Environmental Engineering Reference
In-Depth Information
well-established separation or distillation technologies not previously used with water, or
revisiting older methods with advanced technologies. Examples include using forward
osmosis [67], humidiication-dehumidiication cycles [68], and trapping solar energy for
thermal distillation processes [69]. The second approach relies on exploiting transport
phenomena at the nanoscale and therefore employs use of nanoluidics, nanotechnology,
bio-inspired methods, and nanomaterials [5,30,70-79]. Both approaches present their own
sets of pros and cons, including challenges in energy consumption, management of waste
streams, fabrication and cost considerations, and eventual output lux for a given water
quality. Of the various methods being considered, the biotechnology- or nanotechnology-
driven approaches are considered to be particularly promising as these approaches work
at length scales where fundamental physical processes occur. Consequently, with recent
advances in fabrication techniques, it is possible to develop systems that can manipulate
these physical processes and thereby develop the next-generation water desalination sys-
tems working at near the thermodynamic energy consumption limits. Next, a discussion
of some of the emerging biotechnology and nanotechnology approaches is presented.
27.3.1 Forward Osmosis
Desalination by the process of forward or direct osmosis (FO or DO) employs a highly
concentrated solution (also referred to as the draw solution) to create an osmotic pressure
that extracts freshwater from saline water across a semipermeable membrane [80]. Unlike
RO processes, because FO uses osmotic pressure as the driving force, this method operates
at a small or zero hydraulic pressure, which reduces the amount of observed fouling and
allows the membrane to operate with a fairly sparse support structure [29,81]. The reliance
on osmotic pressure necessitates that a major factor in the choice of the draw solution is the
osmotic pressure of the draw solution [29]. The lack of hydraulic pressure means the only
force the low has to overcome is the few bars of pressure created by the physical barrier
of the membrane itself [29].
Possibilities for the draw solution are numerous; for water desalination solutions of
sulfur dioxide, aluminum sulfate, glucose, potassium nitrate, mixtures of glucose and
fructose, and mixtures of ammonia (NH
3
) and carbon dioxide (CO
2
) gases have all been
suggested as draw solution candidates [29]. In addition, nanoparticles such as magnetofer-
ritin (~12 nm in diameter) have also been demonstrated [82]. However, many draw solu-
tions add an aftertaste or an odor requiring post-treatment. Sugar (glucose and fructose
primarily) has been a popular draw solution due to the minimal need for extensive post-
treatment of desalinated water. The major energy requirement in FO is for treatment of
the draw solution. For draw solutions of NH
3
and CO
2
, which both are soluble in water,
low-quality thermal energy can be used to decrease the solubility and energy needed to
volatilize the NH
3
and CO
2
to remove as a gas and reuse upstream. Magnetic nanopar-
ticles such as magnetoferritin are readily iltered out by magnetic ilters as developed by
industrial metal processing [82].
A major hindrance to this technology has been the development of an appropriate mem-
brane [83]. Ideally, FO membranes should allow a high lux of water while retaining a
high rejection rate of dissolved solids, demonstrate compatibility with both the feedwa-
ter and the draw solution, and handle the mechanical stresses imposed by the osmotic
pressures experienced during the desalination process [83]. Currently available commer-
cial membranes such as cellulose triacetate membranes are not compatible with many of
the preferred draw solutions. Owing to the low pressure required across the membrane
for FO processing, fragile membranes made from aquaporins, which are nature's perfect
