Membrane and Desalination Technologies - TCE Moodle Website

Membrane Technologies for Point-of-Use and Point-of-Entry Applications 615
(FO) and pressure-retarded osmosis (PRO). In FO process, the osmotic pressure differential
(Dp) across the membrane is used. PRO can be viewed as an intermediate process between
FO and RO, where hydraulic pressure is applied in the opposite direction of the osmotic
pressure gradient (similar to RO). However, the net water flux is still in the direction of the
concentrated draw solution (similar to FO).
Forward osmosis has been used to treat industrial wastewaters (at bench scale) (32–34), to
concentrate landfill leachate (at pilot and full scale) (35–37), and to treat liquid foods in the
food industry (at bench scale) (38–46). FO is also being evaluated for reclaiming wastewater
for potable reuse in life support systems (at demonstration scale) (47–50), for desalinating
seawater (51, 52), and for purifying water in emergency relief situations (53). PRO, a closely
related process, has been tested and evaluated since the 1960s as a potential process for power
generation (54–65).
The general equation describing water transport in RO, FO, and PRO is as follow (31):
JW ¼ AðsDp DPÞ; (1Þ
where Jw is the water flux, A is the water permeability constant of the membrane, s is the
reflection coefficient, and DP is the applied pressure.
For FO, DP is zero; for RO, DP > Dp; and for PRO, Dp > DP. The flux directions of the
permeating water in FO, PRO, and RO are illustrated in Fig. 14.3 (31).
Flux directions and driving forces for the three processes were characterized in the early
1980s by Lee et al. (56). The FO point, PRO zone, and RO zone, along with the flux reversal
point, are illustrated in Fig. 14.4 (56).
From Fig. 14.4, FO takes place when the hydraulic pressure difference is zero, the PRO
zone is where the applied pressure difference is between zero and the flux reversal point, and
the RO zone is where the applied pressure difference is greater than the osmotic pressure
difference.
In general, the osmotic pressure of the dissolved chemical causes pure water to pass
through the membrane from the dilute to the more concentrated solution, as shown in
Feed brine
FO
Δπ
Force (P)
PRO
Force (P)
Fig. 14.3. Solvent flows in FO, PRO, and RO (31). Note: For FO, DP is approximately zero, and water
diffuses to the more saline side of the membrane. For PRO, water diffuses to the more saline liquid that
is under positive pressure (Dp > DP). For RO, water diffuses to the less saline side due to hydraulic
pressure (DP > Dp).
RO