Typically, asymmetric membranes are composed by a
dense layer and a support layer. Dense layer thickness is
about 0.1–1 µm, whereas the support layer is highly porous
and 100–200 µm thick. The separation effectiveness is
influenced by the chemical structure of the dense layer, the
size of the pores (usually between 0.4 and 1 nm), the pore
structure and the thickness of the support layer (Haupt and
Lerch 2018).
The desired characteristics of membranes for forward
osmosis applications are high density of the active layer for
high solute rejection and minimum porosity of the support
layer which supposes low concentration polarization (Cath
et al. 2006). A typical membrane structure can be seen in
Fig. 4.
The first commercial FO membrane was made of cellulose triacetate (CTA), and it was developed by hydration
technologies (Wang et al. 2018). From this moment, a high
number of CTA-based forward osmosis studies have been
carried out by several researchers (Jin et al. 2012; Bell et al.
2017; Zuo et al. 2017; Wang et al. 2016; Bensaadi et al.
2016). CTA membranes are obtained by phase inversion
through immersing casted or spun polymer dope into
coagulants. Thus, asymmetric membrane structure featured
with a skin layer integrally supported by a porous substrate
is obtained as product.
CTA membranes show a low fouling propensity since
their hydrophilic nature allows the water flux to pass without
keeping big molecules trapped. Nevertheless, this characteristic also provokes a higher salt flux from the draw
solution to the feed solution, which is equal to lower rejection to salt than other membranes. For this reason, many
efforts have been put into reducing the salt flux in CTA
membranes. Some of the improvements proposed are
exploring new materials and additives, tuning fabrication
conditions and introducing new additives. However, not
remarkable improvements were achieved and the efforts
were focalized to thin-film composite (TFC) membranes
(Haupt and Lerch 2018).
TFC membranes are the most extended commercial type
of forward osmosis membranes because the characteristics
previously explained are achieved by these membranes. In
this kind of membranes, the porous layer is an integrally
skinned membrane which resulted from a non-solventinduced phase separation process. TFC membranes are
promising in terms of both manufacturing cost and water
flux. The polymer needed for CTA membrane production is
among 15–18%, whereas for TFC membranes this range is
about 8–11% (Haupt and Lerch 2018; Wang et al. 2016).
TFC membranes are fabricated mainly by HTI, Porifera
and Oasys Water and typically are embedded in hollow fiber
configurations. TFC membrane drawback is the moderate
water flux in comparison with CTA membranes, which
could be overcome by increasing the draw solution concentration (Ismail et al. 2015). For example, in case of using
NaCl as draw solution, water flux differences between TFC
and CTA are corrected by increasing NaCl concentration
from 0.5 to 1 M (Alsvik and Hägg 2013; Lau et al. 2012).
Nowadays, studies for TFC FO membrane improvements
focused on making the support later more interconnected
and porous as well as thinner. Moreover, reducing the tortuosity is another factor which needs to be optimized. Furthermore, fouling problems are under strict evaluation
continuously in new applications for enhancing the overall
performance (Tarboush et al. 2008; Alsvik and Hägg 2013).
4 Forward Osmosis Applications
in Industrial Areas
Traditionally, forward osmosis has been employed for water
desalination. Nevertheless, its use can be engineered and
adapted for many other industrial processes. Nowadays, the
benefits previously exposed presented by forward osmosis
application have benefited its implementation in many sectors such as the following ones (Mi and Elimelech 2010;
Haupt and Lerch 2018):
• Chemical industry
• Desalination of drinking water
• Food industry
• Pharmaceutical industry
• Textile industry
• Coal processing industry
• Electronic industry
• Heavy metal industry.
