Forward osmosis draw solution

An interesting alternative development is that of forward osmosis. Whereas in reverse osmosis a high pressure is required to oppose the natural tendency of freshwater to move across such a membrane via osmosis to dilute the seawater, in forward osmosis the system takes advantage of this natural tendency. Here, salt water sits on one side of the membrane, but the freshwater on the opposite side is transformed into a high-concentration solution by adding NH3 and CO2. Water naturally flows from the salt water to what is now the draw solution, which can have a solute concentration as high as 10 times that of the salt water. There is no need for an external pressure. The diluted draw solution is then heated to evaporate off the CO2 and NH3 for reuse, leaving behind freshwater. (See Patel-Predd, 2006). 

S. Zhao, L. Zou, D. Mulcahy, Brackish water desahnation by a hybrid forward osmosis-nanofiltration system using divalent draw solute. Desalination 2012, 284, 175-181. 

A number of alternatives to reverse osmosis are being considered. Two promising alternatives are membrane distillation [97] and forward osmosis [98]. Membrane distillation relies on vapor pressure differences across a membrane, arising from a temperature difference, to drive water transport. The process utilizes low temperature heat sources and operates at low pressure which can reduce operating costs relative to reverse osmosis. Forward osmosis relies on water permeation across a water selective membrane to a draw solution - the reverse of reverse osmosis. The water must then be separated from the draw solution but this may be less expense than reverse osmosis because the process operates at low pressure. 

Forward osmosis (FO) is a membrane-separation process that uses osmotic pressure difference between a concentrated draw solution and a feed stream to drive water across a semipermeable membrane [63]. The basis of FO is osmosis, a natural and spontaneously occurring process. It is strictly direct osmosis across an RO membrane. A draw solute of high osmotic pressure, e.g., ammonium carbonate passes across one side of the FO membrane, and a high salinity solution, e.g., seawater flows across the other side of the membrane, as shown in Figure 1.17. Water transfers from the seawater to the draw solute side due to osmotic flow. It is then necessary to regenerate the draw solute and recover the water transferred by the FO process, e.g., in a distillation unit. The primary challenge is... 

Figure 1.17 Schematic flow diagram of a forward osmosis system with ammonia carbon dioxide draw solution (https //www.google.com/search q=cellulose+acetate+membrane). Source [63].

In contrast to a neutral membrane, the positively charged forward osmosis membrane provides double electric repulsions to the salt transfer through the membrane in the active layer facing feed water configuration. This results in a reduction of the salt penetration, while in the active layer facing draw solution config-... 

It is known that the forward osmosis and pressure-retarded osmosis are mostly effected by osmotic pressure difference rather than by applied pressure difference like reverse osmosis. In addition, the early FO studies have already proved that the ICP predominately can reduce the solvent permeating flux by more than 80 % [4, 5]. Meanwhile, compared to the ICP, the ECP plays a more important role in determining the performance of reverse osmosis due to the high applied pressure. Consequently, only ECP effects are considered for reverse osmosis in which the DSL is commonly arranged to face against the draw solution, while for forward osmosis and pressure-retarded osmosis, both the ECP and the ICP effects should be taken into account. 

Osmosis and Its Applications, Fig. 1 Forward osmosis profiles of the solution concentration and the osmotic pressure under the effects of membrane stmetures and orientations, (a) A symmetric dense semipermeable membrane with only ECP effects, i.e., concentrative ECP and dilutive ECP. (b) An asymmetric membrane with the dense-selective layer (DSL) facing against draw solution (normal mode) with the profile of the solution concentration illustrating concentrative ICP and dilutive ECP. (c) An asymmetric membrane with the porous support layer (PSL) facing against draw solution (reverse mode) with the profile of the solution concentration illustrating dilutive ICP and concentrative ECP. The key parameters Ch... 

The term draw solution denotes the concentrated solution of forward osmosis or the dilute solution of reverse osmosis. Further studies of draw solutions should mainly focus on the field of forward osmosis due to its important role in separating processes as abovementioned applications in the previous section. Effective draw solutions showing high osmotic pressure, good regeneration property, and less fouling properties are needed. 

Xu, Y., Peng, X., Tang, C.Y. et al. (2010) Effect of draw solution concentration and operating conditions on forward osmosis and pressure retarded osmosis performance in a spiral wound module. Journal of Membrane Science, 348 (1-2), 298-309. 

Forward osmosis (FO) is an emerging membrane technology for water treatment and energy production. In an FO process, a semipermeable membrane is placed in between two solutions of different osmotic pressures. One is a less concentrated feed solution (FS) with lower osmotic pressure, while the other is a more concentrated draw solution (DS) with higher osmotic pressure. The water in the FS permeates through the membrane into the DS due to the osmotic pressure gradient across the membrane, but the solutes in the FS are selectively rejected by the membrane. 

Phillip, W.A., Yong, J.S. Elimelech, M. (2010) Reverse draw solute permeation in forward osmosis modeling and experiments. Environmental Science Technology, 44 (13), 5170-5176. 

Figure 16.12 Forward Osmosis Feed water flows on the active side of the membrane, while the draw solution with high osmotic presstue flows on the support side of the membrane. Water passes from the feed side of the membrane into the draw solution.

Figure 16.13 Example of a continuous forward osmosis application with recovery and recycle of the draw solution.

Figure 16.18 Flux performance of 2 commercially available RO membranes from GE Water (AG, CE) and a cellulose triacetate forward osmosis membranes from Hydration Technology Innovations (CTA). The draw solution used was a 6M ammonia-carbon dioxide solution and the feed solution was a 0.5M sodium chloride solution. The temperature of the test was 50°G Reprinted from REF16-B7. Tests were done in a custom built, crossflow, benchtop FO testing system.

Forward osmosis relies on the osmotic pressure differential across a membrane to drive water through the membrane RO relies on the hydraulic pressure differential to drive water through the membrane. A draw solution is used on the permeate side of the membrane to osmotically drive water from the feed side of the membrane into the draw solution, which becomes more dilute. The draw solution is then treated (sometimes by heating followed by membrane distillation or by RO) to recover the water and to regenerate the draw solution for reuse. 

The opposite case of reverse osmosis is termed forward osmosis. In this case, the membrane is employed to separate water from a feed stream containing dissolved solutes, but the permeate stream contains a draw solution of higher solute... 

J.R. McCutcheon, R.L. McGinnis, M. Elimelech, Desalination by ammonia-carbon dioxide forward osmosis Influence of draw and feed solution concentrations on process performance. Journal of Membrane Science 278 (2006) 114-123. 

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