Electrodialysis (ED): It uses an "ion sieve" to separate salt from water in an electric field, and then employs bipolar membranes to convert the salt back into acid and alkali.

Electrodialysis (ED): Using an "Ion Sieve" to Separate Salt from Water in an Electric Field, Then Bipolar Membranes to Convert Salt Back into Acid and Base

Reverse Osmosis (RO) relies on pressure to "strain" out salt and struggles with high salinity; evaporation crystallization relies on steam to "boil" out salt and consumes significant energy. Electrodialysis (ED) takes a third path—using ion exchange membranes and a direct current electric field to make salt ions "migrate" themselves into the concentrate compartment, separating water and salt at ambient temperature and pressure in one step; Bipolar Membrane Electrodialysis (BMED) goes further, hydrolyzing inorganic salts directly into their corresponding acids and bases. This article draws on peer-reviewed journals and multiple real-world engineering accounts to clarify the mechanisms, parameter windows, and current efficiency/energy consumption in one go.

Contributed by Gaowutong · Industrial Water Treatment Technology Series · For Industry Technical Personnel · All Data Referenced to Public Literature and Engineering Sources

First, let's draw the line—ED is not "just another pressure-driven membrane." RO/NF/UF rely on pressure differentials, acting like sieves to block ions; ED's driving force is a direct current electric field, relying on the selective permeability of ion exchange membranes: cation exchange membranes allow only cations to pass, while anion exchange membranes allow only anions to pass. Anion and cation membranes are arranged alternately, salt ions are "driven" by the electric field into the concentrate compartment, and water remains in the diluate compartment, thus separating salt and water at ambient temperature without phase change. BMED adds a bipolar membrane (one side anion, one side cation, with the interface in between dissociating water into H⁺ and OH⁻) to the membrane stack, enabling the one-step conversion of salt into acid and base—this is the key leap from ED's "desalination" to "acid/base production."

1. Principle: Ion Exchange Membranes + DC Electric Field, Alternating Diluate and Concentrate Compartments

The core device of ED is the membrane stack: dozens to hundreds of pairs of anion and cation exchange membranes stacked alternately, with the compartments formed between adjacent membranes; electrodes (anode/cathode) are at both ends, connected to a DC power supply. Under the electric field:

Cations (Na⁺, etc.) migrate toward the cathode, passing through cation exchange membranes but being blocked by anion exchange membranes, ultimately accumulating in the "concentrate compartment";
Anions (Cl⁻, SO₄²⁻, etc.) migrate toward the anode, passing through anion exchange membranes but being blocked by cation exchange membranes, also entering the concentrate compartment;
Salt in the diluate compartment is continuously "removed," and the outlet stream is low-salinity fresh water; salinity in the concentrate compartment continuously increases.

In engineering practice, ED is often coupled with RO: RO first removes the bulk of the salt, then the RO brine is fed to ED for further concentration and volume reduction, improving water recovery while lowering overall energy consumption. Membrane stacks typically contain 200–400 pairs of membranes with a membrane spacing of 0.5–2 mm; electrodes are commonly made of corrosion-resistant materials such as titanium coated with ruthenium/iridium, and the operating voltage per membrane pair is generally controlled at 0.8–1.5 V, with a current density of 100–300 A/m² being appropriate.

The operating mode of ED directly determines the concentration limit and energy consumption: batch mode (feed solution circulates through the membrane stack until the target is reached, then is discharged) suits small to medium scales; overflow mode (small concentrate tank with circulating overflow for gradual concentration) suits medium to large scales; continuous mode (feed passes through the membrane stack once) requires multiple membrane stages to achieve sufficient desalination and suits large-scale industrial applications. Multi-stage/overflow modes can suppress the concentration difference between diluate and concentrate compartments and limit water migration, enabling NaCl concentration up to the order of 18%–20%—this is precisely why ED can continue as the "backup concentrator" after RO.

