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Selective Salt Recovery from Reverse Osmosis Concentrate Using Interstage Ion Exchange

Project: 06-10 (Phase E)
Year Released: 2013
Type: Report

Program: Principal
Funding Partners: Bureau of Reclamation, California Department of Water Resources, Sandia National Laboratories
Total Investment: $354,157.55 (Cash: $279,157.55, In-Kind: $75,000)

Principal Investigator: Kerry J. Howe, University of New Mexico

Background

Concentrate management is a major concern for inland users of reverse osmosis (RO) because of cost and environmental regulations. A seawater RO facility may be able to discharge its concentrate stream into the ocean, but inland brackish water RO facilities must use alternative disposal options. Traditional concentrate disposal methods include discharge to surface water, evaporation, discharge to wastewater treatment plants, and deep well injection. High costs are driving RO users to look for better options, such as the recovery of salts for beneficial reuse. The sale of such products may subsidize the RO process and the reduction in fouling potential allows water to be treated further by RO or another process, resulting in increased water recovery.

Goals and Objectives

The project tested the use of sequential cation and anion exchange between two RO stages. The goals of interstage ion exchange are (1) to remove sparingly soluble salts from RO concentrate so that it can be treated by another RO stage without scaling, and (2) to concentrate sparingly soluble salts during ion exchange column regeneration so that they will spontaneously precipitate when cation and anion exchange regeneration solutions are mixed.

Research Approach

Task 1. Literature Review: Conduct a thorough literature review (e.g., peer reviewed, grey literature, international literature). Identify common desalination concentrate salts and characterize and parameterize the range of concentrates. Identify target salts by their potential marketability.

Task 2. Develop Recovery Processes: Develop and test optimized or new approaches needed to provide better separations of saleable salts; identify chemical reaction parameters, determine scope and limits of process application.

Task 3. Construct Desalination Concentrate Salt Recovery Model: Construct and test, either from scratch or building on previous programs, a well-designed model for salt recovery for any desalination concentrate quality specified.

Task 4. Evaluation of Purification Techniques and Model: Perform lab and bench-scale results to compare new separations techniques against existing ones. Evaluate purification techniques, including practicality, scalability, and applicability. Verify the developed models.

Task 5. Analysis: Consider the addition of scale inhibitors in the desalination process and how this will modify the recovery processes. Assess potential hazardous trace contaminants or other impurities that, upon concentration, could limit use of recovered salts or require expensive additional selective removal. Develop techniques that maximize the value of the product and minimize requirements. Consider applicability to different scale installations. Identify potential practical demonstration projects.

Findings and Conclusions

The study concludes that sequential ion exchange has the potential to generate salts from RO concentrate and to increase water recovery from RO systems. Bench-scale tests showed that ions of interest can be recovered from higher-ionic-strength solutions using ion exchange, despite some selectivity decrease. Modeling and regression relationships developed from batch isotherms tests can be used to predict breakthrough curves. Calcium and magnesium selectivity are too similar for ion separation by regeneration variation, but gypsum can be recovered by mixing cation and anion regeneration solutions from an optimized system.

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