Environmental Engineering Reference
In-Depth Information
513.00
51.55%
Thermoelectric power
304.00
30.55%
Irrigation
103.74
10.42%
Public supply
56.62
5.69%
Industrial
10.03
1.01%
Aquaculture
4.71
0.47%
Mining
2.84
0.29%
Livestock
0.22
0.02%
Domestic
-
100
200
300
400
500
600
Cubic meters/day (million)
Figure 9.4
Freshwater withdrawals in the United States from different sectors during the year 2000.
From “Surface water use in the United States,” n.d.
Most aquifers around the world are exploited at a rate much higher than their rate of
recharge, known as overdraft, which eventually leads to the depletion of aquifers. Unfortunately,
what happens with aquifers is similar to what is happening with petroleum. It is known that
these resources are going to come to an end, but very little is being done to devise a more
sustainable use of these resources.
Use of surface water
Surface water coming from lakes, rivers, and reservoirs are the main source of freshwater used
for public consumption, irrigation, mining, thermoelectric power, and industrial applications.
In 2000 in the United States, 74 percent of the total freshwater came from surface water that
was used according to the distribution shown in Figure 9.4 (US Geological Survey, 2004).
About 51 percent of the total was used in thermoelectric power production. Typically coal-
fired power plants have efficiencies less than 40 percent, or close to 60 percent for gas turbine/
steam turbine combined cycles (Bessette, 2003); the rest is lost as heat that needs to be
removed. A portion of the residual heat goes to the flue gas stack and the remainder is removed
with heat exchangers at the low pressure side of steam turbines. These heat exchangers can
be fed with cool water from a river, lake, or reservoir, circulated once and discharged (once-
trough cooling), or can be cooled down with water that is recirculated in a closed loop and
cooled in a cooling tower. Closed-loop cooling is less efficient in terms of water consumption
than once-through cooling systems because water is evaporated in cooling towers (Table 9.1).
The second largest consumer sector is irrigation, which takes about 30 percent of the sur-
face water total use, followed by public consumption and industrial uses.
Even when surface water is “safer” to deplete than groundwater, its utilization is not
immune to threats in the near future. Bodies of water that are fed by aquifers are affected by