Electrodialysis (ED) desalination membrane stack mechanism cross-section: alternating anion/cation exchange membranes, DC electric field, directional ion migration, diluate and concentrate compartments — Fig. 1 ED desalination mechanism: anion/cation exchange membranes arranged alternately, electric field drives salt ions into the concentrate compartment while fresh water remains in the diluate compartment, at ambient temperature without phase change (Illustration by Gaowutong)

2. Key Operating Parameter Windows

Extensive pilot and industrial studies have largely mapped out ED's "operating envelope." Below is a set of practical ranges compiled from multiple literature sources and engineering projects:

Core Parameter Windows (Multi-source Summary): Voltage per membrane pair 0.8–1.5 V, current density 100–300 A/m²; desalination rate 30%–99%, water recovery 40%–90%; membrane stack 200–400 pairs of membranes, membrane spacing 0.5–2 mm; maximum NaCl concentration up to 18%–20%; homogeneous membranes offer lower resistance and better selectivity than heterogeneous membranes (heterogeneous membranes exhibit more pronounced co-ion leakage). Industry-standard operating energy consumption is approximately 3–8 kWh/m³ (RO is approximately 15–25 kWh/m³; this comparison is based on manufacturer/encyclopedia data, see the verification note at the end of the article), membrane life 3–5 年, electrodes approximately 5–8 年.

0.8–1.5V Operating voltage per membrane pair

100–300A/m² Current density range

30–99%Salt Rejection Rate (by stage/mode)

40–90%Water Recovery Rate

Current density is not necessarily better the higher: the closer it approaches the "limiting current density," the more severe the concentration polarization at the membrane surface and the greater the water migration (freshwater being "carried" by the electric field into the concentrate), which instead dilutes the concentration effect and increases energy consumption. The limiting current density given by the ED pilot test for printing and dyeing wastewater is only about 11.57 m A/cm² (corresponding to a limiting current of 25 A), and sufficient margin must be reserved in actual operation.

III. Real Engineering Cost Accounting (ED Desalination/Concentration, All from Public Literature and Engineering Reports)

Industry / Scenario	Process & Key Operating Conditions	Effluent / Performance	Source
Wastewater from retired lithium-battery hydrometallurgical workshop / Full-scale	Pretreatment + ED + evaporation synergy; workshop wastewater conductivity 104.6 mS/cm, Na⁺ 2.3 g/L	Conductivity reduced to 22.09 mS/cm, suitable for leaching recycle; Na⁺ increased to 42.30 g/L; current efficiency 71.41%, energy consumption 0.70 kWh/kg; treatment cost 100→89.89 元/t	Liu Shuifa et al. Environmental Engineering, 2023 Suppl. (S1) (listed in Jiancai Zhihui Net)
Printing and Dyeing Wastewater / Pilot	Effluent from high-density clarifier as feed; limiting current 25 A, limiting current density 11.57 mA/cm², optimal voltage 80 V	Fresh water <1500 μS/cm; desalination rate 78.07%, hardness removal 85.88%, chloride removal 88.50%	Water Purification Technology (National Major Science and Technology Project 2012ZX07101-003)
RO concentrate / Overflow concentration	Current density 350 A/m²	NaCl concentrated to 185 g/L; energy consumption only 0.12 kWh/kg NaCl	Reig et al. (cited in Chemical Industry and Engineering Progress 2019 review)
Coal chemical/coking wastewater advanced treatment / Full-scale	Operating voltage 10 V	Current efficiency 89.2%, energy consumption 1.03 kWh/kg, feed-chamber COD reduced to 42 mg/L; treatment cost approx. 1 元/t (lower than RO)	Li Xing, Gou Mangmang. Improved Electrodialysis for Advanced Treatment of High-Salt Pharmaceutical Wastewater. Water Treatment Technology, 2018, 44(10):106-109 (cited in review)
RO–ED integration / Research	ED-RO recirculation integration	Water recovery up to 95%; operating cost as low as 0.19 EUR/m³	Zhang et al. (cited in Chemical Industry and Engineering Progress 2019 review)
Coal chemical RO concentrate / Homogeneous membrane separation	Self-developed homogeneous anion/cation exchange membranes, multi-stage ED	Conductivity reduced to <10 mS/cm; COD rejection 85.3%–91.4%; salt re-concentrated to 15%–20%	Wang Yaoming et al. (cited in Chemical Industry and Engineering Progress 2019 review)

Supplementary note: ED employs high-density homogeneous ion-exchange membranes that, under an electric field, allow only ions to pass while retaining molecules — enabling effective separation of "salt" from "COD present in molecular form." This is precisely its value over evaporation (which first separates salt and then concentrates) in high-COD, high-salinity wastewater streams (coal chemical, pharmaceutical, pesticide). However, membrane fouling and scaling remain key engineering constraints.

