Once both osmotic pressures are equalized, equilibrium is
reached in which the water flux is zero. The water flux is
defined by the general equation (Eq. 1) which describes
water transport in osmosis processes (Cath et al. 2006):
J w ¼ Aðr Á Dp À DPÞ
ð 1Þ
J w is the water flux; A is a constant related to the pure water
permeability of the membrane; r is the reflection coefficient;
Dp is the osmotic pressure differential; and DP is the applied
pressure in those applications needed (reverse osmosis and
pressure-retarded osmosis).
Compared to conventional water separation technologies,
FO includes the following advantages (Zhao et al. 2012;
Cath et al. 2006; Chung et al. 2012):
• Low energy consumption
• Possibility of treating two problematic effluents in the
same equipment
• High-purity water recovery
• Treatment of liquids which are highly difficult to treat
using other membrane processes (due to their fouling
tendency).
2.2 Draw Solutions
The DS selected for each application is the key for obtaining
a valuable result. Indeed in FO processes, draw solutions are
considered a fundamental choice for a successful performance. Thus, the selection of an appropriate DS has been the
purpose of some studies (Chekli et al. 2012; Achilli et al.
2010; Ge et al. 2013). The main criterion for selecting an
adequate draw solution is the osmotic pressure. This last one
has to be higher than the feed solution to produce the highest
water flux.
The second more important parameter to take into
account for selecting a suitable FO draw solution is the
availability for re-concentrating after FO stage. Re-utilizing
the DS is necessary to keep the overall economic performance of the process (Ge et al. 2013; Chekli et al. 2012).
Furthermore, the reverse flux of the draw solution solute
through the membrane to the feed solution should be considered (Shaffer et al. 2015; Lutchmiah et al. 2014).
For these reasons, several draw solutions have been tested
by experts in this area (Shaffer et al. 2015; Chekli et al.
2012). Many researchers have put their efforts on finding
novel draw solutions, and as a result multitude of compounds have been proposed in the literature. These draw
solutions present some advantages as the same time that
there are aspects which need to overcome (Ge et al. 2013).
Table 1 presents the most typically employed draw solutions
which can serve as a guide for those looking for the correct
draw solution.
As can be seen in Table 1, MgCl 2 and CaCl 2 present the
highest osmotic pressures. Nevertheless, their flux is not the
best ones showed in the literature by some researches (Ge
et al. 2013; Achilli et al. 2010).
Nevertheless, many studies have employed NaCl in a
wide range of applications since saline water is abundant on
earth and easy to obtain. Moreover, NaCl can be easily
re-concentrated with RO processes or MD with few energy
consumptions. Additionally, NaCl presents high water solubility which is a clear advantage since there is no need of
using organic solvents. Therefore, food production and
wastewater treatment have applied NaCl as draw solution at
industrial scale (Akther et al. 2015).
Not as typical as the employment of inorganic salts as
previously exposed but during the last years, some studies
have proposed ethanol, sucrose, glucose and fructose as
organic draw solutions with acceptable results. Experimental
water flux obtained range from 0.24 to 7.5 LMH for these
organic solvents and their solubility is higher due to
hydrogen bonding.
2.3 Concentration Polarization Phenomenon
An essential phenomenon in pressure osmotic-driven processes is the concentration polarization phenomena, which
highly influence in water flux across the membrane area.
This phenomenon is carried out by the increase of the feed
solution concentration over the membrane surface (Kim
et al. 2010; McCutcheon and Elimelech 2006). Figure 3
represents a schematic explanation of this process.
Although this phenomenon takes place in many membrane processes such as nanofiltration, membrane distillation
and reverse osmosis, the effect is harder in FO processes
(Chen et al. 2004). Due to the concentration polarization, the
osmotic pressure gradient decreases compared to the normal
operation, which negatively affects water flux (Kim et al.
2010). Therefore, permeated water is lower than expected
and bigger membrane areas are needed to keep constant the
desired flux (van den Berg and Smolders 1992).
