Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.
Microbial electrodialysis cells (MEDCs) offer simultaneous wastewater treatment, water desalination, and hydrogen production. In a conventional design of MEDCs, the overall performance is retarded by the accumulation of protons on the anode due to the integration of an anion exchange membrane (AEM). The accumulation of protons reduces the anolyte pH to become acidic, affecting the microbial viability and thus limiting the charge carrier needed for the cathodic reaction. This study has modified the conventional MEDC with an internal proton migration pathway, known as the internal proton migration pathway-MEDC (IP-MEDC). Simulation tests under abiotic conditions demonstrated that the pH changes in the anolyte and catholyte of IP-MEDC were smaller than the pH changes in the anolyte and catholyte without the proton pathways. Under biotic conditions, the performance of the IP-MEDC agreed well with the simulation test, showing a significantly higher chemical oxygen demand (COD) removal rate, desalination rate, and hydrogen production than without the migration pathway. This result is supported by the lowest charge transfer resistance shown by EIS analysis and the abundance of microbes on the bioanode through field emission scanning electron microscopy (FESEM) observation. However, hydrogen production was diminished in the second-fed batch cycle, presumably due to the active diffusion of high Cl¯ concentrations from desalination to the anode chamber, which was detrimental to microbial growth. Enlarging the anode volume by threefold improved the COD removal rate and hydrogen production rate by 1.7- and 3.4-fold, respectively, owing to the dilution effect of Cl¯ in the anode. This implied that the dilution effect satisfies both the microbial viability and conductivity. This study also suggests that the anolyte and catholyte replacement frequencies can be reduced, typically at a prolonged hydraulic retention time, thus minimizing the operating cost (e.g., solution pumping). The use of a high concentration of NaCl (35 g L-1) in the desalination chamber and catholyte provides a condition that is close to practicality.
Microbial electrosynthesis (MES) is an emerging electrochemical technology currently being researched as a CO2 sequestration method to address climate change. MES can convert CO2 from pollution or waste materials into various carbon compounds with low energy requirements using electrogenic microbes as biocatalysts. However, the critical component in this technology, the cathode, still needs to perform more effectively than other conventional CO2 reduction methods because of poor selectivity, complex metabolism pathways of microbes, and high material cost. These characteristics lead to the weak interactions of microbes and cathode electrocatalytic activities. These approaches range from cathode modification using conventional engineering approaches to new fabrication methods. Aside from cathode development, the operating procedure also plays a critical function and strategy to optimize electrosynthesis production in reducing operating costs, such as hybridization and integration of MES. If this technology could be realized, it would offer a new way to utilize excess CO2 from industries and generate profitable commodities in the future to replace fossil fuel-derived products. In recent years, several potential approaches have been tested and studied to boost the capabilities of CO2-reducing bio-cathodes regarding surface morphology, current density, and biocompatibility, which would be further elaborated. This compilation aims to showcase that the achievements of MES have significantly improved and the future direction this is going with some recommendations. Highlights - MES approach in carbon sequestration using the biotic component.- The role of microbes as biocatalysts in MES and their metabolic pathways are discussed.- Methods and materials used to modify biocathode for enhancing CO2 reduction are presented.