4. Bipolar Membrane Electrodialysis (BMED): Converting Salt Directly into Acid and Base in One Step

BMED introduces bipolar membranes into the membrane stack: its intermediate interface dissociates water directly into H⁺ and OH⁻ under an electric field. The typical three-compartment configuration is "salt compartment | bipolar membrane | acid/base compartment": NaCl (or Na₂SO₄) in the salt compartment is split, with Na⁺ migrating to the base compartment to combine with OH⁻ to form NaOH, and Cl⁻ (or SO₄²⁻) migrating to the acid compartment to combine with H⁺ to form HCl (or H₂SO₄). Thus, mixed salts are no longer low-value solid waste but are converted into reusable acids and bases.

Furthermore, there is Bipolar Membrane Selective Electrodialysis (BMSED): first, ion selectivity is used to separate monovalent salts (NaCl) from divalent salts (Na₂SO₄) in the mixed salt stream, and then the monovalent salt is converted into acid and base using bipolar membranes. This process has achieved mixed salt resource recovery with NaOH 2.2 m mol/L, HCl 1.9 m mol/L, and purity of 99.99% for both (as cited by Chen et al. in the 2019 review in Chemical Industry and Engineering Progress). For "mixed salts" rather than single salt species, BMSED is more aligned with engineering practice than pure BMED.

Bipolar Membrane Electrodialysis (BMED) three-compartment configuration: salt compartment + bipolar membrane water dissociation H+/OH- + acid compartment + base compartment — Fig. 2 BMED three-compartment configuration: the bipolar membrane dissociates water into H⁺/OH⁻, converting salt directly into the corresponding acid and base (illustration by Gaowutong)

Fact Check ①: The theoretical voltage for water dissociation in a bipolar membrane is only approximately 0.83 V, far lower than the cell voltage for electrolyzing brine to produce acid and base (typically above 2–3 V), and the process does not generate chlorine gas — this is the fundamental chemical basis for BMED being "green, energy-saving, and free of chlorine by-products" compared to chlor-alkali electrolysis (as stated in the original text of the 2019 review in Chemical Industry and Engineering Progress). However, the conversion of "salt to acid/base" is not a 100% transformation: proton back-diffusion (H⁺ leakage) and co-ion leakage reduce current efficiency, with engineering current efficiencies typically in the 40%–80% range, so estimates should not assume "complete recovery."

