From Challenges to Opportunities: Exploring Minimum Liquid Discharge and Zero Liquid Discharge Strategies for Wastewater Management and Resource Recovery

Zero liquid discharge (ZLD) and minimum liquid discharge (MLD) have garnered substantial global interest as these are aimed at eliminating liquid waste and improving water usage efficiency in the wastewater. The adoption of ZLD and MLD presents advantages such as reduced water pollution and enhanced water supply. However, their implementation is hindered by high costs and significant energy consumption. This review is primarily focused on reviewing various studies demonstrating wastewater treatment and resource recovery using ZLD and MLD technologies. ZLD technologies aim to achieve 100% water recovery through the utilization of both membrane-based and thermal-based processes. On the other hand, MLD is a promising alternative that enables up to 95% water recovery, primarily relying on membrane-based technologies. The conventional ZLD and MLD systems based on thermal evaporation exhibit significant energy consumption per unit volume of treated effluent. However, the introduction of membrane-based technologies helps to pre-concentrate the effluent prior to thermal evaporation, thereby reducing the load on evaporators. The membranes employed in ZLD and MLD must possess specific characteristics capable of withstanding high organic and total dissolved solids concentrations, as well as operating under elevated pressure, while simultaneously mitigating fouling. This review critically examines the use of ZLD and MLD technologies in wastewater management and resource recovery, elucidating their unique properties, performance attributes, and prospects for future research. Additionally, it has been observed that novel approaches such as osmotically assisted reverse osmosis and low-salt-rejection reverse osmosis are explored as emerging alternatives for ZLD applications.