Development of electrically conductive membrane distillation for zero liquid discharge by mitigating inorganic scaling
Membrane distillation (MD) is a promising technique in water treatment, known for perfect removal efficiency, low energy use, and low sensitivity to feed salinity. However, high recovery rates lead to challenges like foulant concentration and scaling. Electrically conductive membrane (ECM) technology has been applied to address this challenge. Unlike conventional physical and chemical fouling control techniques, the ECM utilizes electrochemical mechanisms in conjunction, providing excellent fouling mitigation performance and synergistic effects with conventional fouling control techniques, which can lead to further performance improvement. ECM can mitigate fouling through mechanisms such as preventing contact with the membrane or transforming contaminants into a state with low fouling potential by leveraging the electrical properties of pollutants. ECM technology was applied to MD for electrically conductive membrane distillation (ECMD) operation, and scaling inhibition during operation overcame the concentration limit of the conventional process, resulting in a dramatic improvement in final water recovery. Another concept of ECM, electrochemical cleaning (EC), has been proposed by applying it to membrane scaling cleaning, showing excellent performance without efficiency degradation even after repeated cleaning cycles at scaling stage 3. An extensive review of ECM cases currently used for fouling control was performed in this study. New challenges were identified for the practical application of ECM technology in fouling control, such as a new understanding of foulant characteristics from an ECM perspective, optimal membrane material selection, and customized ECM operation conditions optimization (Chapter 3). Furthermore, fundamental research on MD scaling mechanisms was conducted for accurate understanding prior to applying ECM technology to MD scaling control, resulting in identifying a new scaling stage (Chapter 4). Subsequently, an evaluation of the applicability of ECM technology for actual MD scaling control was performed (Chapters 5, 6). This study demonstrated the feasibility and development of ECMD AC operation and EC application for MD scaling control. The transition to ECM technology overcomes the limitations of conventional physical and chemical approaches and increases water productivity and energy efficiency in MD operation by more than two times using new electrochemical mechanisms. As a result, the excellent potential of ECM technology for MD scaling control has been sufficiently confirmed. However, as ECM technology is in its early stages of research and development, additional studies, such as optimization of operating conditions and modules, are necessary for practical application.
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