5. The Real Cost-Benefit of BMED (Salt → Acid/Base, All Data from Public Literature)

Feedstock / Configuration	Key Operating Conditions	Products & Efficiency	Source
Sulfanilic acid wastewater / Three-compartment BP-A-C	Current density 40 mA/cm², initial salt 80 g/L, membrane surface velocity 4.0 cm/s	Desalination rate 97%; acid 1.17 mol/L, base 1.18 mol/L; current efficiency 58.9%; energy consumption 1.86 kWh/kg (as NaOH)	Dissertation/Journal (BMED treatment of sulfanilic acid wastewater)
Coking high-salinity concentrate / Three-compartment	Salt compartment conductivity ≥130 mS/cm, current density 800 A/m², single cycle 200 min	Produces 6.9% HCl and 7.5% NaOH; current efficiency 42.0%; total energy consumption 3 kWh/kg NaOH	Guangdong Chemical Industry, 2026, 53(7):14-19
Na₂SO₄ waste solution / Three-compartment constant current	1.30 M Na₂SO₄, 360 A/m²	H₂SO₄ recovery 77.35%, NaOH 75.18%; energy consumption 1.43 kWh/kg; current efficiency acid 51.1%/base 49.7%	Constant-current BMED of Na₂SO₄ (English journal)
Sodium sulfate wastewater / Multi-unit	Current density 70 mA·cm⁻², circulation rate 80 mL·min⁻¹	SO₄²⁻ recovery 94.2%, Na⁺ 71.2%; specific energy consumption 3.10/1.80 kWh·mol⁻¹; current efficiency 52.4%/50.5%	CNKI journal publishing platform (BMED recovery of H₂SO₄ and NaOH)
Salicylic acid wastewater / BMED	Current density 40 mA/cm², flow rate 4.0 cm/s, initial Na₂SO₄ 80 g/L	Desalination rate 96.3%; H₂SO₄ 0.97 mol/L, NaOH 1.56 mol/L; current efficiency acid 55.2%/base 50.2%; energy consumption 2.89/3.98 kWh/kg	Desalination, 2022, 537:115866 (doi:10.1016/j.desal.2022.115866)
Acid mine drainage / EDBM	450 A/m², volume ratio 2:2:2	Current efficiency 93%, specific energy consumption 2.26 kWh/kg NaOH; obtains 0.5 mol/L NaOH and 0.4 mol/L H₂SO₄	Separation and Purification Technology / Elsevier, 2025
Na₂SO₄ pilot / 30 membrane pairs 0.02 m²	20 wt% Na₂SO₄, 750 / 550 A/m²	H₂SO₄ 10.4 wt%, NaOH 7.4 wt%; purity 96.1%/99.6%; energy consumption 1.3 / 0.9 kWh/kg	ScienceDirect, 2025 (pilot-scale BMED)

Economic note: The energy consumption for alkali production via BMED above typically ranges from 1–4 kWh/kg NaOH. At 0.12 USD/kWh, the total treatment cost is approximately 250 USD/ton-Na₂SO₄ (including CAPEX/OPEX, based on the ScienceDirect 2025 pilot-scale assessment). Given the recent upward trend in caustic soda prices, the economic return of BMED by-product acid and alkali must be evaluated in detail based on salt concentration, product purity requirements, and electricity prices — not simply on the ability to "produce acid and alkali."

6. Process Comparison: ED / BMED / RO / MD / Evaporation

Dimension	Electrodialysis (ED)	Bipolar Membrane Electrodialysis (BMED)	Reverse Osmosis (RO)	Membrane Distillation (MD)	Evaporation Crystallization
Driving Force	DC electric field	DC electric field + water dissociation	Pressure difference	Vapor pressure difference (waste heat)	Steam/heat
Core Products	Fresh water + concentrated brine	Acid + base (+ fresh water)	Fresh water + concentrate	High-purity permeate + concentrated solution	Permeate + crystalline salt
Maximum Salt Tolerance	Concentration up to ~18%–20% NaCl	Medium-high salinity (salt chamber concentration limited)	~7% TDS	25%–30%+ TDS	Any (up to crystallization)
Typical Energy Consumption	0.12–1.0 kWh/kg salt (desalination)	1–4 kWh/kg NaOH	~2–4 kWh/t (salt-limited)	Low-temperature heat from single-digit kWh/t	>200 kWh/t (high)
Key Limitations	Polarization/water transport limits concentration; scaling	Acid/base crossover reduces current efficiency/purity	Not tolerant of high salinity; concentrate difficult to dispose of	Low flux, membrane wetting, heat source required	High energy consumption, scaling and corrosion

Positioning in one sentence: ED is the step for "ambient-temperature electric-field desalination and concentration," while BMED is the step for "converting mixed salts into valuable acid and base." Both should be coupled with RO and evaporation/crystallization rather than operating standalone — ED/BMED only make sense when integrated into the chain of "pretreatment → RO → ED concentration → BMED/evaporation crystallization."

VII. 5 Practical Truths That Must Be Watched in Engineering

Truth 1 — Concentration Polarization and Water Migration Cap the Concentration. Concentration polarization raises the membrane surface concentration, and current efficiency drops sharply as current density approaches the limit; meanwhile, "water migration" drags fresh water into the concentrate chamber, diluting the concentration effect. Single-stage ED can concentrate NaCl to approximately 18%–20%; higher concentrations require multi-stage/overflow parallel configurations or transfer to evaporation.