Therefore, many studies have been published in different
branches and this number has been increased considerably
(Haupt and Lerch 2018). To further knowledge of the different applications, in this section we have included four
points to give the reader a wide comprehension of the
Fig. 4 Membrane structure
Forward Osmosis for Sustainable Industrial Growth
5
dense layer and a support layer. Dense layer thickness is
about 0.1–1 µm, whereas the support layer is highly porous
and 100–200 µm thick. The separation effectiveness is
influenced by the chemical structure of the dense layer, the
size of the pores (usually between 0.4 and 1 nm), the pore
structure and the thickness of the support layer (Haupt and
Lerch 2018).
The desired characteristics of membranes for forward
osmosis applications are high density of the active layer for
high solute rejection and minimum porosity of the support
layer which supposes low concentration polarization (Cath
et al. 2006). A typical membrane structure can be seen in
Fig. 4.
The first commercial FO membrane was made of cellulose triacetate (CTA), and it was developed by hydration
technologies (Wang et al. 2018). From this moment, a high
number of CTA-based forward osmosis studies have been
carried out by several researchers (Jin et al. 2012; Bell et al.
2017; Zuo et al. 2017; Wang et al. 2016; Bensaadi et al.
2016). CTA membranes are obtained by phase inversion
through immersing casted or spun polymer dope into
coagulants. Thus, asymmetric membrane structure featured
with a skin layer integrally supported by a porous substrate
is obtained as product.
CTA membranes show a low fouling propensity since
their hydrophilic nature allows the water flux to pass without
keeping big molecules trapped. Nevertheless, this characteristic also provokes a higher salt flux from the draw
solution to the feed solution, which is equal to lower rejection to salt than other membranes. For this reason, many
efforts have been put into reducing the salt flux in CTA
membranes. Some of the improvements proposed are
exploring new materials and additives, tuning fabrication
conditions and introducing new additives. However, not
remarkable improvements were achieved and the efforts
were focalized to thin-film composite (TFC) membranes
(Haupt and Lerch 2018).
TFC membranes are the most extended commercial type
of forward osmosis membranes because the characteristics
previously explained are achieved by these membranes. In
this kind of membranes, the porous layer is an integrally
skinned membrane which resulted from a non-solventinduced phase separation process. TFC membranes are
promising in terms of both manufacturing cost and water
flux. The polymer needed for CTA membrane production is
among 15–18%, whereas for TFC membranes this range is
about 8–11% (Haupt and Lerch 2018; Wang et al. 2016).
TFC membranes are fabricated mainly by HTI, Porifera
and Oasys Water and typically are embedded in hollow fiber
configurations. TFC membrane drawback is the moderate
water flux in comparison with CTA membranes, which
could be overcome by increasing the draw solution concentration (Ismail et al. 2015). For example, in case of using
NaCl as draw solution, water flux differences between TFC
and CTA are corrected by increasing NaCl concentration
from 0.5 to 1 M (Alsvik and Hägg 2013; Lau et al. 2012).
Nowadays, studies for TFC FO membrane improvements
focused on making the support later more interconnected
and porous as well as thinner. Moreover, reducing the tortuosity is another factor which needs to be optimized. Furthermore, fouling problems are under strict evaluation
continuously in new applications for enhancing the overall
performance (Tarboush et al. 2008; Alsvik and Hägg 2013).
4 Forward Osmosis Applications
in Industrial Areas
Traditionally, forward osmosis has been employed for water
desalination. Nevertheless, its use can be engineered and
adapted for many other industrial processes. Nowadays, the
benefits previously exposed presented by forward osmosis
application have benefited its implementation in many sectors such as the following ones (Mi and Elimelech 2010;
Haupt and Lerch 2018):
• Chemical industry
• Desalination of drinking water
• Food industry
• Pharmaceutical industry
• Textile industry
• Coal processing industry
• Electronic industry
• Heavy metal industry.
Therefore, many studies have been published in different
branches and this number has been increased considerably
(Haupt and Lerch 2018). To further knowledge of the different applications, in this section we have included four
points to give the reader a wide comprehension of the
Fig. 4 Membrane structure
Forward Osmosis for Sustainable Industrial Growth
5