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 CIX-mended 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.