Environmental Engineering Reference
In-Depth Information
Table 9.2
Classification of water based on saline content and palatability as a function
of total dissolved solids.
Classification of water according to salt content
a
• Fresh, less than 1 g/l
• Slightly saline, 1-3 g/l
• Moderately saline, 3-10 g/l
• Highly saline, 10-35 g/l
Palatability of drinking water
b
• Excellent, less than 300 mg/l
• Good, 300-600 mg/l
• Fair, 600-900 mg/l
• Poor, 900-1200 mg/l
• Unacceptable, greater than 1200 mg/l
a
“Thirsty? How 'bout a cool, refreshing cup of seawater?”, n.d.
b
World Health Organization (WHO), 1996.
Table 9.3
Energy requirements for different desalination processes.
Process heat input
Heat input (kJ/kg
of product)
Mechanical power
input (kWh/m
3
of
product)
Prime energy
consumption (kJ/kg
of product)
a
Multistage flash (MSF)
294
2.5-4 (3.7)
b
338.4
Multiple effect boiling (MEB)
123
2.2
149.4
Vapor compression (VC)
—
8-16 (16)
192
Reverse osmosis (RO)
—
5-13 (10)
120
RO with energy recovery
(ER-RO)
—
4-6 (5)
60
Electrodialysis (ED)
—
12
144
Solar
2330
0.3
2333.6
a
Assumed conversion efficiency for electricity generation of 30 percent.
Reproduced from Kalogirou, 2005, with permission of Elsevier.
seawater that then is condensed into drinkable water. Reverse osmosis uses semipermeable
membranes and high pressures to overcome the osmotic pressure of seawater and force the
pure solvent (water) through.
Desalinization by distillation is more energy intensive than reverse osmosis (Table 9.3).
Energy expense for distillation systems can be improved by using waste heat from power
generation to evaporate water (cogeneration). But in this case, dedicated plants need to be
constructed on coastal areas. Another problem with cogeneration is that power plants never
have a 100-percent capacity factor. Capacity factors can reach 73 percent for coal-fired plants,
42 percent for natural gas, and 91 percent for nuclear plants (Department of Energy [DOE],
2010). As a consequence, there are periods of time with no generation of freshwater from
these plants.
The cost of water desalinization using membranes is around $3.67 per thousand gallons
($0.96/m
3
), from which 44 percent goes to electricity, 37 percent to facilities depreciation,
11 percent to labor, 5 percent to membranes, and 3 percent to chemicals (Voith, 2009). In
contrast, the cost of potabilization of surface water in the United States averages $0.5/m
3
(Pimentel and Pimentel, 2008).
Even though desalination is technically and economically feasible in places where
freshwater is scarce, it does not come without major direct and indirect environmental
