A pre-treatment concept for increasing the recovery ratio of coastline BWRO plants, while providing Mg2+in the product water

The maximal recovery ratio of brackish water RO desalination plants located on the coastline can be significantly increased by exchanging Ca2+ with Mg2+ and Na+ ions in the raw water by passing the water through a cationic ion-exchange (CIX) resin which is thereafter regenerated with seawater. The concept allows also enriching the product water with Mg2+, at no additional cost. The Ca2+ fraction (equivalent-based) attached to the CIX resin at equilibrium with seawater was merely -10% of its total capacity, much lower than Na+ (59%) and Mg2+ (28%). The process was simulated on the Sabha-Eilat BWRO plant (Israel) water with the goal of reducing [Ca2+] from -850 to -450 mg/L using a breakthrough/seawater-regeneration sequence. The goal was met by applying a 60 bed-volume (BV) breakthrough-step followed by 20-BV seawater regeneration (HRT = 6 min). -2% of the CIXmended water bypassed the RO step to provide 10 mg Mg/L in the product water. Simulating the maximal recovery ratio showed that it could be raised to 89.2%. A general design and cost-analysis revealed that the cost of each additional m3 of desalinated water produced by the new method was -$0.23, -14% more water is produced and retentate production is -50% lower.

Automated summary

The study focused on increasing the maximum recovery ratio of brackish water RO desalination plants by using a cationic ion-exchange resin to exchange Ca2+ with Mg2+ and Na+ ions in the raw water, and regenerating it with seawater. Through simulations, the goal of reducing [Ca2+] and enriching the product water with Mg2+ was achieved at no additional cost. The maximal recovery ratio was shown to be raised to 89.2% with a cost reduction of -$0.23 per additional m3 of desalinated water produced.