2010). This concept could be quite useful for the recovery of salts and water from the
blowdown of thermal desalination plant. One of the potential applications of the
membrane distillation in water treatment besides recovering water is to reduce the
temperature of return cooling water being disposed into the environment (Jansen
et al. 2007). Membrane distillation can be coupled with renewable energy (Blanco
Gálvez et al. 2009), particularly solar, for the concentration of solutions including
wastewater (Walton et al. 2004). Membrane distillation can be used for
de-moisturization of wet steam as moisture can get condensed on the retentate side
while dry steam can be sent for appropriate use. Condensation assisted by membrane
represents a new source of water (Drioli et al. 2015).
8.4.4 Ultrafiltration
Ultrafiltration membranes were commercially developed later than reverse osmosis
membranes. The applications have encompassed many areas including wastewater
treatment and water purification. Being a low-pressure technique, the energy consumption is low compared to other pressure-driven processes, and the virtue of
ultrafilters is its amenability for backwashing and possibility of dead-end operation
mode, which provides nearly 100% recovery of the fluid or solids. The nominal pore
size of ultrafilters may be in the range of about 20 nm to 0.1 microns. Commercially
ultrafiltration systems are available in different ranges of pore sizes, specified in
terms of molecular weight cutoff from 5 kilo Dalton to 1.2 lakh kilo Dalton. Size
exclusion is the basic philosophy of separation, and osmotic pressure is normally not
a limitation to the process. As the filtration proceeds, the pressure drop across the
membrane, i.e., transmembrane pressure drop, would increase leading to reduction
in flux. The flux can be nearly restored by backwashing, i.e., by allowing the water to
flow from product to feed side for about a minute. The commercially operating
system has a backwash cycle for about 1–2 min, for every 40–50 min of service
cycle. Most of the seawater reverse osmosis desalination plants use ultrafiltration for
the pretreatment as it gives high-quality treated water. After many pilot studies
between 1995 and 2005, ultrafiltration has been installed as pretreatment system in
many large-scale seawater reverse osmosis plants (Busch et al. 2009).
Size-enhanced ultrafiltration is a technique whereby the size of the desired
species is enhanced and separated through ultrafiltration. This method is limited to
small concentration of solutes present in bulk solution such as the presence of very
small amounts of heavy metal species. Increase in size can be due to complexation,
coprecipitation, and adsorption. Because of the size, these species are retained by the
ultrafiltration membranes. The advantages of this technique include:
1. Isolation and removal of trace metal species in the presence of bulk component
based on size exclusion principle.
2. The process is not limited by osmotic pressure constraints as in reverse osmosis
and nanofiltration.
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A. Kapoor et al.
blowdown of thermal desalination plant. One of the potential applications of the
membrane distillation in water treatment besides recovering water is to reduce the
temperature of return cooling water being disposed into the environment (Jansen
et al. 2007). Membrane distillation can be coupled with renewable energy (Blanco
Gálvez et al. 2009), particularly solar, for the concentration of solutions including
wastewater (Walton et al. 2004). Membrane distillation can be used for
de-moisturization of wet steam as moisture can get condensed on the retentate side
while dry steam can be sent for appropriate use. Condensation assisted by membrane
represents a new source of water (Drioli et al. 2015).
8.4.4 Ultrafiltration
Ultrafiltration membranes were commercially developed later than reverse osmosis
membranes. The applications have encompassed many areas including wastewater
treatment and water purification. Being a low-pressure technique, the energy consumption is low compared to other pressure-driven processes, and the virtue of
ultrafilters is its amenability for backwashing and possibility of dead-end operation
mode, which provides nearly 100% recovery of the fluid or solids. The nominal pore
size of ultrafilters may be in the range of about 20 nm to 0.1 microns. Commercially
ultrafiltration systems are available in different ranges of pore sizes, specified in
terms of molecular weight cutoff from 5 kilo Dalton to 1.2 lakh kilo Dalton. Size
exclusion is the basic philosophy of separation, and osmotic pressure is normally not
a limitation to the process. As the filtration proceeds, the pressure drop across the
membrane, i.e., transmembrane pressure drop, would increase leading to reduction
in flux. The flux can be nearly restored by backwashing, i.e., by allowing the water to
flow from product to feed side for about a minute. The commercially operating
system has a backwash cycle for about 1–2 min, for every 40–50 min of service
cycle. Most of the seawater reverse osmosis desalination plants use ultrafiltration for
the pretreatment as it gives high-quality treated water. After many pilot studies
between 1995 and 2005, ultrafiltration has been installed as pretreatment system in
many large-scale seawater reverse osmosis plants (Busch et al. 2009).
Size-enhanced ultrafiltration is a technique whereby the size of the desired
species is enhanced and separated through ultrafiltration. This method is limited to
small concentration of solutes present in bulk solution such as the presence of very
small amounts of heavy metal species. Increase in size can be due to complexation,
coprecipitation, and adsorption. Because of the size, these species are retained by the
ultrafiltration membranes. The advantages of this technique include:
1. Isolation and removal of trace metal species in the presence of bulk component
based on size exclusion principle.
2. The process is not limited by osmotic pressure constraints as in reverse osmosis
and nanofiltration.
268
A. Kapoor et al.