary amines	Homogeneous
Poly-vinylidene fluoride blend with poly-vinyl benzyl chloride	Different diamines	Cross-linked
Poly-ether sulfone	Quaternary amines	Homogeneous
Poly-methyl methacylate-co-glycidyl methacrylate	Quaternary amines	-

2.2. CEMs and AEMs in the BMED Process

In BMED, CEMs and AEMs operate alongside bipolar membranes (BPMs) under an applied electric field to achieve simultaneous ion separation, acid/base generation, and chemical recovery. CEMs and AEMs are made of semi-permeable polymers with immobilized ionic functional groups added to the polymer matrix to impart perm-selectivity toward ions of opposite charge [27]. CEMs have negatively charged functional groups, while IEMs have positively charged functional groups [28]. The mobile ions called counter-ions have opposite charges of fixed group charges on the polymer. The mobile ions called co-ions have the same charges as that of fixed groups [29]. The fixed functional groups of CEMs consist of sulfonate/sulfonic acid, phosphonic acid, phosphoric acid ester, carboxylic acid, and various sulfonamides/sulfonimides [27]. CEMs preferentially allow the transport of positively charged ions (counter-ions, such as Na⁺, K⁺, Ca²⁺, and Mg²⁺) while repelling negatively charged species (co-ions) via electrostatic repulsion [27].

For AEMs, the positively charged functional groups embedded within the matrix polymer of the membrane typically include primary, secondary, and tertiary amines; quaternary ammonium; imidazolium; pyridinium, or phosphonium, tertiary sulfonium; and quaternary phosphonium and cobaltocenium groups [27,30]. Commercially, the commonly functional groups used in AEMs are quaternary ammonium groups, while those used in CEMs are sulfonic acid and carboxylic acid groups. However, quaternary ammonium fixed groups of AEMs can undergo degradation via nucleophilic displacement (SN₂) or Hoffmann elimination reactions (E₂) when AEMs regularly operate in alkaline local medium during BMED. Consequently, recent developments focus on ether-free backbones and sterically hindered cationic groups to enhance alkali durability [27]. Figure 2a,b show migrations of cations and anions through the cation exchange membrane and anion exchange membrane, respectively, under an electrical field.

Figure 2. (a) Migration of cations through the cation exchange membrane; (b) migration of anions through the anion exchange membrane.

2.3. Membrane Fouling and Scaling Mecanisms in BMED

Bipolar membrane electrodialysis (BMED) has emerged as an efficient, chemical-free technology for salt conversion, ion separation, and resource recovery [4]. However, operational efficiency is fundamentally constrained by membrane deposition, comprising organic and biological fouling alongside inorganic scaling [4]. Fouling involves the deposition of insoluble biomass, colloids, and organic macromolecules such as proteins, carbohydrates, humic acids, and surfactants onto or within the membrane matrix [14,31,32]. Conversely, scaling refers to the surface precipitation or pore crystallization of sparingly soluble salts and oxides such as CaCO₃, CaSO₄, Mg(OH)₂, Ca₃(PO₄)₂, Fe₂O₃, SiO₂, and Al₂O₃), present in the feed stream [14,31,32]. Over operational time, the accumulation of foulants and scalants forms a surface gel layer on the membrane [33]. The gel layer undergoes an increase with an increase in charge accumulation, subsequently becoming thinner, denser, and increasingly resistive [32]. This accumulation escalates electrical resistance, causes substantial hydrodynamic pressure losses, impairs flow distribution, and induces irreversible physical damage to the membrane structure [34]. High feed salt concentrations and operation at elevated current densities significantly accelerate these phenomena [32,33].

