Environmental Engineering Reference
In-Depth Information
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Active Figure 11-24
Natural capital:
dilution and decay of degradable, oxygen-demanding wastes and
heat in a stream, showing the oxygen sag curve and the curve of oxygen demand. Depending on flow rates
and the amount of pollutants, streams can recover from such pollution if they are given enough time and are
not overloaded.
See an animation based on this figure and take a short quiz on the concept.
it flowed through Cleveland. The highly publicized
image of this burning river prompted elected officials
to enact laws limiting the discharge of industrial
wastes into the river and sewage systems, and to pro-
vide funds to upgrade sewage treatment facilities. To-
day the river is cleaner and is widely used by boaters
and anglers. This accomplishment illustrates the
power of bottom-up pressure by citizens, who prod-
ded elected officials to change a severely polluted river
into an economically and ecologically valuable public
resource. Individuals matter!
On the other hand, large fish kills and drinking
water contamination still occur in parts of developed
countries. Two causes of these problems are accidental
or deliberate releases of toxic inorganic and organic
chemicals by industries or mines and malfunctioning
sewage treatment plants. A third cause is nonpoint
runoff of pesticides and excess plant nutrients from
cropland and animal feedlots.
Stream pollution from discharges of untreated sew-
age and industrial wastes is a serious and growing
problem in most developing countries. According to
a 2003 report by the World Commission on Water in
the 21st Century, half of the world's 500 rivers are
heavily polluted; most of them running through de-
veloping countries. Most of these countries cannot
afford to build waste treatment plants and do not
have—or do not enforce—laws for controlling water
pollution.
Industrial wastes and sewage pollute more than
two-thirds of India's water resources and 54 of the 78
streams monitored in China. Only 10% of the sewage
produced in Chinese cities is treated. In Latin America
and Africa, most streams passing through urban or in-
dustrial areas suffer from severe pollution.
Science: Pollution Problems of Lakes
Dilution of pollutants in lakes is less effective than in
most streams because most lake water is not mixed
well and has little flow.
In lakes and reservoirs, dilution of pollutants often is
less effective than in streams for two reasons.
First,
lakes and reservoirs often contain stratified layers
Global Outlook: Stream Pollution
in Developing Countries
Stream pollution in most developing countries is a
serious and growing problem.
