1. INTRODUCTION Water scarcity has beenrecognised as a global crisis. (P. Aldhous, Nature, 2003) while wastewater purification partiallyrelieves water shortages, only desalination technologies can extend our supplyto the ocean, which is the major source of water. One of the prevalentseawater desalination technologies is reverse osmosis (RO) whose energyconsumption has been greatly reduced after decades of development (M. Elimelech and W.
A. Phillip, Science, 2011). RO (reverse osmosis) process is most widelyused for desalination.
In RO process, a semi-permeable membrane utilizes toproduce clean water from saline solution and in this process, high hydraulicpressure is necessary for RO process because of large osmotic pressure of seawater, RO is an energy and cost intensive process (K.P. Lee, T.C.Arnot, D. Mattia, 2011). By forward osmosis, desalination emerges with promise ofovercoming the challenges of pressure-driven membrane processes (T.Y.
Cath, A.E. Childress, M. Elimelech, 2006). world-leading FO technologyused in a number of membrane process applications, forward osmosis fordesalination emerges with many advantages. In FO process, water passes throughsemi-permeable membrane from high osmotic potential (water potential) to lowerosmotic potential.
Since in FO process, No external hydraulic pressurenecessary (M. Elimelech, Yale, 2007), FO offers advantages of lower energydemands and that reduce the capital cost of process and less fouling onmembrane (less solution or particle is deposited on membrane surface or inmembrane pores).Thus FO process showed many advantages, forward osmosis hasattracted to many researchers, therefore many research have been doneespecially on FO technology over the past decade, progress in FO membranedevelopments started since 2010.FO technology depends on membrane, Many membrane design andhybrid FO system have developed in order to progress in forward osmosisprocess, more research are going on the thin film composite (TFC) membranes afterdeveloped hybrid technology successfully, TFC membranes are composed of multiple layers. Membranesdesigned for desalination use an active thin-film layer of polyamide layered with polysulfone as a porous support layer. The three layer configuration gives thedesired properties of high rejection of undesired materials (like salts), highfiltration rate, and good mechanical strength.
The polyamide top layer isresponsible for the high rejection (filmtech corporation ,1997) In a hybrid FO system, asillustrated schematically above, (In membrane distillation, a hydrophobic andmicroporous membrane is employed to separate the aqueous feed stream and thepermeate) the outputs are a concentratedfeed solution and permeate consisting of reusable water. In a hybrid FO system,the FO part still functions as an energy-efficient water extractor, extractingwater from a feed stream, which is difficult and expensive to treat withtraditional membrane technologies, to a draw stream that is considerably easierand less expensive to treat when it is diluted by the FO process (mark perry,2014).The FO membrane sub-system aswell as the other sub-systems of the hybrid, it is also possible to recover lowmolecular weight solutes such as NaCl from the feed stream. This is especiallyof value in the textile industry where large amounts of salts released in wastewater streams (J.
R. McCutcheon, R.L. McGinnis, M.
Elimelech,2006). To create a DS with a higher osmotic pressure than FS, organicdraw solutes used and create draw solute which is easily recoverable duringprocess, ammonia and carbon dioxide solution as an easily re-generable DS whichcame out by research. A ‘smart’ draw solute in forward osmosis processes for waterreuse without losing performance efficiency, Smart materials exhibit advantages as FO draw agents tolower the energy cost of regeneration. Therefore, increasing research effortshave been devoted to the design of smart draw solutes. Smart drawsolutes are responsive towards temperature, pH, electro-magnetic field orlight (Y. Cai, X.