The specific mechanisms of deposition are highly membrane-dependent, driven by unique electrostatic interactions and extreme local pH gradients characteristic of BMED:

AEMs: AEMs are exceptionally vulnerable to organic fouling compared with CEMs [35,36]. Organic matter (OM), humic substances, proteins, polysaccharides, and surfactants predominantly carry net negative charges in neutral to alkaline BMED streams [31]. Strong electrostatic attraction between these anionic species and the positively charged functional groups of the AEM matrix drives rapid surface adhesion and internal pore blocking, leading to drastic resistance increases and flux decline [4,31].

CEMs: While CEMs resist negatively charged organics due to electrostatic repulsion [35], they are predominantly impacted by inorganic scaling, colloidal matter, and cationic or amphoteric proteins [31,32,37]. In the presence of multivalent and alkaline cations (Ca²⁺, Mg²⁺, Ba²⁺, Fe³⁺) and species like sulfate and bicarbonate, severe concentration polarization and local alkalinity near the membrane interface induce supersaturation. This leads to the surface crystallization of sparingly soluble inorganic scales and oxides such as CaCO₃, CaSO₄, Mg(OH)₂, Ca₃(PO₄)₂, Fe₂O₃, Al₂O₃, and SiO₂ forming dense layers that cause significant ohmic loss and structural damage [31,38,39].

BPMs: BPMs face unique threats within their catalytic intermediate junction layer. Fine colloidal particulates, heavy metals, suspended silicates, and organic foulants can penetrate or adhere to the bipolar interface [39]. Solute deposition inside this interfacial layer impedes water diffusion, deactivates catalytic water-splitting sites, sharply increases stack operational voltage requirement, and can trigger irreversible structural delamination under high current densities [40]. The extent and severity of BMED membrane degradation depend on the interplay between foulant physical–chemical properties, membrane surface charges, and feed solution characteristic.

2.4. Prevention and Mitigation of Membrane Deposition

To mitigate membrane deposition and preserve long-term stack performance during BMED operation, several proactive and reactive strategies have been developed, ranging from feed pretreatment to advanced cleaning methods:

Feed pretreatment and hydrodynamic optimization: The preliminary treatment of the feed solution using ultrafiltration (UF), microfiltration (MF), or activated carbon adsorption prior to the BMED process effectively removes suspended solids, colloids, and macromolecules [4]. Operationally, maintaining current densities below critical thresholds prevents rapid field-induced aggregation and minimizes concentration polarization at membrane surfaces [4,32].

Pulsed electric fields (PEFs) and fluid-loop swapping: To mitigate concentration polarization and membrane deposition in BMED without alternating current direction, non-stationary electrical regimes and hydraulic switching are employed: Pulsed electric fields (PEF) replace continuous direct current by applying active pulse intervals (T_on) alternated with zero-current pause periods (T_off), allowing the boundary layer near the membrane–solution interface to relax and homogenize during pauses [41,42]. This temporary relaxation suppresses localized pH spikes and concentration polarization, allowing electrostatically bound organic foulants (such as proteins or humic acids) and scaling precursors to detach under tangential cross-flow shear forces before forming an irreversible gel layer, especially when coupled with higher feed flow rates that promote electroconvective mixing [41]. In parallel, compartment fluid-loop swapping utilizes automatic external manifold valves to periodically alternate hydraulic feed channels (swapping concentrate/diluate or acid/base streams) without altering the directional electrical potential across the bipolar membranes, thereby exposing accumulated scalants to alternating pH and ionic strength regimes to dissolve early-stage inorganic precipitates in situ [43].

Chemical cleaning: The selection of a chemical compound to be applied for the purpose of membrane cleaning is contingent upon the membrane’s type, the intensity of the contamination, and the nature of the pollutants present. The following chemicals are typically advised by membrane producers: weak and strong acid solutions, alkali solution (NaOH), cleaning agents (detergents), complexing agents (EDTA), disinfectants (H₂O₂, NaOCl), and enzymes [36]. However, this chemical cleaning triggers secondary membrane degradation to AEMs, CEMs, and BMs, proving that a perfect anti-fouling solution remains elusive. Consequently, recent research is shifting toward chemical free, reagentless alternatives, focusing on pretreatment, pulsed electric fields (PIFs), optimized hydrodynamic channel design, and optimized hydrodynamic channel design and long-lasting membrane surface modifications [44]. Hence, the choice of cleaning strategy must consider membrane chemical tolerance, foulant composition, and the degree of fouling irreversibility [36]. A summary of typical foulants, their charge characteristics, and corresponding mitigation/cleaning strategies is provided in Table 3.

