A review of desalting process techniques and economic analysis of the recovery of salts from retentates

Disposal of saline effluent from desalination plants, agricultural drainage water, and other industries is an increasing problem worldwide. Saline effluent has long been considered waste brine and traditional approaches to its disposal have included evaporation ponds, deep wells, and coastal discharge. However, such effluent is now being considered as a saline resource; therefore, alternative approaches have been developed to extract available salts and to recover purified water. In this study, techniques to dispose of rejected brine were investigated and alternative salt recovery methods that have been developed and widely employed were reviewed. Evaporation and cooling to extract salts from retentate has been widely used worldwide, but use of the membrane separation technique has been increasing rapidly owing to the development of cheaper and higher performance membranes. Electrodialysis, ion-exchange, eutectic freezing, and chemical processing can also be used to recover salts from retentate. Currently, hybrid systems that combine two or more techniques, such as the nanofiltration (pretreatment)-reverse osmosis (concentration)-thermal processes (crystallization), are being developed and used actively to increase the amount of extracted salt and reduce the final volume of rejected brine. Economic analysis of concentrate utilization in comparison with disposal was performed in this study. Based on the cost of producing salts, the multi-stage flash (MSF) distillation and electrodialysis and Dow chemical process were relatively expensive when compared to nanofiltration and membrane crystallization, evaporation, and ion-exchange techniques. However, technical developments for increasing the performance efficiency must be further investigated to reduce the cost of desalting processes. (C) 2010 Elsevier B.V. All rights reserved.