Evaluating Pretreatment Strategies with Modeling for Reducing Scaling Potential of Reverse Osmosis Concentrate: Insights from Ion Exchange and Activated Alumina

Reverse osmosis concentrate (ROC) treatment is critical for enhancing water recovery, especially in regions facing water scarcity. The high concentrations of total dissolved solids (TDS), hardness (Ca2+ and Mg2+) and silica, present significant challenges for ROC treatment due to their high scaling potential. Developing effective pretreatment strategies, such as ion exchange (IX) and activated alumina (AA), to mitigate scaling is essential for improving water recovery. This study investigates the effectiveness of dif-ferent ion exchange (IX) resins and activated alumina (AA) in removing hardness and silica from brackish water ROC, through batch isotherm studies, continuous column experiments, regeneration, impact of operating conditions, model simulation, and pre-liminary costs analysis. The ROC is characterized by TDS 12,445 mg/L, Ca²⁺ 847 mg/L, Mg²⁺ 212 mg/L, and silica 160 mg/L. Hardness removal was evaluated using two types of strong acid cation (SAC) IX resins and silica removal using three types of strong base anion (SBA) IX resins and three types of AA. While IX was effective in removing hardness and silica during the initial stages of column experiments, hardness breakthrough occurred at 17–33 bed volumes (BV) for SAC and 5 BV for silica using SBA, due to their high con-centrations in ROC. In contrast, AA demonstrated superior and sustained silica removal, reducing silica concentration in ROC by up to 65% and maintaining performance for up to 800 BV without reaching saturation. Model simulation of a secondary RO treating ROC after the IX and AA pretreatment indicated an additional water recovery of ~70% using antiscalants. While this study demonstrates the potential for achieving higher water recovery, significant challenges remain, including limited capacity of IX resins for ROC treatment, which necessitates frequent regeneration and adds operational costs, as well as the restricted regeneration capacity of AA. These findings emphasize the critical need for developing advanced materials and optimized strategies to enhance the efficiency of ROC treatment processes.