(Ismail et al. 2019; Duarte and Bordado 2016). Asymmetric
membranes can be categorized into integrally skinned
asymmetric cellulosic membranes, non-cellulosic polymer
membranes, and thin-film interfacial composite membranes
depending on whether the membrane structure consists of
the same or different materials (Wang and Wang 2019). In
addition, several other BWRO membranes have also been
prepared from some other materials during the last few
years. Research advancements regarding the development
and modification of commercially important BWRO
desalination membranes to enhance their desalination performance are briefly described here.
3.1 Integrally Skinned Cellulosic Membranes
The earliest defect-free high-performance anisotropic
BWRO desalination membrane was prepared from cellulose
acetate via Loeb–Sourirajan method (Loeb and Sourirajan
1963; Kimura and Sourirajan 1967; Loeb 1981). Discovery
of this first asymmetric membrane composed of layered
structure proved to be a revolutionary achievement resulting
into the commercialization of BWRO desalination process
(Ulbricht 2006; Glater 1998). In spite of being one of the
oldest BWRO membranes, cellulosic membranes still
occupy a small size of the market due to its salient features
including easy manufacturing, good mechanical toughness
and comparatively high resistance against chlorine attack as
compared to interfacial composite membranes. Sterilization
of infected feed water by chlorine or other oxidizing agent
can be easily accomplished via these membranes since they
can resist perpetual exposure of 1 ppm chlorine.
Being highly sensitive to acetyl content contained by
them, water flux and salt rejection characteristics of cellulose
acetate membranes can be significantly improved by
increasing the acetylation content of these membranes (Reid
and Breton 1959; Rosenbaum et al. 1967; Lonsdale 1966).
Cellulose diacetate and cellulose triacetate membranes
exhibiting isotropic morphology were subsequently developed to improve permeate flux and salt rejection properties
(Sidney and Srinivasa 1964; Kucera 2010; Loeb and
Sourirajan 1963). An increase in degree of acetylation of
cellulose
acetate
membranes generally
improves
water-to-salt flux ratio at the expense of reduced water flux
(Lonsdale et al. 1965). In contrast to their diacetate counterpart, cellulose triacetate membranes are thermally,
chemically, and biologically more stable. As compared to
cellulose triacetate membrane, a blend membrane of cellulose diacetate and cellulose triacetate was prepared to further
improve water flux, salt rejection and more importantly
densification resistance when operated under high pressures
(Sudak 1990; Lee et al. 2011). Advantageous features of
cellulose acetate membranes encompass decent fouling
resistance, reasonable chlorine resistance, and low preparation cost (Kucera 2010; El-Saied et al. 2003; Singh 2015).
Practical limitations of cellulose acetate membranes include
low silica rejection, degradation susceptibility towards bacteriological attack, and low thermal stability. Normally,
cellulose acetate membranes are used below an operating
temperature of 35 °C (Kucera 2010). Furthermore, these
membranes can only be safely used in a narrow operating
range of pH of feed water; acetate groups tend to gradually
hydrolyze outside the pH range of 4–6 and leads to shortening of membrane lifetime (Vos et al. 1966). Key information of some of the commercially available cellulose
acetate membranes is reported in Table 1.
3.2 Non-cellulosic Polymer Membranes
Integrally skinned isotropic membranes can also be commercially prepared from non-cellulosic polymers like aliphatic and aromatic polyamides. Asymmetric membranes
prepared from aliphatic polyamides offered mild water fluxes
and low salt rejections. Various asymmetric membranes
composed of aromatic polyamide in the form of hollow fiber
rendered low water fluxes and reasonable salt rejections
(Endoh et al. 1977; McKinney and Rhodes 1971; Richter
and Hoehn 1971). Aromatic polyamide membranes having
trade name Permasep
® B-9 were prepared by Du Pont to
commercially desalinate brackish water on a large scale
(Mehta and Loeb 1978; Hoehn and Richter 1973). As
compared to cellulose membranes, separation performance
of polyamide membranes was found to be better and
remained a first priority of BWRO plants until early 1990s.
Noticeable features of polyamide membranes include good
permeate flux, high salt rejection, moderate resistance
against hydrolysis and biological attack, good thermal and
mechanical stability, and wide pH range operability (Soltanieh and Gill 1981). Du Pont, however, abandoned the
production of polyamide membranes on account of their
susceptibility to disinfectants on prolonged exposure to
chlorine or ozone (Lee et al. 2011).
With the passage of time, other integrally skinned
asymmetric BWRO desalination membranes were also prepared from polybenzimidazole and polybenzimidazoline
materials (Sawyer and Jones 1984; Goldsmith et al. 1977).
These membranes not only had inherently low salt rejection
but were also susceptible to chlorine attack. Aiming to
overcome this difficulty, membranologists prepared polypiperazinamides membranes but could not be commercialized
owing to their low salt rejection (Credali et al. 1974; Parrini
1983; Credali and Parrini 1971). Other integrally skinned
asymmetric membranes giving low permeability and low salt
rejection were prepared from polyimide, polyoxadiazole,
polyvinylchloride/cellulose acetate blend, and cellulose
Recent Trends in Membrane Processes for Water Purification …
43
membranes can be categorized into integrally skinned
asymmetric cellulosic membranes, non-cellulosic polymer
membranes, and thin-film interfacial composite membranes
depending on whether the membrane structure consists of
the same or different materials (Wang and Wang 2019). In
addition, several other BWRO membranes have also been
prepared from some other materials during the last few
years. Research advancements regarding the development
and modification of commercially important BWRO
desalination membranes to enhance their desalination performance are briefly described here.