Table 3. Foulants and cleaning methods of IEMs [31,32,41].

Foulant Type	Foulant Formula	Net Charge/Properties	Primary Targeted Membrane	Cleaning Methods
Scale	CaCO3, SrSO4, CaSO4.2H2O, SiO2, BaSO4	Neutral	CEM and BPM (outer cationic layer)	- Adjustment of pH levels - Utilization of citric acid or EDTA
Molecular colloids	SiO2, Fe(OH)3, Al(OH)3, Cr(OH)3	Negative	AEM and CEM	- Pretreatment with MF, UF, higher flow rate - Adjustment of pH levels
Organics	Macromolecules, proteins, whey, polyelectrolytes, humate	Negative at neural or alkaline pH	AEM and BPM (anionic side)	- Pretreatment with MF, UF, activated carbon - Clean with NaOH
Interfacial Foulants	Metal hydroxides, organo-metallic complexes	Variable/Catalytic site blockers	BPM Junction (internal catalytic interface)	Sub-limiting current operation pulsed electric fields (PEF), or non-oxidative chelating/acidic CIP flushes

3. Applications of BMED Process

3.1. Acid and Base Production from Liquid Waste by BMED

The BMED process appears more attractive in wastewater treatment due to its ability to move ions of a salt solution and to convert them into bases and acids (Figure 3). Many previous studies investigated the acid and base recovery from wastewater. Gao et al. [45] examined the recovery of hydrochloric acid and sodium hydroxide from sodium chloride contained in wastewater using a BMED system. They indicated that the initial concentration of sodium hydroxide had little or no effect on the acid production, whereas the current density has a significant impact. BMED has also been used to product and recover organic acids including, citric acid [46], acetic acid [47], lactobionic acid [48], succinic acid [49], malic acid [50], tartaric acid [51], propionic acid [52], gluconic acid [53], vitamin C [54], amino acids [55], formic acid [56], and salicylic acid [57]. The flow sheet depicted in Figure 3 shows the BMED process for acid and base recovery.

Figure 3. Flow diagram of BMED for the recovery of acid and base from liquid waste.

In BMED, faradaic reactions (oxidation and reduction) occur at the outer anode and cathode immersed in an electrode rinse solution, supplying the electric current needed to drive catalytic water splitting and ion migration across the membrane stack [16,58]. At the anode, water undergoes oxidation via the oxygen evolution reaction, yielding oxygen gas and hydrogen ions:

2H₂O → O₂(g) + 4H⁺ + 4e⁻

(10)

This reaction causes local acidification of the anolyte solution. Concurrently, at the negative cathode, water reduction occurs via the hydrogen evolution reaction, producing hydrogen gas and hydroxide ions:

2H₂O + 2e⁻ → H₂(g) + 2OH⁻

(11)

This process leads to local alkalinization in the cathode compartment [58]. To mitigate extreme pH gradients and prevent scaling or corrosion, BMED systems typically use a neutral sodium sulfate solution (Na₂SO₄) rinse, allowing generated H⁺ and OH⁻ ions to neutralize each other [16]. It is worth emphasizing that while these electrode reactions generate gases and alter local electrode pH, the primary production of acid and base products in BMED occurs within the catalytic interface of the BPM via catalytic water dissociation without gas generation [16].