Truth 2 — Scaling Is the Number One Enemy. Ca²⁺, Mg²⁺, SiO₂, and organics can cause scaling and fouling on the membrane surface. If hardness removal pretreatment is not performed before the bipolar membrane, scaling on the cation exchange membrane will directly reduce the base chamber performance — engineering practice generally applies "hardness removal/softening" before ED/BMED (e.g., induced crystallization, ion exchange softening).

Truth 3 — Current Efficiency Is Not 100%. Co-ion leakage (ions that should not pass through the membrane sneaking across) and proton back-diffusion cause ED/BMED current efficiency to fluctuate mostly in the 40%–93% range, as itemized in the accounting above. When designing capacity and calculating acid/alkali production, always use measured current efficiency — do not calculate at theoretical full value.

Truth 4 — BMED Acid-Base Cross-Leakage Reduces Purity. H⁺ readily "leaks" through the cation membrane back into the base chamber, reducing both NaOH purity and current efficiency (the ScienceDirect 2025 pilot study observed that acid chamber current efficiency was slightly higher than that of the base chamber, precisely due to proton back-diffusion). For applications with high product purity requirements (e.g., reuse in production processes), purity must be verified (pilot H₂SO₄ 96.1%, NaOH 99.6%).

Truth 5 — Commercial Data Requires Secondary Verification. Claims by manufacturers/encyclopedias such as "fresh water recovery above 80%, energy consumption 3–8 kWh/m³, saving 30%–50% compared to RO" fall under commercial/encyclopedic claims; patent sources claiming bipolar membrane "acid current efficiency 69%, alkali 80%, energy consumption 5.5/4.8 kWh/kg" are also patent statements. Before formal design, calculate based on actual water quality, membrane model, electricity price, and concentrated brine disposal cost; it is recommended to supplement with manual sources and mark as [To Be Verified].

Industrial high-salinity wastewater membrane desalination salt recovery toolbox positioning: ED desalination and concentration, BMED salt-to-acid/alkali conversion, RO ambient-temperature desalination, MD waste-heat concentration, evaporation crystallization — Fig. 3 Process positioning: ED desalination/concentration / BMED salt-to-acid/alkali / RO ambient-temperature desalination / MD waste-heat concentration / evaporation crystallization — each performing its own role, coupled into a Zero Liquid Discharge chain (drafted by Gaowutong)

VIII. One-Sentence Selection Recommendations

Suitable for: Scenarios with existing RO concentrate/high-salinity wastewater requiring "volume reduction + salt value enhancement," desalination at ambient temperature without phase change, or conversion of mixed salts into reusable acid/alkali (coking/coal chemical/lithium battery recycling/mining/printing and dyeing/pharmaceutical); especially when low-cost electricity is available on site and the salt species is relatively simple — ED/BMED economics are most stable under these conditions.

Not suitable for: Cases requiring high-purity crystalline salt in a single step, or where salt concentration is already near saturation without downstream evaporation as a safety net — in such cases ED is only responsible for "concentration and volume reduction," while crystallization and salt separation must still be handed to evaporation; BMED is only suitable for closed loops where "salt → corresponding acid/alkali has a practical reuse destination," otherwise the produced acid/alkali becomes a burden with nowhere to go.

Figure Notes: This article has generated 3 figures — Fig. 1 Electrodialysis desalination membrane stack mechanism (alternating ion exchange membranes + electric field + diluate/concentrate chambers), Fig. 2 Bipolar membrane three-compartment configuration (salt → acid + alkali), Fig. 3 High-salinity wastewater membrane desalination salt recovery "toolbox" positioning diagram. The figures are schematic/trend graphics based on real mechanisms and literature data, not measured raw charts.