Typically, concentration polarization has been split into
two types: external concentration polarization (ECP) and
internal concentration polarization (ICP). The first one
occurs very frequently in both FO and RO processes, and it
takes place into the surface of the active layer due to the
difference in the concentration of the solution respecting to
the bulk solution. ECP phenomenon has decreasing effects
on the osmotic gradient, and hence it caused the inhibition of
the water flux across the membrane. Nevertheless, the
impact of ICP during FO operation is much higher than
Forward Osmosis for Sustainable Industrial Growth
3
reached in which the water flux is zero. The water flux is
defined by the general equation (Eq. 1) which describes
water transport in osmosis processes (Cath et al. 2006):
J w ¼ Aðr Á Dp À DPÞ
ð 1Þ
J w is the water flux; A is a constant related to the pure water
permeability of the membrane; r is the reflection coefficient;
Dp is the osmotic pressure differential; and DP is the applied
pressure in those applications needed (reverse osmosis and
pressure-retarded osmosis).
Compared to conventional water separation technologies,
FO includes the following advantages (Zhao et al. 2012;
Cath et al. 2006; Chung et al. 2012):
• Low energy consumption
• Possibility of treating two problematic effluents in the
same equipment
• High-purity water recovery
• Treatment of liquids which are highly difficult to treat
using other membrane processes (due to their fouling
tendency).
2.2 Draw Solutions
The DS selected for each application is the key for obtaining
a valuable result. Indeed in FO processes, draw solutions are
considered a fundamental choice for a successful performance. Thus, the selection of an appropriate DS has been the
purpose of some studies (Chekli et al. 2012; Achilli et al.
2010; Ge et al. 2013). The main criterion for selecting an
adequate draw solution is the osmotic pressure. This last one
has to be higher than the feed solution to produce the highest
water flux.
The second more important parameter to take into
account for selecting a suitable FO draw solution is the
availability for re-concentrating after FO stage. Re-utilizing
the DS is necessary to keep the overall economic performance of the process (Ge et al. 2013; Chekli et al. 2012).
Furthermore, the reverse flux of the draw solution solute
through the membrane to the feed solution should be considered (Shaffer et al. 2015; Lutchmiah et al. 2014).
For these reasons, several draw solutions have been tested
by experts in this area (Shaffer et al. 2015; Chekli et al.
2012). Many researchers have put their efforts on finding
novel draw solutions, and as a result multitude of compounds have been proposed in the literature. These draw
solutions present some advantages as the same time that
there are aspects which need to overcome (Ge et al. 2013).
Table 1 presents the most typically employed draw solutions
which can serve as a guide for those looking for the correct
draw solution.
As can be seen in Table 1, MgCl 2 and CaCl 2 present the
highest osmotic pressures. Nevertheless, their flux is not the
best ones showed in the literature by some researches (Ge
et al. 2013; Achilli et al. 2010).
Nevertheless, many studies have employed NaCl in a
wide range of applications since saline water is abundant on
earth and easy to obtain. Moreover, NaCl can be easily
re-concentrated with RO processes or MD with few energy
consumptions. Additionally, NaCl presents high water solubility which is a clear advantage since there is no need of
using organic solvents. Therefore, food production and
wastewater treatment have applied NaCl as draw solution at
industrial scale (Akther et al. 2015).
Not as typical as the employment of inorganic salts as
previously exposed but during the last years, some studies
have proposed ethanol, sucrose, glucose and fructose as
organic draw solutions with acceptable results. Experimental
water flux obtained range from 0.24 to 7.5 LMH for these
organic solvents and their solubility is higher due to
hydrogen bonding.
2.3 Concentration Polarization Phenomenon
An essential phenomenon in pressure osmotic-driven processes is the concentration polarization phenomena, which
highly influence in water flux across the membrane area.
This phenomenon is carried out by the increase of the feed
solution concentration over the membrane surface (Kim
et al. 2010; McCutcheon and Elimelech 2006). Figure 3
represents a schematic explanation of this process.
Although this phenomenon takes place in many membrane processes such as nanofiltration, membrane distillation
and reverse osmosis, the effect is harder in FO processes
(Chen et al. 2004). Due to the concentration polarization, the
osmotic pressure gradient decreases compared to the normal
operation, which negatively affects water flux (Kim et al.
2010). Therefore, permeated water is lower than expected
and bigger membrane areas are needed to keep constant the
desired flux (van den Berg and Smolders 1992).
Typically, concentration polarization has been split into
two types: external concentration polarization (ECP) and
internal concentration polarization (ICP). The first one
occurs very frequently in both FO and RO processes, and it
takes place into the surface of the active layer due to the
difference in the concentration of the solution respecting to
the bulk solution. ECP phenomenon has decreasing effects
on the osmotic gradient, and hence it caused the inhibition of
the water flux across the membrane. Nevertheless, the
impact of ICP during FO operation is much higher than
Forward Osmosis for Sustainable Industrial Growth
3