Performance Evaluation of BMED Cells

The performance of BMED cells can be evaluated using different key operational indicators including current efficiency (Ie), specific power consumption (SPC), removal efficiency (Re), and recovery efficiency (β) [59,60]:

$$\mathit{Ie}={\displaystyle \frac{\left(C_i{V}_i-C_f{V}_f\right).96500}{\mathit{Id}.S.t}}$$

(12)

$$\mathit{SPC}={\displaystyle \frac{E{\int }_0^t\mathit{Idt}}{V}}$$

(13)

$$\mathrm{\beta }={\displaystyle \frac{\left(\frac{C_{\mathit{rt}}}{n}\right)}{C_i}}.100$$

(14)

$$\mathrm{Re}={\displaystyle \frac{\left(C_i-C_f\right).100}{C_i}}.100$$

(15)

where Ie: current efficiency; SPC is the specific power consumption (kWh/m³); Re: removal efficiency (%); β: recovery efficiency (%); E: electrical potential (V); I: electrical current (A); V: volume of the sample compartment (m³); t: time (s); C_i: initial concentration of the sample compartment (mg/L); C_f: final concentration of sample solution (mg/L); C_rt: final concentration in recovery compartment (mg/L); V_i: initial volume of sample solution (L); V_f: final volume of sample solution (L); n: ratio of initial volume of sample to initial volume of acid and base compartments; Id: current density (A/m²); and S: active membrane area (m²).

Chen et al. [59] suggested a novel technology called BPM selective electrodialysis (BMSED) to separate Na⁺/Ca²⁺ and Cl⁻/SO₄²⁻ ions found in concentrated saline solution. This process consists of a monovalent AEM and a monovalent CEM, which only permit monovalent ions to migrate through in order to produce a high purity acid/base. The results of this study have shown that the purity of HCl and NaOH were both close to 99.9% [59]. Table 4 shows acids and bases produced from different salt solutions and liquid waste using different BMED systems.

Table 4. Acid and base production from liquid waste and salt solutions by BMED.

BMED System	Wastewater Type	Membrane Properties				Acid Production	Base Production	References
BMED: BM-AEM-CEM	Saline textile wastewater (Na2SO4)	Characteristics	BM	AEM	CEM	66.9% of SO42− converted in H2SO4	72.2 of Na+ converted in NaOH	[60]
		Thickness (μm)	-	140–160	180–210
		Surface resistance (Ω/cm2)	-	3.5–4.5	2.5–3.5
		Transfer number	-	> 0.93	> 0.93
		IEC (meq/g)	-	0.5–0.6	0.8–1.0
BMED: BM-AEM-CEM	NH4Cl and NaCl	Thickness (μm)	200–350	160–200	160–200	60% of Cl- converted in HCl	80% of NH4+ and Na+ converted in NH4OH and NaOH	[61]
		Surface resistance (Ω/cm2)	-	2	2.5
		Transfer number	>0.95	>0.95	>0.95
		Water splitting voltage (%)	0.8–1	-	-
BMED: BM-CEM-AEM	RO brines	Resistance Ω(cm2)	-	<8	<9	60–90% of HCl	60–90% of NaOH	[62]
		Transfer nº counter-ion	-	>0.98	>0.98
		Water splitting efficiency (%)	> 99	-	-
		Exchange capacity (mval/g)	-	1.8	2.2
		Burst strength (kg cm−2)	6	-	-
RO-ED-BMED	Cold-rolling wastewater	Tickness (μm)	160–230	120–180	140–200	2.35 M H2SO4	2.03 M NaOH	[63]
		Ion-exchange capacity (meq g −1)	-	1.5–1.8	1.5–1.8
		Area resistance (Ω cm2)	-	2.0–3.5	1.8–3.8
		Permeaselectivity (%)	-	>96	>96
BMED: BM-AEM-CEM	Solid waste residue (CH3COONa)	Thickness (mm)	0.18–0.20	0.11–0.15	0.13–0.15	87.7% of CH3COOH	99.0% of NaOH	[64]
		Burst strength (MPa)	-	0.16	0.16
		Area resistance (Ω cm2)	<3	1.5−3.0	2.5–3.0
		Transport number (%)	-	>96	>96
		Chemical stability (pH)	1–14	-	-