3.1 Integrally Skinned Cellulosic Membranes
The earliest defect-free high-performance anisotropic
BWRO desalination membrane was prepared from cellulose
acetate via Loeb–Sourirajan method (Loeb and Sourirajan
1963; Kimura and Sourirajan 1967; Loeb 1981). Discovery
of this first asymmetric membrane composed of layered
structure proved to be a revolutionary achievement resulting
into the commercialization of BWRO desalination process
(Ulbricht 2006; Glater 1998). In spite of being one of the
oldest BWRO membranes, cellulosic membranes still
occupy a small size of the market due to its salient features
including easy manufacturing, good mechanical toughness
and comparatively high resistance against chlorine attack as
compared to interfacial composite membranes. Sterilization
of infected feed water by chlorine or other oxidizing agent
can be easily accomplished via these membranes since they
can resist perpetual exposure of 1 ppm chlorine.
Being highly sensitive to acetyl content contained by
them, water flux and salt rejection characteristics of cellulose
acetate membranes can be significantly improved by
increasing the acetylation content of these membranes (Reid
and Breton 1959; Rosenbaum et al. 1967; Lonsdale 1966).
Cellulose diacetate and cellulose triacetate membranes
exhibiting isotropic morphology were subsequently developed to improve permeate flux and salt rejection properties
(Sidney and Srinivasa 1964; Kucera 2010; Loeb and
Sourirajan 1963). An increase in degree of acetylation of
cellulose
acetate
membranes generally
improves
water-to-salt flux ratio at the expense of reduced water flux
(Lonsdale et al. 1965). In contrast to their diacetate counterpart, cellulose triacetate membranes are thermally,
chemically, and biologically more stable. As compared to
cellulose triacetate membrane, a blend membrane of cellulose diacetate and cellulose triacetate was prepared to further
improve water flux, salt rejection and more importantly
densification resistance when operated under high pressures
(Sudak 1990; Lee et al. 2011). Advantageous features of
cellulose acetate membranes encompass decent fouling
resistance, reasonable chlorine resistance, and low preparation cost (Kucera 2010; El-Saied et al. 2003; Singh 2015).
Practical limitations of cellulose acetate membranes include
low silica rejection, degradation susceptibility towards bacteriological attack, and low thermal stability. Normally,
cellulose acetate membranes are used below an operating
temperature of 35 °C (Kucera 2010). Furthermore, these
membranes can only be safely used in a narrow operating
range of pH of feed water; acetate groups tend to gradually
hydrolyze outside the pH range of 4–6 and leads to shortening of membrane lifetime (Vos et al. 1966). Key information of some of the commercially available cellulose
acetate membranes is reported in Table 1.
3.2 Non-cellulosic Polymer Membranes
Integrally skinned isotropic membranes can also be commercially prepared from non-cellulosic polymers like aliphatic and aromatic polyamides. Asymmetric membranes
prepared from aliphatic polyamides offered mild water fluxes
and low salt rejections. Various asymmetric membranes
composed of aromatic polyamide in the form of hollow fiber
rendered low water fluxes and reasonable salt rejections
(Endoh et al. 1977; McKinney and Rhodes 1971; Richter
and Hoehn 1971). Aromatic polyamide membranes having
trade name Permasep
® B-9 were prepared by Du Pont to
commercially desalinate brackish water on a large scale
(Mehta and Loeb 1978; Hoehn and Richter 1973). As
compared to cellulose membranes, separation performance
of polyamide membranes was found to be better and
remained a first priority of BWRO plants until early 1990s.
Noticeable features of polyamide membranes include good
permeate flux, high salt rejection, moderate resistance
against hydrolysis and biological attack, good thermal and
mechanical stability, and wide pH range operability (Soltanieh and Gill 1981). Du Pont, however, abandoned the
production of polyamide membranes on account of their
susceptibility to disinfectants on prolonged exposure to
chlorine or ozone (Lee et al. 2011).
With the passage of time, other integrally skinned
asymmetric BWRO desalination membranes were also prepared from polybenzimidazole and polybenzimidazoline
materials (Sawyer and Jones 1984; Goldsmith et al. 1977).
These membranes not only had inherently low salt rejection
but were also susceptible to chlorine attack. Aiming to
overcome this difficulty, membranologists prepared polypiperazinamides membranes but could not be commercialized
owing to their low salt rejection (Credali et al. 1974; Parrini
1983; Credali and Parrini 1971). Other integrally skinned
asymmetric membranes giving low permeability and low salt
rejection were prepared from polyimide, polyoxadiazole,
polyvinylchloride/cellulose acetate blend, and cellulose
Recent Trends in Membrane Processes for Water Purification …
43