Environmental Engineering Reference
In-Depth Information
nanochannels and permit only water to low through, can potentially be used for FO.
Other membrane materials such as cellulose acetate, polybenzimidazole, and aromatic
polyamides have also been used for FO puriications [84].
Other challenges to FO technology are overcoming the coupled effects of internal con-
centration polarization (ICP) and fouling. Internal concentration polarization occurs as a
permeate dilutes the strength of the draw solution in the supporting layer of the mem-
brane. This is especially detrimental to FO membranes because the osmotic pressure is
the driving force for low through the membrane [85]. One method of combating ICP is
the development of the FO membrane without a fabric support layer, but instead using a
supportive mesh integrated with the selective polymer layer. This solution has been shown
to reduce ICP, but not eliminate the effects altogether [86]. A major advantage of FO is that
many of the FO membranes used for desalination processes are compatible with chlo-
rination; thus, less membrane maintenance and biofouling tends to occur [87]. There is
the potential with FO membranes to produce a higher amount of product water from the
feedwater if scaling and precipitation can be prevented. If this percentage is high enough,
the salt could be left to precipitate out, thus reducing the amount of waste produced [86].
FO membrane technology has also been used to develop water puriication for emer-
gency water supply in the event of water supply disruption or natural disaster. Hydration
Technology Innovations LLC (HTI, Scottsdale, AZ) commercially produces emergency
pouches that produce a clean energy drink from any available water source. In addition,
the irst commercial FO desalination plant went into operation in 2010 in Al Khaluf, Oman.
The plant design employs an FO cell to draw clean water from the feedwater and then
treats the drawn solution with a RO process to produce the inal product water [87]. The
FO/RO cells are sized so that the waste from the RO system is recycled back as the draw
solution for the FO system, thus cutting down on waste and chemicals needed for process-
ing, which is a common practice in FO setups with inal membrane processing step for
product water.
27.3.2 Microfluidic and Nanofluidic Concentration Polarization for Desalination
Concentration polarization in most water desalination systems is considered to be a loss
term, and immense resources and effort have been employed in better understanding
and mitigating polarization effects [25,88,89]. Recent research has revealed that the imbal-
ances created by concentration polarization (enrichment and depletion regions) can be
sustained for extended periods of time when triggered across a nanochannel connecting
two microchannels [90]. On the basis of this fact, researchers demonstrated the ability to
desalinate water using a setup as shown in Figure 27.13, thereby exploiting polarization for
separations. However, for eficient separation, pretreatment was needed for the removal
of Ca 2+ ions and physical iltration for elimination of precipitation and large debris. The
membrane-less system that was tested was primarily driven by electrokinetic lows and
achieved a 99% salt rejection ratio [5]. In addition to achieving seawater desalination, this
system was shown to remove most solid particles, microorganisms, and biomolecules
owing to their charged nature [5]. The energy consumption of this system for the actual
separation process is modest due to the delection of ions rather than physical displace-
ment across a membrane and the low low resistance of microchannels and energy con-
sumption of approximately 5 Wh/l for low rates of 0.25 μl/min [5]. Although considerable
challenges lie ahead for the scaling up of a system using the concentration polarization
technique, massive parallelization of the proof-of-concept device is estimated to deliver
approximately 180-288 ml/min for a small-scale system [5].
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