3.2. Nutrients Separation and Recovery from Wastewater by BMED

The growing urban, rural, and industrial water supply increase the amount of wastewater containing different contaminants and nutrients [2]. The management of this wastewater has become one of the environmental and economic challenges. The contamination of surface waters and groundwater by these nutrients is a serious problem around the world. Excessive amount of nitrogen and phosphorous in water causes methemoglobinemia in infants [65] and eutrophication [66], respectively. Ingesting an elevated amount of nitrogen in drinking water leads to its bacterial reduction to nitrite (NO₂⁻) in the gastrointestinal tract, which oxidizes functional ferrous iron (Fe²⁺) in hemoglobin to ferric iron (Fe³⁺); this forms methemoglobin, impairing oxygen transport and causing infantile methemoglobinemia (blue baby syndrome) alongside severe tissue hypoxia [67,68]. On the other hand, the over-enrichment of receiving water bodies with these reactive nitrogen and phosphorus species accelerates the proliferation of harmful algal blooms (HABs); as these dense blooms die, their microbial decomposition severely depletes dissolved oxygen, triggering widespread aquatic hypoxia, mass fauna mortality, and systemic water ecosystem degradation [67,68].

BMED is an electro-membrane process which has been used to remove or recover these nutrients from liquid waste to avoid this problem. The separation and recovery of ammonium (NH₄⁺) and phosphate ions (HPO₄²⁻, H₂PO₄⁻) into high-value chemicals (NH₄OH} and H₃PO₄) can be achieved using a three-compartment bipolar membrane electrodialysis (BMED) configuration composed of alternating BPM, AEM, and CEM stacked between electrodes in a repeating unit (Figure 4). Under an applied electric field, electrocatalytic water splitting at the intermediate layer of BPM continuously generates protons (H⁺) into the adjacent acid compartment and hydroxyl ions (OH⁻) into the base compartment. Concurrently, selective electro-migration drives NH₄⁺ cations from the feed stream across the CEM into the base compartment, where they recombine with generated OH⁻ to produce aqueous ammonium hydroxide.

NH₄⁺ + OH⁻ → NH₄OH

(16)

Figure 4. Flow diagram of BMED for the simultanous separation and recovery of nutrients.

Simultaneously, phosphate species migrate across the AEM into the acid compartment, where H⁺ drives stepwise protonation (Equations (1) and (2)), effectively concentrating and transforming the recovered nutrients into phosphoric acid (H₃PO₄) and ammonium hydroxide (NH₄OH) streams.

HPO₄²⁻ + H+ → H₂PO₄⁻

(17)

HPO₄²⁻ + H+ → H₂PO₄⁻

(18)

3.2.1. Nitrogen Separation and Recovery from Wastewater by BMED

BMED has been utilized frequently in recent years to separate and recover ammonia from aqueous waste. Li et al. [69] treated simulated NH₄Cl of wastewater by BMED and recovered HCl and NH₃ solutions. Their findings showed that the energy consumption remained constant at 19 MJ/kg of nitrogen, whereas the ammonia removal efficiency ranged from 85% to 91%. Zhang et al. [70] have investigated the effects of humic acid, Mg²⁺, and Ca²⁺ on the extraction of ammonium from a leachate, and their findings showed a significant effect with more than 86% of ammonium removal. Gao et al. [66] recovered ~77% nitrogen by a hybrid electrochemical process that combines membrane capacitive deionization (CDI) and BMED. Wu et al. [11] used BMED with a monovalent anion exchange membrane to separate and recover NH₄ ⁺, PO₄³⁻, and NO₃⁻ from wastewater. They indicated that the separation rates of NH₄⁺, PO₄³⁻, and NO₃⁻ increased with current density. At 1.0 mA/cm², the separation rates of NH₄⁺ and NO₃⁻ after 140 min were 100% and 100%, while their recoveries were 79.1% and 34.2%, respectively. They explained that this difference in the separation and the concentration rates is caused by membrane adsorption during the process.

