0714_Volume Reduction of High-Silica RO Concentrate

1. Title and journal
"Volume Reduction of High-Silica RO Concentrate Using Membranes and Lime Treatment."
University of Texas at El Paso; Desalination and Water Purification Research and Development Program Report No. 108.

2. Background of the author
Anthony J. Tarquin, Ph. D. is professor for civil engineering at the University of Texas.  His focus is on water treatment and, to some extend, on desalination.

3. Summary
The research objective is a new option for RO-brine management, i.e. volume reduction of the brine (RO reject concentrate) that is produced.
Option A
Sea Water - RO concentrate     - NF                    - RO - product water
Option B
Sea Water - RO concentrate     - Lime treatment - RO - product water

In both options the initial desalination process is reverse osmosis.  The water intake source in the case of this study is a brackish ground water source with high levels of silica in the Southwestern United States.  Due to fouling problems large volumes of RO concentrate are created.  The maximum intake water silica concentration in this study was 140 mg/l.

In Option A, the membrane process could reduce the volume by up to 55%.  Although the NF was capable of high recoveries the overall recovery was limited by fouling occurring in the second RO of the NF permeate.

In Option B, the lime treatment was very effective in silica removal, with the silica removal appearing as a first order reaction.  The silica removal was more effective at high pH.  A pilot plant for the lime treatment has shown recoveries of 60%, however much higher recoveries are expected after optimization of the process.

A cost analysis has been performed, comparing the best "throw away" option (deep-well injection) with the contemplated process.  The cost analysis showed that this brine treatment technology can lead to substantial cost reductions for the overall process: "At a lime dosage of 750 mg/L, there would be a net savings of $1.6 million per year compared to injection, and 5 million gallons per day (0.218 cubic meters per second) of project water would be produced at a cost of $1.43 per thousand gallons ($0.375 per cubic meter)."

4. Originality and Contribution to the lab's work
The solubility of silica tends to increase at above pH 9.  However effects of adsorption can lead to co-precipitation during the lime treatment.  Which one of these two contradicting processes is the dominant one has clearly been shown by this study.  The silica can be removed by lime treatment.
More general, the idea of reducing overall cost by brine management techniques as opposed to brine disposal is very interesting for the desalination field.

5. Reviewer contact information
Michael Hoyer, Michael.hoyer@student.tu-freiberg.de

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