Environmental Engineering Reference
In-Depth Information
TABLE 27.1
Worldwide Desalination Capacity
Region
Desalination Capacity (10
6
m
3
)
% Desalination Capacity
Middle East
31.2
52%
North America
9.6
16%
Europe
7.8
13%
Asia
7.2
12%
Africa
2.4
4%
Central America
1.8
3%
Australia
0.18
0.3%
Source:
Data from Humplik, T., J. Lee, S.C. O'Hern, B.A. Fellman, M.A. Baig, S.F.
Hassan, M.A. Atieh, F. Rahman, T. Laoui, R. Karnik, and E.N. Wang,
Nano-
technology
, 22, 1, 2011.
27.2 Review of Existing Water Desalination Methods
A wide variety of water desalination methods exist. In this section, a brief review of the
most common methods is presented; therefore, not all techniques were included in this
chapter. However, the reader is pointed to several other articles and topics along with a
broad variety of literature cited throughout this chapter to allow further reading given the
reader's speciic interests.
27.2.1 Theoretical Minimum Energy Requirement for Water Desalination
Consider an equilibrium analysis for the process of water desalination. From the second
law of thermodynamics, for reversible processes the amount of energy used for such a pro-
cess is independent of the method used [12]. As a consequence, given the starting and tar-
geted salinity of the water being treated, it is possible to calculate a theoretical minimum
amount of energy required for water desalination. This exercise may appear academic, but
is of value since the energy-water nexus [13,14] is a major consideration toward evaluat-
ing the current state of water desalination technologies and identifying new technologies.
Consider an ideal compressor for moving water vapor from a tank of seawater (typically
assumed at ~35,000 ppm or mg/l salinity) to an ininitely large tank of freshwater (typi-
cally ~500 ppm salinity) [12]. Using these conditions, the minimum energy requirement
for water desalination at 25°C with a recovery rate of zero (i.e., negligibly small amount
of water produced from a near-ininite amount of seawater) is 2.5 kJ/l [12]. Recovery rate
is deined as the ratio of freshwater produced to the inlet salt water. For a viable system,
recovery ratios must be maximized in contrast to the waste or brine streams. Increase of
the recovery rates to 25%, 50%, and 75% requires theoretical energy minimums of 2.9, 3,5,
and 4.6 kJ/l of freshwater product, respectively [1,15]. Therefore, in principle, it should be
possible to remove salt from water in an eficient manner with lower energy consumption
than being achieved currently. However, several challenges exist to achieving these theo-
retical limits.
