A Novel Process for Concurrent Desalination and Boron Removal

A Novel Process for Concurrent Desalination and Boron Removal

Süer Kürklü¹, Sadiye Velioglu^1,2, M. Göktug Ahunbay¹, S. Birgül Tantekin-Ersolmaz¹, William B. Krantz^3,4

¹Department of Chemical Engineering, Istanbul Technical University, Turkey

²Department of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore

³Department of Chemical and Biological Engineering, University of Colorado, U.S.A.

⁴Singapore Membrane Technology Center, Nanyang Technological University, Singapore

Desalination of saline water is a critical technology for supplying the water needed for the rapidly increasing world population and to sustain economic development, particularly in the agricultural industry. The boron content of seawater averages 4.6 ppm and is as high as 9.6 ppm in the Eastern Mediterranean. A small amount of boron is required for all plants; however, it affects the yield and can become toxic to plants at levels ranging between 0.5 and 20 ppm. The World Health Organization (WHO) recommends that the boron concentration in drinking water and irrigation water be 0.5 ppm or less. Since boron is present in water as boric acid, its concentration cannot be reduced adequately by single-stage reverse osmosis (SSRO) using commercially available reverse osmosis (RO) membranes. This can be done using commercially available RO membranes by using two or more SSRO stages in series at the cost of reduced water recovery. Increased water recovery is possible by raising the pH that causes boric acid to dissociate into hydrated borate ions for which commercially available RO membranes have a good rejection. However, this requires adding chemicals and an additional process step to reduce the pH again. Desalination followed by ion exchange or dialysis can also reduce the boron concentration at the cost of increased process complexity. None of these processes can achieve desalination and boron removal with a high water recovery using just commercially available membranes.

This paper describes the Concurrent Desalination and Boron Removal (CDBR) process for water desalination and reduction of the boron concentration to 0.5 ppm or less using commercially available membranes.  The saline water is fed to one or more SSRO stages in series whose retentate is fed to a countercurrent membrane cascade with recycle (CMCR) and whose permeate is fed to a low pressure membrane stage (LPMS) for further boron removal. The permeate from the LPMS is blended with that from the CMCR to obtain the desired product water. This process configuration employs countercurrent flow of the retentate and permeate, permeate recycle, and retentate recycle in the CMCR to reduce the required osmotic pressure differential (OPD) significantly. This reduces the pumping, maintenance, pretreatment and brine disposal costs, and can provide a highly concentrated brine that can be used as the draw solution for Pressure-Retarded Osmosis (PRO) to harvest the osmotic potential energy in the brine.

The performance metrics for this novel CDBR process are compared with those for using two SSRO stages in series for CDBR. The assumptions are a seawater feed containing 35,000 ppm salt and 10 ppm boron producing water containing 0.5 ppm boron and less than 100 ppm salt and, pump and energy recovery device (ERD) efficiencies of 85% and 90%, respectively. For these conditions the CDBR process employing commercially available RO membranes with a 90% boron rejection can reduce the OPD relative to two SSRO stages in series by 35% at a water recovery of 55% and 37% at a recovery of 75%, and can reduce the net specific energy consumption (SEC_net) by 0.87% at 55% recovery and 39% at 75% recovery.

(This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) through Project No. 114Y165.)