Environmental Engineering Reference
In-Depth Information
threatened resource is underground water contained in aquifers, which is pumped at rates
higher than the rate of replenishment, or when aquifers that get contaminated as a result of
human activity.
The largest amounts of water are used for power generation and agriculture. Thermoelectric
power plants rely on cooling water to condensate steam at the turbines' exhaust. There are two
types of cooling systems: open-loop and closed-loop. Open-loop systems withdraw large
amounts of water from surface bodies and water is returned after use with little loss to evapo-
ration (less than 1 percent). Closed-loop systems withdraw less than 5 percent of open-loop
systems, but it is almost all lost to evaporation (Department of Energy [DOE], 2006).
Agriculture uses surface water, when available, and if not, extracts from underground aqui-
fers. As a consequence, levels are declining all around the world. Rivers are becoming casual-
ties of overpumping, too. Some rivers do not reach the ocean anymore because water is
excessively intercepted and used in agriculture or for human consumption in cities.
Where inexpensive electricity is available, salt from saline water can be removed in desalina-
tion plants and used for human consumption and low volume industrial use. However, this
process is too expensive for its use in agriculture and it is likely that it will never be cost-effective.
Agriculture is a vital activity because food is produced in the fields, and the lack of water
in the future will compromise the food security, especially in countries who are net importers
of food. Many fertile lands are located in low precipitation areas and depend exclusively on
ground water. As underground water levels decline, so does the availability for irrigation and
crop yields. In the industrial sector, the food industry is particularly intensive in the use of
water and generation of wastewater.
As water sources decline, a water crisis is imminent and there is a need for conservation
strategies, which also produce a cut in energy use. The main water conservation initiatives are
reduction, reuse, and recycling. Reduction can be achieved by optimizing water utilization
with better technologies and calibration of equipment and devices. Reuse is about using the
water multiple times in the same process, and recycling consists of using the effluent from one
process into another when possible.
Materials
Materials in this section refer to resources that are needed besides energy and water to manu-
facture goods that have become essential to modern life and the production of food. The most
essential materials are metals, minerals, polymers, and plant nutrients. In terms of abundance,
aluminum and iron are plentiful in the ground. However, it takes energy to extract and process
them, especially aluminum, which is an energy-intensive operation. Other metals that are
important to manufacture of goods are not as abundant. Table 15.1 shows some metals that can
reach depletion in the next few decades unless new deposits are discovered. The advantage of
metals is that if collected they can be recycled.
Of plant nutrients, phosphorus is a limiting nutrient in most soils, thus needing supplemen-
tation. At the current mining rate or 167
×
10
6
metric tons a year and considering confirmed
10
9
metric tons of premium reserves, it would take around 100 years to reach
depletion (
see
Table 3.2). Although considered marginal reserves and subeconomical resources,
phosphorus reserves climb to 47
reserves of 15
×
10
9
metric tons, which would extend the potential depletion
to 280 years. The problem with phosphorus is that there is no substitution in agriculture.
Declining of phosphorus reserves could be mitigated by avoiding overfertilization by
substituting industrial phosphorus use with other alternatives and by recovering it from waste-
water streams. Unfortunately, no cost-effective technologies for phosphorus recovery are
currently available.
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