non-cellulosic polymeric membranes, and thin-film interfacial composite membranes are also discussed in detail.
Furthermore, a general description of a complete commercial
BWRO plant along with different types of commercial
membrane modules is thoroughly discussed. Lastly, general
design principles of BWRO process, optimization strategies,
and pertinent energy and economic considerations are also
summarized in this chapter.
2 Theoretical Background of BWRO Process
2.1 Reverse Osmosis Process
Water molecules spontaneously permeate from a dilute
solution to a concentrated one through a semipermeable
membrane without any externally applied pressure in the
naturally occurring osmosis phenomena (Fig. 1a). In this
situation, the partially permeable membrane does not let the
solute to pass through it. This normal process persists until
the chemical potentials of brine solutions on either side of
the membrane osmotically equilibrate (Fig. 1b). Permeation
of water molecules can be halted or even reversed by
exerting an external pressure on the concentrated side of the
membrane (Chen et al. 2011). If the level of externally
applied pressure (p) on the concentrated side of membrane is
higher than natural osmotic pressure (p) of concentrated
solution, water molecules would move from a concentrated
solution to a dilute solution (Fig. 1c). This unnatural process
known as reverse osmosis is used to produce clean water
from brackish water.
2.2 Transport Through BWRO Membranes
Transport of water and salt permeating through BWRO
membranes takes place via solution-diffusion mechanism
(Wijmans and Baker 1995). According to this model, water
flux (J w ) and solute flux (J s ) are, respectively, proportional to
net transmembrane pressure difference (Dp > Dp) and solute
concentration difference (C F − C P ) for BWRO systems
operating at steady state (Wang et al. 2014):
J w ¼ A Dp À Dp
ð
Þ
J s ¼ B C F À C P
ð
Þ
where A and B are membrane permeability coefficients for
water and salt, respectively. Dp is the transmembrane
applied pressure difference while Dp is the transmembrane
osmotic pressure difference between feed and permeate
sides. C F and C P are feed-side and permeate-side solute
concentrations, respectively.
2.3 Desalination Performance of BWRO
Membranes
Recovery (R) of a BWRO desalination process is the fractional volume of feed water permeated through the membrane. Percentage recovery (%R) of a BWRO system, in
view of Fig. 2, can be estimated as follows:
R %
ð Þ ¼
q P
q F
 100
where q F and q P are respective volumetric flow rates of
feed and permeate streams. Recovery of a BWRO system
generally depends on quality and salinity level of feed water,
process design arrangement, pretreatment process, and
concentrate handling (Greenlee et al. 2009). Typical recovery values of most BWRO systems vary from 50 to 85%
(Kucera 2010).
Salt or solute rejection (SR) refers to the fractional
rejection of a specific solute by BWRO membrane as given
by following expression:
SR ¼ 1 À
C P
C F
 100%
where C F and C P , respectively, denote feed and permeate
solute concentrations normally expressed as mol/L or mg/L.
Salt rejection is a strong function of molecular size,
molecular weight, and nature of solute molecules, as well as
solute–membrane interaction determined by the type of
BWRO membrane (Kucera 2010).
Another relevant term, opposite to SR, called solute or
salt passage (SP) expresses fraction of a certain permeating
solute passed through BWRO membrane.
SP ¼
C P
C F
 100%
Both the opposing terms SR and SP can be correlated as:
SP %
ð Þ ¼ 100% À SR %
ð Þ
3 BWRO Membranes
The heart of BWRO desalination process is the selective
semipermeable membranes which preferentially permit
water molecules to permeate through them while rejecting
solutes (salts and other dissolved solids) to produce freshwater product. Desalination performance of BWRO plants
measured in terms of permeate water flux and salt rejection
strongly depends on chemical, morphological, and structural
properties of the employed membrane (Kucera 2010).
Continuous development of unique membrane materials is
Recent Trends in Membrane Processes for Water Purification …
41
Furthermore, a general description of a complete commercial
BWRO plant along with different types of commercial
membrane modules is thoroughly discussed. Lastly, general
design principles of BWRO process, optimization strategies,
and pertinent energy and economic considerations are also
summarized in this chapter.
2 Theoretical Background of BWRO Process
2.1 Reverse Osmosis Process
Water molecules spontaneously permeate from a dilute
solution to a concentrated one through a semipermeable
membrane without any externally applied pressure in the
naturally occurring osmosis phenomena (Fig. 1a). In this
situation, the partially permeable membrane does not let the
solute to pass through it. This normal process persists until
the chemical potentials of brine solutions on either side of
the membrane osmotically equilibrate (Fig. 1b). Permeation
of water molecules can be halted or even reversed by
exerting an external pressure on the concentrated side of the
membrane (Chen et al. 2011). If the level of externally
applied pressure (p) on the concentrated side of membrane is
higher than natural osmotic pressure (p) of concentrated
solution, water molecules would move from a concentrated
solution to a dilute solution (Fig. 1c). This unnatural process
known as reverse osmosis is used to produce clean water
from brackish water.
2.2 Transport Through BWRO Membranes
Transport of water and salt permeating through BWRO
membranes takes place via solution-diffusion mechanism
(Wijmans and Baker 1995). According to this model, water
flux (J w ) and solute flux (J s ) are, respectively, proportional to
net transmembrane pressure difference (Dp > Dp) and solute
concentration difference (C F − C P ) for BWRO systems
operating at steady state (Wang et al. 2014):
J w ¼ A Dp À Dp
ð
Þ
J s ¼ B C F À C P
ð
Þ
where A and B are membrane permeability coefficients for
water and salt, respectively. Dp is the transmembrane
applied pressure difference while Dp is the transmembrane
osmotic pressure difference between feed and permeate
sides. C F and C P are feed-side and permeate-side solute
concentrations, respectively.
2.3 Desalination Performance of BWRO
Membranes
Recovery (R) of a BWRO desalination process is the fractional volume of feed water permeated through the membrane. Percentage recovery (%R) of a BWRO system, in
view of Fig. 2, can be estimated as follows:
R %
ð Þ ¼
q P
q F
 100
where q F and q P are respective volumetric flow rates of
feed and permeate streams. Recovery of a BWRO system
generally depends on quality and salinity level of feed water,
process design arrangement, pretreatment process, and
concentrate handling (Greenlee et al. 2009). Typical recovery values of most BWRO systems vary from 50 to 85%
(Kucera 2010).
Salt or solute rejection (SR) refers to the fractional
rejection of a specific solute by BWRO membrane as given
by following expression:
SR ¼ 1 À
C P
C F
 100%
where C F and C P , respectively, denote feed and permeate
solute concentrations normally expressed as mol/L or mg/L.
Salt rejection is a strong function of molecular size,
molecular weight, and nature of solute molecules, as well as
solute–membrane interaction determined by the type of
BWRO membrane (Kucera 2010).
Another relevant term, opposite to SR, called solute or
salt passage (SP) expresses fraction of a certain permeating
solute passed through BWRO membrane.
SP ¼
C P
C F
 100%
Both the opposing terms SR and SP can be correlated as:
SP %
ð Þ ¼ 100% À SR %
ð Þ
3 BWRO Membranes
The heart of BWRO desalination process is the selective
semipermeable membranes which preferentially permit
water molecules to permeate through them while rejecting
solutes (salts and other dissolved solids) to produce freshwater product. Desalination performance of BWRO plants
measured in terms of permeate water flux and salt rejection
strongly depends on chemical, morphological, and structural
properties of the employed membrane (Kucera 2010).
Continuous development of unique membrane materials is
Recent Trends in Membrane Processes for Water Purification …
41