Environmental Engineering Reference
In-Depth Information
presented. The purpose of this chapter is to highlight the water-energy nexus, with a
focus on generating freshwater from salt water and identifying emerging technologies,
materials, and methods that can provide potentially more energy-eficient pathways than
existing methods.
To discuss developing technologies for water puriication and supply increase, it is
important to review current water treatment processing as well as the energy cost of this
treatment. In the United States, water is generally sourced from a surface source such
as a lake or harvested from available groundwater supplies. Surface water puriication is
typically achieved by a series of processes, as shown schematically in Figure 27.2. Water is
pumped in from the source through a screen and treated to remove odor and organisms.
Next, water is transferred to the coagulation area and mixed with alum and other chemi-
cals to help large particles like dirt form “locs” that are heavy enough to settle out. The
water is then transferred to sedimentation basins where the locs are allowed to settle to
the bottom while the clear water travels to the iltration step, where water travels through
a ilter of sand, gravel, and charcoal to remove smaller particles. Finally, the water is disin-
fected and then distributed through the system. This example is a typical treatment pro-
cess; however, depending on incoming source water and regulations, additional steps may
be added. For plant sizes ranging from 1 to 100 million gallons per day (MGD), the electri-
cal energy requirement ranges from 1483 to 1407 kWh/million gallons treated. Pumping
accounts for 80%-85% of these energy requirements. Groundwater treatment requires less
processing than surface water; water is pumped into the plant and disinfected then dis-
tributed to the system. Energy costs for producing this water are approximately 1824 kWh/
million gallons treated, almost 30% higher than surface water production due to the large
energy consumption of the pumps needed to retrieve the water [9]. The total electricity
demand for water puriication of the public supply in the United States was 32 billion kWh
in 2005 [9].
Water for desalination processing is usually obtained from either open surface sea
intakes or underground beach wells [8]. Worldwide desalination capacity grew by 12.4%
in 2011 to 71.9 million m 3 of seawater processed per day [10]. This desalted water was sup-
plied by 14,451 desalination plants, while another 244 were being planned or built.
Table 27.1 summarizes the 2009 desalination capacity by region [7]. Desalination plant
development is expected to continue growing, with an estimated worldwide desalination
capacity of 104 million m 3 projected to cost $17 billion total by 2016; the majority of that
investment (~$13 billion) is expected to be toward reverse osmosis (RO) plant development
[8,11].
Filtration
media
Intake and
screening
Distribution
Coagulation and
flocculation
Sedimentation
Filtration
Disinfection
FIGURE 27.2
Schematic depicting basic water treatment of surface waters from source to distribution. Intake water is
screened and coagulants are added, and locs are allowed to form. These locs settle in the sedimentation sec-
tion. Water is subsequently iltered through gravel, sand, and charcoal ilters to remove remaining particles,
then disinfected and distributed.
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