the solvent, leading to the formation of membranes in thermally induced phase
separation. Among these techniques, non-solvent-induced phase separation and
thermally induced phase separation are mostly used for the production of commercial membranes (Liu et al. 2011; Lalia et al. 2013). In stretching, the polymer is
heated above the melting point and then extruded into a thin film, which is subsequently stretched to form a porous matrix (Sadeghi et al. 2007). This technique does
not need any solvent. In sintering, the polymer powder is pressed into a thin film and
is sintered at a temperature just below the melting point. Electrospinning is a
developing technology which produces nano-fibers (Ray et al. 2016) under the
application of an electric field. Track-etched membranes are prepared by
bombarding a thin nonporous film with accelerated heavy ions, followed by etching
(Apel 2001).
Membranes used in water treatment are required to exhibit good solute rejection
and water-flux characteristics. Accordingly, the membranes should have less resistance for water flow without compromise on the strength of the membrane to
withstand the operating conditions. Reverse osmosis and nanofiltration membranes
by design have pores less than 2 nm and have to withstand operating pressures
ranging from 20 to 70 bars. In order to achieve these properties, membranes are
prepared in a two-step process. First, a porous support layer is prepared by phase
inversion, and then a very thin active layer is formed on its surface by in situ
polymerization of two reactive monomers (Petersen 1993).
Phase inversion techniques allow the preparation of porous membranes with
different ranges of pore-size distribution by varying the dope composition, the
casting conditions, and the posttreatment of the membranes. The average pore size
can be controlled by the variation of size and quantum of pore-inducing additives in
the membrane dope solution. The pore formation in the phase inversion technique is
a stochastic process, and hence, there would be a distribution of pore sizes, which is
assumed to follow normal distribution. On the other hand, stretching leads to
somewhat uniform pore size. Track etching gives nearly uniform pore size, but the
pore density would be very low, and the cost is very high. Charged membranes are
nonporous and prepared using resins mixed with some binding materials and cast
into films (Drioli and Giorno 2010).
Depending on the chemicals used and conditions of casting, membranes can be
prepared with hydrophilic or hydrophobic characteristics. The membrane processes
relevant to water treatment are pressure-driven ultrafiltration, nanofiltration and
reverse osmosis, concentration-driven forward osmosis and diffusion dialysis, thermally driven membrane distillation and electrically driven electrodialysis, electrodeionization, and bipolar membrane electrodialysis. Capacitive deionization is also
an electrically driven desalination technique but without the use of membranes and is
not to be discussed further.
Membranes used in pressure-driven membrane processes are on the hydrophilic
side. Reverse osmosis and nanofiltration membranes operate at high pressures and
are asymmetric in nature to reduce the resistance for water flow. Electrodialysis and
bipolar membrane use nonporous charged membranes and remove ions from solution. Forward osmosis and diffusion dialysis are passive processes using neutral
8 Role of Membranes in Wastewater Treatment
251
separation. Among these techniques, non-solvent-induced phase separation and
thermally induced phase separation are mostly used for the production of commercial membranes (Liu et al. 2011; Lalia et al. 2013). In stretching, the polymer is
heated above the melting point and then extruded into a thin film, which is subsequently stretched to form a porous matrix (Sadeghi et al. 2007). This technique does
not need any solvent. In sintering, the polymer powder is pressed into a thin film and
is sintered at a temperature just below the melting point. Electrospinning is a
developing technology which produces nano-fibers (Ray et al. 2016) under the
application of an electric field. Track-etched membranes are prepared by
bombarding a thin nonporous film with accelerated heavy ions, followed by etching
(Apel 2001).
Membranes used in water treatment are required to exhibit good solute rejection
and water-flux characteristics. Accordingly, the membranes should have less resistance for water flow without compromise on the strength of the membrane to
withstand the operating conditions. Reverse osmosis and nanofiltration membranes
by design have pores less than 2 nm and have to withstand operating pressures
ranging from 20 to 70 bars. In order to achieve these properties, membranes are
prepared in a two-step process. First, a porous support layer is prepared by phase
inversion, and then a very thin active layer is formed on its surface by in situ
polymerization of two reactive monomers (Petersen 1993).
Phase inversion techniques allow the preparation of porous membranes with
different ranges of pore-size distribution by varying the dope composition, the
casting conditions, and the posttreatment of the membranes. The average pore size
can be controlled by the variation of size and quantum of pore-inducing additives in
the membrane dope solution. The pore formation in the phase inversion technique is
a stochastic process, and hence, there would be a distribution of pore sizes, which is
assumed to follow normal distribution. On the other hand, stretching leads to
somewhat uniform pore size. Track etching gives nearly uniform pore size, but the
pore density would be very low, and the cost is very high. Charged membranes are
nonporous and prepared using resins mixed with some binding materials and cast
into films (Drioli and Giorno 2010).
Depending on the chemicals used and conditions of casting, membranes can be
prepared with hydrophilic or hydrophobic characteristics. The membrane processes
relevant to water treatment are pressure-driven ultrafiltration, nanofiltration and
reverse osmosis, concentration-driven forward osmosis and diffusion dialysis, thermally driven membrane distillation and electrically driven electrodialysis, electrodeionization, and bipolar membrane electrodialysis. Capacitive deionization is also
an electrically driven desalination technique but without the use of membranes and is
not to be discussed further.
Membranes used in pressure-driven membrane processes are on the hydrophilic
side. Reverse osmosis and nanofiltration membranes operate at high pressures and
are asymmetric in nature to reduce the resistance for water flow. Electrodialysis and
bipolar membrane use nonporous charged membranes and remove ions from solution. Forward osmosis and diffusion dialysis are passive processes using neutral
8 Role of Membranes in Wastewater Treatment
251