adaptable to industries in a profitable manner even though the process is slow and is
limited to acid/base recovery.
8.4.8 Synergism of Different Membrane Processes in Water
Treatment
Each membrane process has its unique advantages and challenges. The pressuredriven membrane processes, ultrafiltration, nanofiltration, and reverse osmosis, have
complementary characteristics. The processes, in pairs or sometimes all together, are
used in water treatment applications. The process sequence ultrafiltration–
nanofiltration–reverse osmosis may have to be maintained to get the synergism. In
fact, ultrafiltration is a standard pretreatment for reverse osmosis in most instances.
Ultrafiltration–reverse osmosis and ultrafiltration–nanofiltration–reverse osmosis are
combinations which can provide fractionation of the solute species besides recovery
of water. Since the operating pressure increases along the sequence, booster pumps
are necessary between each membrane operation.
The advantages of using hyphenated membrane systems include satisfactory
pretreatment through ultrafiltration to improve the sustainability of the nanofiltration
or reverse osmosis systems. The use of nanofiltration in combination with reverse
osmosis has the following advantages:
(a) The operating pressure can be reduced because of the poor rejection of monovalent species by nanofiltration and the consequent reduction in net osmotic
pressure.
(b) Reverse osmosis can operate at lower pressures due to less concentration of
nanofiltration permeate leading to better recovery and better quality of the
product (Helal 2009).
(c) Since the reject of reverse osmosis plant would be having mostly monovalent
solutes and less in concentration corresponding to the seawater feed, it can be
blended with feed to bring down the salinity and hence obtain higher recovery
(Song et al. 2015b).
(d) The intangible advantage of nanofiltration–reverse osmosis could possibly be
less fouling and less maintenance requirements because of slight charge on
nanofiltration membrane surface.
Ahunbay (2019) has indicated that by a combination of reverse osmosis and
nanofiltration in a multistage configuration, the specific energy consumption can be
brought lower than single-stage seawater desalination system. However, one may
have to assess the specific energy consumption of seawater reverse osmosis versus
nanofiltration–reverse osmosis system, as the energy recovery component would be
less for nanofiltration–reverse osmosis system. For example, in seawater desalination, the combination of nanofiltration and reverse osmosis can lead to recovery of
value. The concentrate of reverse osmosis would be rich in sodium chloride with
8 Role of Membranes in Wastewater Treatment
273
limited to acid/base recovery.
8.4.8 Synergism of Different Membrane Processes in Water
Treatment
Each membrane process has its unique advantages and challenges. The pressuredriven membrane processes, ultrafiltration, nanofiltration, and reverse osmosis, have
complementary characteristics. The processes, in pairs or sometimes all together, are
used in water treatment applications. The process sequence ultrafiltration–
nanofiltration–reverse osmosis may have to be maintained to get the synergism. In
fact, ultrafiltration is a standard pretreatment for reverse osmosis in most instances.
Ultrafiltration–reverse osmosis and ultrafiltration–nanofiltration–reverse osmosis are
combinations which can provide fractionation of the solute species besides recovery
of water. Since the operating pressure increases along the sequence, booster pumps
are necessary between each membrane operation.
The advantages of using hyphenated membrane systems include satisfactory
pretreatment through ultrafiltration to improve the sustainability of the nanofiltration
or reverse osmosis systems. The use of nanofiltration in combination with reverse
osmosis has the following advantages:
(a) The operating pressure can be reduced because of the poor rejection of monovalent species by nanofiltration and the consequent reduction in net osmotic
pressure.
(b) Reverse osmosis can operate at lower pressures due to less concentration of
nanofiltration permeate leading to better recovery and better quality of the
product (Helal 2009).
(c) Since the reject of reverse osmosis plant would be having mostly monovalent
solutes and less in concentration corresponding to the seawater feed, it can be
blended with feed to bring down the salinity and hence obtain higher recovery
(Song et al. 2015b).
(d) The intangible advantage of nanofiltration–reverse osmosis could possibly be
less fouling and less maintenance requirements because of slight charge on
nanofiltration membrane surface.
Ahunbay (2019) has indicated that by a combination of reverse osmosis and
nanofiltration in a multistage configuration, the specific energy consumption can be
brought lower than single-stage seawater desalination system. However, one may
have to assess the specific energy consumption of seawater reverse osmosis versus
nanofiltration–reverse osmosis system, as the energy recovery component would be
less for nanofiltration–reverse osmosis system. For example, in seawater desalination, the combination of nanofiltration and reverse osmosis can lead to recovery of
value. The concentrate of reverse osmosis would be rich in sodium chloride with
8 Role of Membranes in Wastewater Treatment
273