Scaling formation and mitigation during electrodialysis for desalination and nutrient recovery

• Main Author: Liu, Hong
• Other Authors: She Qianhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering
• Online Access: https://hdl.handle.net/10356/176491
• Institution: Nanyang Technological University

Electrodialysis (ED) stands as a promising and versatile separation technology, flexible to customization for diverse applications such as desalination, nutrient recovery, wastewater treatment, energy generation, and so on. Despite its wide-ranging utility, ED is not devoid of challenges. In particular, the formation of scaling emerges as an obstacle, significantly impacting the performance of ED. Addressing this concern constitutes a key area of focus for future advancements in this field. In this work, we investigated conventional ED desalination of seawater and revealed the formation of scaling in the conventional ED stack (ConED) undergoes 3 phases. In phase 1 during the early stage of ED desalination, divalent cations (e.g., Mg2+ and Ca2+) transport through the CEM into the cathode chamber and accumulate in the electrolyte solution, and meanwhile, OH− ions are generated via water electrolysis at the cathode, both of which synergistically increase the tendency of electrode scaling. In phase 2 after a period of desalination, fast reactions between divalent cations and OH− ions result in the occurrence of scaling in the cathode chamber. Under the typical constant-current operating mode, scaling on the CEM surface facing the cathode reduces effective membrane area, which leads to an increase in the local current density through the CEM. In phase 3, when the local current density exceeds the limiting current density, water splitting occurs on the surface of the CEM facing the dilute chamber, which induces the generation of OH− and thereby enhances the crystallization on this surface. Eventually, scaling in the cathode chamber further enhances scaling on ion exchange membranes in the adjacent dilute chamber. The mechanisms of scaling formation and evolution provide important implications for scaling mitigation during ED desalination. Subsequently, three ED stack designs were designed to prevent the transport of scaling precursor cations from the treated solution to the electrode solution for mitigating scaling in the electrode system in ED. The BED stack and the SED stack were designed by employing a bipolar membrane and a monovalent selective cations exchange membrane in the ED stack to form the cathode chamber, respectively. Unfortunately, both the BED and SED configurations failed to prevent cathode scaling formation due to the transport of divalent cations into the cathode solution. However, a novel EDM stack design, which introduced two mediating solution (MS) chambers near each electrode chamber in the ED stack, can effectively prevent the formation of scaling. In the EDM stack, there were two anion exchange membranes between the introduced MS chambers and the dilute/concentrate solution chambers, respectively, which impeded the migration of cations between the MS and the dilute/concentrate solutions. Thus, the mediating solution chambers successfully blocked the transport of divalent cations from the dilute/concentration into the electrode solution, thereby resulting in the absence of scaling. Accordingly, the gap between the specific energy consumption (SEC) for the non-scaling test and the scaling test by the EDM configuration was the lowest among all these four configurations (ConED, BED, SED, EDM) due to the absence of scaling formation, with SEC at 4.55 and 5.06 kJ/(mS/cm) for the non-scaling test and the scaling test, respectively. Our results suggest that the EDM stack design is a promising strategy for electrode scaling mitigation. Furthermore, the investigation was extended to the formation and mitigation of scaling during electrodialysis for nutrient recovery. Remarkably, electrodialysis exhibited applicability for the simultaneous recovery of ammonium and phosphate from dewatered centrate to a stripping solution, achieving removal ratios of approximately 85% for ammonium and 60% for phosphate. The EDM configuration effectively avoided the ammonium leakage observed in conventional electrodialysis (ConED), but enhanced scaling formation on the membrane surfaces exposed to the stripping solution (SS). Subsequently, the SEDM configuration was designed by substituting CEMs in the EDM by monovalent selective cation exchange membranes, resulting in the enhancement in the recovery of both ammonium and phosphate. This achievement can be attributed to the reduced transport of multivalent cations, effectively slowing the rise in their concentration within the stripping solution, and thereby deferring the onset of pronounced scaling. Consequently, at the end of the nutrient recovery process, the SEDM displayed a lower specific energy consumption (8.7±1.4 kWh/(kg NH4+-N) and 26.4±3.8 kWh/(kg phosphate)) than the ConED and the EDM, suggesting the enhancement in energy efficiency. In sum, this thesis elucidates the mechanism of scaling during electrodialysis for seawater desalination. Furthermore, this research also presents that the optimization of the electrodialysis performance was obtained through the prevention of electrode scaling by introducing mediating chambers. Moreover, this thesis highlights the potential of electrodialysis to recover ammonium and phosphate simultaneously from dewatered centrate, and the ED performance was further optimized by scaling mitigation through the stack design. This work contributes to the improvement of electrodialysis and facilitates the industrial applications of electrodialysis process.