Recently, Cao et al. [71] recovered 78.3% of nitrogen (NH₄⁺) from simulated wastewater by using BMED. They also noted that the separation and recovery rates of NH₄⁺ increase both with time and current density. By increasing the current density, the amount of energy consumed steadily rises. The separation and recovery rates of ammonium carried out using different types of BMED processes depend on the characteristics of ion exchange membrane, sample solution, treatment conditions, and the configuration of the stack. Table 5 shows different separation and recovery rates of nitrogen reported in the literature.

Table 5. Separation and recovery of nitrogen from wastewater using BMED.

System	Wastewater Type	Membrane Characteristics				Nitrogen Separation Rate (%)	Nitrogen Recovery Rate (%)	References
Proposed BMED	Simulated wastewater	Characteristics	BM	AEM	CEM	96.5	78.3	[71]
		Thickness (mm)	0.16–0.23	0.12–0.20	0.16–0.23
		Surface resistance (Ω/cm2)	2.0–2.8	1.5–2.0	1.8–2.4
		Transfer number (%)	95–99	>98	90–95
		IEC (meq/g)	2.0–2.8	1.5–2.0	1.8–2.4
SBMED		Thickness (μm)	170–260	420	50.8	100	79.1	[11]
		Exchange capacity (mol/kg)	-	2.2	0.95–1.01
	Synthetic wastewater	Selective transmission coefficient (%)	-	90	-
		Water decomposition efficiency (%)	>98	-	-
		Rupture strength (MPa)	0.4–0.7	0.6
		Moisture content (%)	-	42	5 ± 3.0
BMED-MCDI	Synthetic wastewater	Membrane type	Fumasep FBM	Neosepta AMX	Neosepta CMX	-	77.0	[66]
		Membrane area (cm2)	17.5	17.5	17.5
		Thickness (mm)	0.5	0.5	0.5
BMED (five cells): BM-AEM-CEM	Leachate	Thickness (μm)	180–220	90–100	140–150	86.17	72.6	[70]
		IEC (meq/g)	0.8–1.0	0.8–1.0	0.8–1.0
		Moisture content (%)	45–55	15–20	35–40
		Surface resistance (Ω·cm2)	-	2.5–3.5	1.5–2.5
		Number of migrations	-	>0.94	>0.98
BMED: BM-AEM-CEM	Simulated NH4Cl wastewater	Thickness (μm)	0.24–0.26	0.24–0.26	0.3	-	63.6	[69]
		IEC (meq/g)	-	1.6–1.7	2±0.02
		Water content (%)	-	20–24	35
		Surface resistance (Ω·cm2)	8–9	7–10	≤4.5
		Number of migrations	-	≥96	>98
		Burst strength (×105 Pa)	2.5–3.0	-	-
BMED: 5 Cell pairs (BM-CEM) and end AEM	Product of wastewater treatment plant	Thickness (μm)	1.1	0.5	0.6	95	87.0	[72]
		IEC (meq/g)	-	≥2.4	≥2.1
		Surface resistance (Ω·cm2)	-	7.0	10.0
		Transport Number(%)	-	82%	95%
		Burst strength (kPa)	>2206.3	1034.2	1034.2
		pH operation range	1–14	<10	1–14
BMED: BM-CEM	Dewatering centrate and low-grade sulfuric H2SO4	Thickness (μm)	1.1	0.5	0.6	-	88.4	[73]
		IEC (meq/g)	-	≥2.4	≥2.1
		Surface resistance (Ω·cm2)	-	7.0	10.0
		Transport Number(%)	-	82%	95%
		Burst strength (kPa)	>2206.3	1034.2	1034.2
		pH operation range	1–14	<10	1–14
Two-stage BMED	Pig manure hydrolysate	Thickness (mm)	0.4	<0.45	<0.45	-	86.0	[74]
		Swell difference (%)	<22	<50	<50
		Area resistance (Ω·cm2)	-	< 8	<8
		Transport number (%)	-	>95	>95
		Selectivity (%)		>90	>90

