Reverse electrodialysis and its hybrid processes for energy-efficient desalination

Abstract

Seawater desalination is widely applied to mitigate the worldwide water scarcity. However, there are still several major challenges, such as the high-energy cost and brine disposal, that need to be further addressed. Controlled mixing of the brine from desalination and treated wastewater in reverse electrodialysis (RED) brings us a new opportunity of energy efficient and environment friendly desalination. This study aims to explore various RED-based hybrid systems for its synergies with the desalination technologies and the enhanced power generation by combining with chemical reactions. The practical constraints of such hybrid systems and the underlying mechanisms are investigated. Furthermore, the potential impacts of membrane chemical cleaning on the membrane properties and the corresponding RED performance are also studied.

Co-locating RED with reverse osmosis (RO) desalination plant was first studied in two different configurations that RED served as the post- or pre-treatment step of the desalination process, respectively. In the former case, the warm feed stream and concentrated brine was demonstrated to significantly enhance the power generation of RED through increasing ionic conductivity, especially for low salinity stream (LS), and electrical driving force, respectively. Furthermore, adjusting the salinity of LS to an intermediate concentration within 0.01 - 0.02 M led to the maximum power density value, attributing to the trade-off between high ionic conductivity and lower electrochemical potential. In the latter case, RED was validated to effectively reduce the salinity of high salinity stream (HS) by 47 %, regarding the equal volumes of HS and LS.

In addition to co-locating RED and desalination facilities, another way of incorporating RED with chemical reactions (e.g., acid and base neutralization) within one reactor was explored to tailor higher salinity gradient for higher power generation. This novel chemical reaction enhanced RED process is named as RED neutralization (i.e., REDn) to differentiate from conventional RED process (i.e., REDc). HCl and NaOH solutions were used as the model acid (AS) and base (BS) in REDn, of which each unit was composed of alternatively arranged AS and BS compartments divided by a neutralization (NS) compartment. Compared to REDc using common NaCl salt solutions, the power density of REDn was almost doubled due to the remarkably enhanced electrical driving force provided by H+/OH-, resulting from their greater salinity gradient achieved by the neutralization reaction in NS. The characterization of ionic transportation in REDn validated that the ionic uphill transport and severe concentration polarization hindered the power performance of REDn. One magnitude higher power density can be obtainable through addressing these issues.

Furthermore, a mathematical model was established on a novel self-sufficient zero energy reverse electrodialysis (REDD) process basing on the combination of reverse electrodialysis and electrodialysis within a single reactor. This model was subsequently verified by lab-scale experiments and applied for investigating the impacts of operation conditions and membrane properties on the desalination performance of REDD. The practical constraints of the trade-off between the generated freshwater quality (i.e., rejection) and quantity (i.e., recovery) was validated, whereas the specific values varied with the operation conditions. Both the raised salinity gradient and higher volumetric ratio in REDcell can offer an enhanced electrical driving force for desalination, leading to simultaneously increased rejection and recovery. Moreover, using ion exchange membranes with lower permselectivity (e.g., a permselectivity valued of around 0.3) resulted in a further improvement on the desalination performance, through maintaining a desirable electrical driving force for the longer time.

Finally, membrane chlorination experiments were performed to investigate the impacts of chemical cleaning (i.e., chlorine exposure) on the membrane chemistry and properties, as well as its performance in RED stack. Membrane characterization results validated that chlorine attacked C-Cl bonds, leading to side chain cleavage of functional group, and thus significant reduction in membrane permselectivity and ionic conductivity. In addition, anion exchange membrane was demonstrated to be more prone to chlorine attack that it suffered the emergent of more extensive cracks and deterioration of properties.

In summary, this study explored a series of potential applications of RED through incorporating with desalination processes and chemical reactions. Both experimental and modelling works were performed to validate the synergies of the RED-based hybrid processes and reveal the underlying mechanisms governing their performance. It offers us the theoretical basis for designing an RED-based hybrid system for self-sufficient desalination and enhanced power generation.