The Global Rise of Zero Liquid Discharge for Wastewater Management
Freshwater scarcity, one of the most critical global challenges of our time, poses a major threat to economic growth, water security, and ecosystem health. The challenge of providing adequate and safe drinking water is further complicated by climate change and the pressures of economic development and industrialization. The public and industrial sectors consume substantial amounts of freshwater while producing vast quantities of wastewater. If inadequately treated, wastewater discharge into the aquatic environment causes severe pollution that adversely impacts aquatic ecosystems and public health.
Zero liquid discharge (ZLD)—a wastewater management strategy that eliminates liquid waste and maximizes water usage efficiency — has attracted renewed interest worldwide in recent years. Although the implementation of ZLD reduces water pollution and augments water supply, the technology is constrained by high cost and intensive energy consumption.
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In recent years, greater recognition of the dual challenges of water scarcity and pollution of aquatic environments has revived global interest in ZLD. More stringent regulations, rising expenses for wastewater disposal, and increasing value of freshwater are driving ZLD to become a beneficial or even a necessary option for wastewater management. The global market for ZLD is estimated to reach an annual investment of at least $100–200 million, spreading rapidly from developed countries in North America and Europe to emerging economies such as China and India.
Early ZLD systems were based on stand-alone thermal processes, where wastewater was typically evaporated in a brine concentrator followed by a brine crystallizer or an evaporation pond. The condensed distillate water in ZLD systems is collected for reuse, while the produced solids are either sent to a landfill or recovered as valuable salt byproducts. Such systems, which have been in successful operation for 40 years and are still being built, require considerable energy and capital.
Reverse osmosis (RO), a membrane-based technology widely applied in desalination,has been incorporated into ZLD systems to improve energy and cost efficiencies. However, RO, although much more energy efficient than thermal evaporation, can be applied only to feedwaters with a limited salinity range. Accordingly, other salt-concentrating technologies that can treat higher salinity feedwaters, such as electrodialysis (ED), forward osmosis (FO), and membrane distillation (MD), have emerged recently as alternative ZLD technologies to further concentrate wastewater beyond RO.