249
make aligned nanotubes. A continuous high-yield CVD prototype has been designed
for producing vertically aligned CNT, paving the way for large-scale production [76].
A post-manufacturing alignment method using magnetic field was also developed [215].
Nanocomposite and TFN membranes have good scalability as they can be fabricated using current industrial manufacturing processes. The high water permeability can reduce the applied pressure or required membrane area and consequently cut
cost. This strategy may greatly improve the energy efficiency for treatment of waters
with low osmosis pressure, but it may have limited advantage in seawater RO,
whose energy consumption is already close to the thermodynamic limit [85]. A
recent review ranked current membrane nanotechnologies based on their potential
performance enhancement and state of commercial readiness [257].
Forward Osmosis
Forward osmosis (FO) utilizes the osmotic gradient to draw water from a lowosmotic- pressure solution to a high-osmotic-pressure one (i.e., the draw solution).
The diluted draw solution is then treated by reverse osmosis or thermal processes to
generate pure water. FO has two major advantages over the pressure-driven reverse
osmosis: it does not require high pressure, and the membrane is less prone to
fouling.
The key to FO is to have a draw solute with high osmolality and that is easily
separable from water. Chemicals currently employed for draw solutions include
NaCl and ammonia bicarbonate. Therefore, RO or thermal treatment, both energy
intensive, is required to recover water from the draw solution. Magnetic nanoparticles were recently explored as a new type of draw solute for its easy separation and
reuse. Hydrophilic coating was employed to aid dissolution and increase osmotic
pressure. An FO permeate flux higher than 10 L/m
2
/h was achieved using 0.065 M
poly(ethylene glycol) diacid-coated magnetic nanoparticles when deionized water
was used as the feed solution [104]. Magnetic nanoparticles were also applied to
recover draw solutes. In a recent study, magnetic nanoparticles (Fe 3 O 4 @SiO 2 ) were
used to recover Al 2 (SO 4 ) 3 (the draw solute) through flocculation [193].
Photocatalysis
Photocatalytic oxidation is an advanced oxidation process for removal of trace contaminants and microbial pathogens. It is a useful pretreatment for hazardous and
nonbiodegradable contaminants to enhance their biodegradability. Photocatalysis
can also be used as a polishing step to treat recalcitrant organic compounds. The
major barrier for its wide application is the slow kinetics due to limited light fluence
and photocatalytic activity. Current research focuses on increasing photocatalytic
reaction kinetics and photoactivity range (Table 13.2).
Current and Potential Applications for Water and Wastewater Treatment
make aligned nanotubes. A continuous high-yield CVD prototype has been designed
for producing vertically aligned CNT, paving the way for large-scale production [76].
A post-manufacturing alignment method using magnetic field was also developed [215].
Nanocomposite and TFN membranes have good scalability as they can be fabricated using current industrial manufacturing processes. The high water permeability can reduce the applied pressure or required membrane area and consequently cut
cost. This strategy may greatly improve the energy efficiency for treatment of waters
with low osmosis pressure, but it may have limited advantage in seawater RO,
whose energy consumption is already close to the thermodynamic limit [85]. A
recent review ranked current membrane nanotechnologies based on their potential
performance enhancement and state of commercial readiness [257].
Forward Osmosis
Forward osmosis (FO) utilizes the osmotic gradient to draw water from a lowosmotic- pressure solution to a high-osmotic-pressure one (i.e., the draw solution).
The diluted draw solution is then treated by reverse osmosis or thermal processes to
generate pure water. FO has two major advantages over the pressure-driven reverse
osmosis: it does not require high pressure, and the membrane is less prone to
fouling.
The key to FO is to have a draw solute with high osmolality and that is easily
separable from water. Chemicals currently employed for draw solutions include
NaCl and ammonia bicarbonate. Therefore, RO or thermal treatment, both energy
intensive, is required to recover water from the draw solution. Magnetic nanoparticles were recently explored as a new type of draw solute for its easy separation and
reuse. Hydrophilic coating was employed to aid dissolution and increase osmotic
pressure. An FO permeate flux higher than 10 L/m
2
/h was achieved using 0.065 M
poly(ethylene glycol) diacid-coated magnetic nanoparticles when deionized water
was used as the feed solution [104]. Magnetic nanoparticles were also applied to
recover draw solutes. In a recent study, magnetic nanoparticles (Fe 3 O 4 @SiO 2 ) were
used to recover Al 2 (SO 4 ) 3 (the draw solute) through flocculation [193].
Photocatalysis
Photocatalytic oxidation is an advanced oxidation process for removal of trace contaminants and microbial pathogens. It is a useful pretreatment for hazardous and
nonbiodegradable contaminants to enhance their biodegradability. Photocatalysis
can also be used as a polishing step to treat recalcitrant organic compounds. The
major barrier for its wide application is the slow kinetics due to limited light fluence
and photocatalytic activity. Current research focuses on increasing photocatalytic
reaction kinetics and photoactivity range (Table 13.2).
Current and Potential Applications for Water and Wastewater Treatment