(686b) Concurrent Desalination and Boron Removal Via Reverse Osmosis

Desalination of saline water is a critical technology for supplying the potable and irrigation water needed for the rapidly increasing world population. 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 a two-pass SSRO 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. (Krantz, 2016; Kürklü, 2017)

In the CDBR process, 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 reflux in the CMCR to reduce the required osmotic pressure differential (OPD) significantly. This reduces the pumping, maintenance, pretreatment and brine disposal costs. The performance metrics of the process have been predicted for operation at the thermodynamic limit.

The performance metrics for this novel CDBR process are compared with those for using two SSRO stages in series for desalination and boron removal. 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. The SEC_net and OPD can be further reduced by recycling and blending the retentate of the LPMS stage with the feed to the SSRO.

A total cost analysis of water production for the CDBR configurations is also carried out in terms of the fixed and operating costs, relative to a two-pass SSRO process. In the comparison, both processes are designed using commercial membranes to yield a water product with less than 0.5 ppm boron concentration from a saltwater feed containing 10 ppm boron and 35000 ppm TDS. While the two-pass RO process yields an overall water recovery of 46%, the yield of the CDBR process is 61%. The gross specific energy consumption of the CDBR process 4.44 kWh/m³ in comparison to 4.76 kWh/m³ for the two-pass RO process. If energy recovery devices are used, then the net specific energy consumption of the CDBR process is reduced to 3.51 kWh/m³ in comparison to 3.19 kWh/m³ for the two-pass RO process. The cost comparison is based on a water production capacity of 10000 m³/day.

A novel CDBR process for achieving concurrent desalination and boron removal has been developed. The CDBR process uses only membrane technology without the need for increasing the pH or using other technologies such as ion exchange that are required for current commercial processes for desalination and boron removal. As such, the CDBR process permits concurrent desalination and boron removal rather than requiring sequential processes.

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

References

Krantz, W.B., S. Velioğlu, S. Kűrklű, M.G. Ahunbay, S.B. Ersolmaz, “Concurrent Desalination and Boron Removal (CDBR) Process,” Patent application no. PCT/TR2016/050387, October 19, 2016.

Kürklü, S.; Velioglu, S.; Ahunbay, M. G.; Tantekin-Ersolmaz, S. B.; Krantz, W. B., Concurrent Desalination and Boron Removal (CDBR) Process, Desalination, 423,79-94, 2017.