3.2.2. Phosphorus Separation and Recovery from Wastewater by BMED

BMED has attracted increasing attention as a sustainable approach for nutrient recovery, particularly in the context of phosphorus scarcity and the development of circular economy strategies for wastewater management [4,11]. It has been demonstrated through experimental studies that BMED is capable of effectively separating and concentrating phosphate ions (PO₄³⁻) from dilute wastewater streams. This, in turn, has been shown to improve the feasibility of subsequent phosphorus recovery processes, such as struvite precipitation or phosphoric acid production [11]. Furthermore, BMED has been successfully applied to the recovery of phosphorus from various aqueous matrices, including synthetic wastewater and industrial by-products, thereby highlighting its versatility and resource recovery potential [75].

Characteristics such as ion-exchange capacity, membrane selectivity, electrical resistance, water dissociation efficiency, and chemical stability directly affect phosphate transport and energy requirements. The utilisation of high-selectivity membranes has been demonstrated to reduce the co-transport of competing ions and enhance phosphorus concentration in the recovery compartment [4].

Additionally, the performance of BMED in terms of phosphorus recovery is strongly dependent on the stack configuration. The arrangement of BMs, AEMs, and CEMs determines ion migration pathways and phosphorus selectivity. As demonstrated by Wu et al. [11], BMED configuration incorporating selective membrane compartments was found to be an effective means of separating phosphate from competing ions, such as nitrate. This process served to enhance phosphorus enrichment, thus facilitating its subsequent recovery as struvite. The authors observed phosphorus transport efficiencies above 80%, thus demonstrating that stack design plays a critical role in nutrient separation. Table 6 shows different separation and recovery rates of phosphorus, membrane properties, BMED configuration, and samples used as reported in the literature.

Table 6. Separation and recovery of phosphorus from wastewater using BMED.

BMED System	Wastewater Type	Membrane Properties				Phosphorus Separation Rate (%)	Phosphorus Recovery Rate (%)	Refere-nces
Tree-compartment-BMED: BM-AEM-CEM-BM	Synthetic wastewater imitating the liquid fraction of animal manure	Characteristics	BM	AEM	CEM	-	96	[76]
		Thickness (mm)	<0.50	<0.70	<0.70
		Specific conductivity (mS/cm2)	-	109.7	117.3
		Water splitting voltage	1.20	-	-
		IEC (meq/g)	2.0–2.8	1.5–2.0	1.8–2.4
Two-stage BMED	Pig manure hydrolysate	Thickness (mm)	0.45	<0.45	<0.45	-	75	[74]
		Swell difference (%)	<22	<50	<50
		Area resistance (Ω·cm2)	-	<8	<8
		Transport number of K+ (%)	-	>95	>95
		Selectivity (%)		>90	>90
Three-compartment BMED-MC-CP	Synthetic AD dew atering sidestream comprising 50 mM NH4Cl, 50 mM NaCl, and NaH2PO4 (0.5–4.0 mM)	Membrane type	Fumasep FBM	Fumasep FAS-PET-75	Fumasep FKB-PK-75	-	> 80	[10]
		Thickness (μm)	340	340	340
		Membrane area (cm2)	29.2	29.2	29.2
BPMED-MCDI	Synthetic wastewater	Membrane type	Fumasep FBM	Neosepta AMX	Neosepta CMX	~89	-	[66]
		Membrane area (cm2)	17.5	17.5	17.5
		Thickness (mm)	0.5	0.5	0.5
BMED with ion exchange absorption T-52H Resin	Synthetic solution of NaH2PO2 solution	Membrane type	Fumasep FBM	Neosepta AHA	Neosepta CMB	-	80.06	[77]
		IEC (mequiv·g−1)	-	1.4–1.7	1.5–1.8
		Area resistance (Ωcm2)	<3.0	2.0–3.5	1.8–3.8
		Thickness (mm)	0.20	0.12–0.18	0.14–0.20
		Transport number (%)	98	95	97
BMED combined with RA	Triethylammonium phosphate wastewater	Effective area of the membrane (cm 2)	21 × 7	21 × 7	21 × 7	-	96	[78]
BMED: BM-AEM-CEM	Electroless nickel plating (ENP) wastewater	Effective membrane area (cm2)	12	12	12	99.3	94.5	[79]
BMED with magnesite tailings added to the TTC	Simulated aquaculture wastewater containing NH4Cl, NaH2PO4 and NaCl	Surface area (cm2)	37.5	37.5	37.5	-	97.9	[80]
		Distance of between different membranes (cm)	5	5	5
BMED: BM-AEM-CEM	Synthetic solution (Na3PO4·12H2O, Na2HPO4· 12H2O and NaH2PO4·2H2O)	Membrane type	Neosepta BP-1	Neosepta AMX	Neosepta CMX	95.8	80.3	[81]
		Thickness (μm)	200–350	120–180	220–260
		IEC (meq·g−1)	-	1.4–1.7	1.5–1.8
		Area resistance (Ω cm2)	-	2.0–3.5	2.0–3.5
		Voltage drop (V)	1.2–2.2	-	-
		efficiency (%)	>98	-	-