References (Authentic Sources)

Liu Shuifa, Hu Jian, Wu Na, et al. Study on electrodialysis concentration of wastewater from wet recycling workshop of retired lithium-ion batteries. Environmental Engineering, 2023 Supplement (S1) (listed on Building Materials Smart Net). Conductivity 104.6→22.09 mS/cm, Na⁺ 2.3→42.30 g/L, current efficiency 71.41%, energy consumption 0.70 kWh/kg, cost 100→89.89 元/t.
Pilot test of electrodialysis desalination of printing and dyeing wastewater and its influencing factors. Water Purification Technology (National Science and Technology Major Project 2012ZX07101-003). Limiting current 25 A, limiting current density 11.57 mA/cm², optimal voltage 80 V, desalination rate 78.07%, hardness removal 85.88%, chloride removal 88.50%.
Yan Haiyang, Wang Yaoming, Jiang Chenxiao, et al. Application, opportunities and challenges of ion-exchange membrane electrodialysis in "zero liquid discharge" of high-salinity wastewater. Chemical Industry and Engineering Progress, 2019, 38(1):672-681. (Review citing Reig et al. on ED concentration of RO brine, Zhang et al. on RO-ED integration, Wang Yaoming et al. on homogeneous membrane separation of RO concentrate from coal chemical industry, etc.).
Li Xing, Gou Mangmang. Study on advanced treatment of pharmaceutical high-salinity wastewater by improved electrodialysis. Water Treatment Technology, 2018, 44(10):106-109. (Review citing: ED of coking/coal chemical wastewater, operating voltage 10 V, current efficiency 89.2%, energy consumption 1.03 kWh/kg, COD reduced to 42 mg/L, cost approx. 1 元/t).
Treatment of sulfanilic acid wastewater by BMED (dissertation/journal article). Three-compartment BP-A-C: 40 mA/cm², 80 g/L → desalination rate 97%, acid 1.17/base 1.18 mol/L, current efficiency 58.9%, energy consumption 1.86 kWh/kg NaOH.
Study on resource recovery from coking high-salinity concentrate by bipolar membrane electrodialysis. Guangdong Chemical Industry, 2026, 53(7):14-19. Three-compartment: ≥130 mS/cm, 800 A/m², 200 min → 6.9% HCl + 7.5% NaOH, current efficiency 42.0%, total energy consumption 3 kWh/kg NaOH.
Constant-current BMED of Na₂SO₄ waste solution (English journal). 1.30 M, 360 A/m² → H₂SO₄ 77.35%, NaOH 75.18%, energy consumption 1.43 kWh/kg, current efficiency acid 51.1%/base 49.7%.
Recovery of sulfuric acid and sodium hydroxide by bipolar membrane electrodialysis (CNKI journal publishing platform). 70 mA·cm⁻², 80 mL·min⁻¹ → SO₄²⁻ 94.2%, Na⁺ 71.2%, energy consumption 3.10/1.80 kWh·mol⁻¹, current efficiency 52.4%/50.5%.
Liu Y, Sun Y, Peng Z. Evaluation of bipolar membrane electrodialysis for desalination of simulated salicylic acid wastewater. Desalination, 2022, 537:115866 (doi:10.1016/j.desal.2022.115866). 40 mA/cm², 80 g/L → desalination rate 96.3%, H₂SO₄ 0.97/NaOH 1.56 mol/L, current efficiency acid 55.2%/base 50.2%, energy consumption 2.89/3.98 kWh/kg.
Integration of ED with bipolar membranes for acidic mine waters valorisation (EDBM). Separation and Purification Technology / Elsevier, 2025. 450 A/m², 2:2:2 → CE 93%, SEC 2.26 kWh/kg NaOH, 0.5 mol/L NaOH + 0.4 mol/L H₂SO₄.
Pilot-scale bipolar membrane electrodialysis of high-concentration Na₂SO₄. ScienceDirect, 2025. 30 membrane pairs 0.02 m², 20 wt% Na₂SO₄ → H₂SO₄ 10.4 wt%/NaOH 7.4 wt%, purity 96.1%/99.6%, energy consumption 1.3/0.9 kWh/kg.
Application of ion-exchange membrane electrodialysis in zero liquid discharge of high-salinity wastewater (dowater technical review, 2022). Membrane stack 200–400 membrane pairs, membrane spacing 0.5–2 mm, limiting concentration 18%–20% NaCl, bipolar membrane water dissociation voltage lower than electrolysis of brine (no Cl₂ by-product), and other statements as sources.