From Concentration to Resource Recovery: The Core Role of Electrodialysis in Industrial Wastewater Zero Liquid Discharge (ZLD)

Driven by the dual-carbon targets and national wastewater resource recovery strategy, zero liquid discharge (ZLD) of industrial wastewater has become an inevitable technical route for water-intensive and heavily polluting industries. The core principle of ZLD is straightforward: fully separate clean water from pollutants in wastewater to realize water reuse, while pollutants are treated as solid residues for disposal or resource recovery. Nevertheless, implementing ZLD faces prominent technical bottlenecks, especially for high-salinity wastewater which constitutes the bulk of industrial effluents. Maximizing brine concentration at minimal energy cost is the decisive factor governing the economic viability of the entire ZLD system.

Against this backdrop, electrodialysis (ED) technology stands out thanks to its unique technical merits and is gaining an increasingly central position in full-scale ZLD processes.

1. Necessity of Electrodialysis for ZLD Systems

Conventional ZLD processes mainly adopt the combined process of reverse osmosis (RO) + evaporative crystallization. RO delivers preliminary desalination and concentration, yet its concentration ratio is constrained by osmotic pressure. Once the total dissolved solids (TDS) of wastewater rise to a threshold, further boosting operating pressure triggers skyrocketing power consumption and raises risks of irreversible mechanical damage to RO membranes. Consequently, large volumes of under-concentrated brine are fed into evaporators. Evaporation is universally recognized as the most energy-intensive and costly unit operation across the whole ZLD workflow.

Electrodialysis precisely fills the concentration gap between RO and thermal evaporation. Unlike pressure-driven RO, ED relies on an applied electric field to drive directional ion migration, enabling high-fold concentration under ambient temperature and atmospheric pressure with far higher tolerance to influent salinity. Installing ED as an intermediate stage after RO and ahead of evaporative crystallization drastically cuts down the brine volume sent to evaporators without excessive incremental energy input.

2. Core Functions of Electrodialysis in ZLD Projects

(1) Deep Brine Concentration

Deep concentration represents ED’s primary function in ZLD facilities. Brine concentrated via pre-treatment processes such as RO flows into ED stacks for secondary or even tertiary concentration, pushing salinity close to saturation before thermal crystallization. This tiered "membrane concentration + thermal concentration" design minimizes the feed volume of high-energy evaporation units, which has become the mainstream engineering paradigm for modern ZLD projects.

(2) Selective Salt Separation and Resource Valorization

Many industrial wastewaters contain mixed dissolved salts. Direct crystallization of mixed salts yields low-value mixed solid waste that must be handled as hazardous waste with substantial disposal costs. Selective electrodialysis separates ions of different valence states under electric potential. For instance, selective ED effectively segregates corrosive chloride ions (Cl⁻) and sulfate ions (SO₄²⁻) in steel mill wastewater. The separated high-purity saline streams can be further refined and recycled as industrial raw materials.

Furthermore, bipolar membrane electrodialysis (BPED) directly converts waste brine into corresponding acids and alkalis. These regenerated chemicals can be recycled on-site for pH adjustment, ion exchange resin regeneration and other production procedures, achieving genuine resource recovery from saline wastewater.

3. Technical Limitations & Practical Engineering Considerations

Electrodialysis is not a universal all-in-one solution. Stable and efficient ED operation requires strict pre-treatment of influent: suspended solids, scale-forming hardness ions and organic contaminants must be removed to meet influent standards; otherwise, severe membrane fouling and scaling will occur and degrade stack performance. Although ED accommodates higher influent TDS compared with RO, excessively high initial salinity reduces current efficiency and weakens concentration performance. Accordingly, process parameters must be customized based on site-specific water quality data. In real-world engineering applications, ED is integrated with chemical softening, nanofiltration (NF), RO, evaporative crystallization and other treatment units into a complete integrated ZLD system, where each unit undertakes dedicated functions and operates synergistically.

4. Conclusion

The core objective of industrial wastewater ZLD is to strike a balanced trade-off between environmental compliance and economic sustainability. The core advantage of electrodialysis lies in substituting a portion of high-cost thermal energy consumption with relatively low-cost electrical energy, enabling cascaded energy utilization throughout the whole ZLD system. With continuous advancement in ion exchange membrane materials and accumulation of field engineering experience, ED has evolved from an optional auxiliary module to an indispensable core unit within ZLD workflows. It cannot eliminate all wastewater treatment challenges, but it perfectly addresses the critical bottleneck of volume reduction for high-salinity brine. For enterprises planning or constructing ZLD facilities, mastering and properly deploying electrodialysis technology is a pivotal step to turn the conceptual goal of zero liquid discharge into feasible industrial practice.