Consequently, phosphorus recovery by BMED is not governed by a single factor but rather by the combined effects of membrane stack configuration, membrane characteristics and wastewater composition, and process operating conditions, all of which must be carefully optimized to achieve high separation efficiency, phosphorus purity, and recovery performance.

4. Conclusions

BMED demonstrates strong potential as a chemical-free, energy-efficient technology for the targeted separation and recovery of nitrogen and phosphorus from complex wastewater streams. Critical analysis of reported performance metrics reveals that the separation and recovery efficiencies of nitrogen (NH₄⁺) and phosphorus (PO₄³⁻, HPO₄²⁻, H₂PO₄⁻) are strongly dictated by stack configuration, operational parameters, and membrane selectivity. In terms of nitrogen removal/separation, efficiencies range between 85 and 100%, with final recovery yields ranging from 63.6% to 88.4% in real and synthetic matrices (such as digestate centrate and swine manure hydrolysate) at specific energy consumptions around 19 MJ/kg-N. On the other hand, phosphate species migrate across anion exchange membranes into the acid compartment, where generated H⁺ drives stepwise protonation to form phosphoric acid (H₃PO₄), reaching separation rates above 95% and recovery efficiencies of 75% to 97.9%. However, long-term process viability is fundamentally constrained by targeted degradation mechanisms. AEMs are selectively vulnerable to organic fouling (humic acids, proteins) via electrostatic attraction and alkaline degradation (SN₂ displacement and E₂ Hofmann elimination), whereas CEMs suffer from inorganic scaling (CaCO₃, Mg(OH)₂, Ca₃(PO₄)₂) driven by severe boundary-layer concentration polarization and localized alkalinization. The internal catalytic junction of the BM risks irreversible delamination and voltage spikes upon colloidal or heavy metal penetration. To preserve long-term stack performance without reversing electric field polarity, reagentless mitigation strategies, specifically pre-filtration (UF/MF), pulsed electric fields (PEFs) to promote boundary-layer relaxation and shear detachment, and compartment fluid-loop swapping, provide sustainable alternatives to chemical cleaning, which itself induces secondary membrane degradation. Hence, BMED offers a robust circular economy pathway for transforming wastewater nutrients into high-purity chemical resources (NH₄OH and H₃PO₄). Future research would focus on developing alkali-stable ion-exchange membranes and optimizing non-stationary electrical regimes (PEF).