Environmental Engineering Reference
In-Depth Information
13.9 Desalination
Thirsty. In the water-short regions of the world, desalination has become one of the best
ways to provide additional water supplies—for drinking and industrial purposes. As a
result, seawater desalination has become an important segment of the water business.
13.9.1 Background
Seawater generally has concentrations of dissolved salts that are from 10,000 parts per
million (ppm) up to 35,000 ppm, according to the USGS [13]. The agency noted that ocean
water salt content is commonly 35,000 ppm. In contrast, freshwater will have <1000 ppm
in saline content.
Desalination involves the use of either thermal distillation technologies or seawater RO
(SWRO) to remove salt content to make ocean or brackish water suitable for drinking or
industrial use. The three primary thermal technologies are multistage lash (MSF), multi-
effect distillation (MED), and vacuum compression (VC).
There are >14,000 desalination plants worldwide that can produce more than 59.9 mil-
lion m 3 /day of treated water, according to a report by the International Desalination & Water
Reuse Quarterly [14]. These plants range from the size of massive desalination facilities in
the Middle East and Australia with capacities in the millions of gallons per day to small
facilities on islands in the Caribbean that treat a few thousand gallons per day. An exam-
ple of one of the world's largest desalination plants is the Kurnell Desalination Plant near
Sydney, Australia, that has a daily capacity of 66 million gal (250 ML). Desalination plants
are also found on military and commercial ships.
13.9.1.1 Energy Use
One criticism of desalination is the high cost of running the pumps for SWRO, or heating
the water in thermal processes. There is no question that desalination is more expensive
than the treatment of freshwater resources. Briely, here are some comparisons of the cost
between membrane and thermal technologies.
N. Voutchkov [15] noted that the energy use by seawater desalination plants is between
12 and 15 kWh/1000 gal of freshwater. That compares to from 3 to 8 kWh/1000 gal for
brackish RO plants. In contrast, thermal desalination can use between 25 and 40 kWh/1000
gal. Of the three principal thermal technologies, MSF uses more energy than MED plants.
In turn, MED facilities consume more energy than VC plants.
Table 13.4 [16] provides a comparison of the energy use of water supply alternatives. As
can be seen, desalination does require more energy than other sources. However, perhaps
nanotechnology can come to play an important role in desalination, even lowering the cost.
13.9.1.2 Developing New Technologies
Another approach to save energy being pursued by some is the development of tech-
nologies that can either save energy or more eficiently remove salt. Nanotubes are one
example. This approach would involve membranes made of carbon nanotubes (CNTs) and
silicon. Researchers at Lawrence Livermore National Laboratory [17] found that liquids
and gases will low rapidly through the tubes. One use they found was that the nanotubes
could be used for desalting or deionizing water.
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