M. Hu, 2016).2.
Forward osmosisdesalination 2.1 Physical concept and phenomenon The concept of osmosis has been known to mankind since thebeginningof human civilization. The physical phenomenon of forwardosmosis(FO) can simply be defined as the movement of watermolecules across a semipermeable membrane due to difference in osmotic pressuredriving force across the membrane. The semipermeable membrane allows only watermolecules to permeate through while the solute or salt molecules are rejected.The FO desalination process simply makes use of a highly concentrated saltsolution (known as the draw solution, osmotic agent, osmotic media, or osmoticengine) with low water chemical potential (high osmotic pressure) to draw thewater molecules from a feed solution (brackish or seawater) with higher waterchemical potential (lower osmotic pressure) compared to the draw solution. Thisis in agreement with the 2nd law of thermodynamics, since transport of water moleculeswill bring chemical potentials in the feed and the draw solution to equilibrium.
During the FO process, the membrane rejects the salt and consequently,the feed solution is concentrated and the draw solution is diluted with time.Pure water has then to be recovered from the diluted draw solution by using anenergy-efficient separation technique. Unlike RO,FO utilizes the natural osmotic pressure difference acrossthe membrane to transport the water molecules and does not require applicationof hydraulic pressure to overcome the osmotic pressure of the feed. A comparison of waterflux direction in FO and RO is shown in Fig. 2.Fig.2. Water flux directions in FO and RO (T.
Y. Cath, A.E.Childress, M. Elimelech,2006). (a) In FO, the feed solution of high waterchemical potential is separated from the draw solution of low water chemical potentialby a semipermeable membrane. (b) Due tothe difference in osmotic pressures, water molecules are transported from thefeed to the draw solution in FO.
(c) InRO, hydraulic pressure is applied to the feed to overcome the osmotic pressure. (d) The application of pressure forcesthe water molecules through the semipermeable membrane in RO. In short, the process of FO desalination can be divided intotwo steps. In the first step, water molecules are permeated from the feed tothe draw solution across the semipermeable membrane.
In the second step of FOdesalination, the draw solution is subsequently recovered by separating pure waterfrom the diluted draw solution obtained in the first step of the process. Asimplified schematic of this two-stage FO desalination process is shown in Fig. 3. Fig.3. Two stages in FOdesalination.
Stage 1 consists of dilution of the draw solution. Stage 2involves regeneration of the draw solution and pure product water recovery 2.2. Advantages of FO desalination FO desalination has a range of potential advantages since itoperatesat low or no hydraulic pressure and simply allows for thepermeation ofwater molecules across the membrane under the influence ofosmoticpressure difference(T.Y.
Cath, A.E. Childress, M. Elimelech, 2006; S. Zhao, L. Zou, C.
Y. Tang, D.Mulcahy,2011). Due to low or no hydraulic pressure application, FOdesalination has potentially less energy consumption compared to theconventional desalination technologies (R.W. Field, J.J.
Wu, 2013). In addition, FO desalination results inlower and reversible membrane fouling, exhibits high salt rejection, andis negligibly affected by a variety of contaminants in the feed solution (V.Yangali-Quintanilla, Z.
Li, R. Valladares, 2011). FO desalination also providesa clean solution to the problem of brine generated from desalination plants. Bygenerating high osmotic pressure gradient across the membrane, FO has thepotential to achieve higher water flux and recovery and therebyreduce the amount of brine generated. FO process is a promising and economic alternativeto the conventional thermal brine concentration processes.
3. Research areas in forward osmosis desalination In the area of desalination, research on FO mainly focuseson the development of new membranes (both flat sheet and hollow fiber) andutilization of new draw solutes in order to overcome the existing challengesfaced by the FO desalination process. In general, any dense, non-porous, andsemipermeable material can be employed a membrane in the FO process (T.Y. Cath, A.E.
Childress, M. Elimelech,2006) However, the membraneshould exhibit high water flux (based on lower structural parameter (S)values), low reverse solute flux, low fouling, high salt rejection, low internalconcentration polarization (ICP is one of themain challenges in FO desalinationProcess development for FO applications, other thandesalination, has also become an important research subject. Since, current FOtechnologies are not feasible enough to establish standalone applications, hybridand retrofitted applications have been investigated worldwide