Fouling in electrodialysis (ED) during resource recovery

• Main Author: Hew, Gong Qi
• Other Authors: She Qianhong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/150147
• Institution: Nanyang Technological University

Electrodialysis (ED) is a cutting-edge membrane technology that is widely deployed in water desalination and resource reclamation. Fouling is the emerging issue that arises in the ED application, and have attracted much research effort to be devoted to curbing this issue to diminish the concomitant consequence. This project was devised to study one of the fouling types, scaling, during the struvite formation to recover NH4+, Mg2+ and PO43- ions via ED application. Domestic wastewater was synthetically prepared with only selected major chemical compounds, such as NH4Cl, KH2PO4, NaHCO3, MgCl2*6H2O and CaCl2*2H2O to deeply delve into the relations of such chemical compositions on scaling. The experiments were staggered into three phases, namely pre-concentration of domestic wastewater, base production to increase the pH and lastly the struvite formation. The current density applied throughout the experiments is slightly larger than the limiting current density to investigate its effect on the fouling behaviour. In the pre-concentration process, the electrolyte was directed in different paths: from cathode to anode (configuration 1) and from anode to cathode (configuration 2). Scaling was discovered at the cathode of the first configuration, as OH- ions produced from the half-reactions provided a basic condition for scaling. Especially at the anode in configuration 2, the acidic condition was present owing to the H+ ions from the half-reactions, which was unfavourable for scaling. The anode of configuration 1 and cathode of configuration 2 displayed no sign of scaling as OH- was consumed or H+ were generated at the stated anode and cathode respectively, which did not contribute to scaling. Therefore, a change in electrolyte’s flow direction is a preferred measure to contain scaling. Additionally, the change in morphology of CEMs and AEMs surface before and after experiments were studied by Field Emission Scanning Electron Microscopy (FESEM) to look into the scaling on membrane surface. The FESEM result shows that for both configurations, the scaling is not serious on diluate side of CEM surface but obvious scaling were formed at concentrate side of AEM surfaces. The change in flow direction of electrolytes does not influence the scaling on IEMs as both membrane morphologies are similar in configuration 1 and 2. On top of that, the use of current density higher than the limiting current density do not cause severe scaling at the electrodes. Moreover, the effectiveness of pre-concentration was found to be unaffected by the flow direction of electrolyte, as both of them achieved similar pre-concentration and dilution ratio for domestic wastewater. Nevertheless, the pre-concentration efficiency in this project is relatively low, but it can be adopted on an industrial scale. The base production process using BPED is also believed to be capable of elevating pH with bipolar membrane, as evidenced by its increasing rate of change of OH- ion concentration. Results and analysis from this were served as a reference on the occurrence of scaling during struvite formation.