Forward osmosis pretreatment of seawater for thermal desalination

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Water Filtration Systems: Processes, Uses and Importance, 2016, pp. 1 - 26
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© 2016 Nova Science Publishers, Inc. Thermal desalination technologies are still widely used for seawater desalination. The main advantages of these technologies is that the capability to treat high salinity seawater and product water TDS is not affected by the feed salinity. Nevertheless, thermal desalination suffers from a number of drawbacks including limited feed conversion ratio and high energy consumption. The latter issue is further aggravated by scale development and deposition on the heat exchangers. Scaling problems can be alleviated by dosing antiscalants in the feed water, though this wouldn't prevent the scaling issue over time. Furthermore, Nanofiltration (NF) filtration of seawater is sometimes carried out for the removal of scaling ions from seawater. NF coupling with thermal desalination may prevent scaling problems, but the cost of NF pretreatment was unjustifiably expensive. Alternatively, Forward Osmosis (FO) pretreatment of seawater was proposed for the removal of scaling ions from the seawater feed to the thermal desalination plants. FO operates by natural osmosis energy created across a semipermeable membrane separating two solutions having different osmotic pressure. Therefore, the cost of seawater pretreatment can be reduced significantly. FO process is relatively new technique and requires more evaluation and investigation before field application. The main challenges of FO application in seawater desalination and pretreatment, in particular, are the draw solution type and FO membrane. Important requirements the selection of the most optimal draw solution is to be inexpensive, easy to regenerate, and non-toxic material. Recent studies proposed brine reject to be the draw solution of FO process; however, brine reject osmotic pressure would induce a limited membrane flux due to its moderately low osmotic pressure. Higher membrane flux can be achieved by manipulating the operating parameters and/or using FO membrane of high water permeability. Applying positive pressure on the feed side of the FO membrane, Pressure Assisted FO (PAFO), promotes water flux across the membrane. The combination of PAFO process and high permeability membrane would enhance the performance of FO process for seawater pretreatment. Additionally, the impact of increasing the feed flow rate, Enhanced Flow FO (EFFO), on the performance of FO process should be investigated and compared with that of the PAFO process. Finally, power consumption associated with PAFO and EFFO would ultimately decide the cost-effectiveness of the pretreatment process.
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