Science of Water : Concepts and Applications

Water Pollution 217
City and suburban
runoff
Rural runoff
Wastewater treatment
plant
Nonpoint
sources
FIGURE 7.2 Point and nonpoint sources of pollution.
Point
sources
Agricultural runoff
Industrial waste
Nutrients are organic and inorganic substances that provide food for microorganisms such as
bacteria, fungi, and algae. Nutrients are supplemented by the discharge of sewage. The bacteria,
fungi, and algae are consumed by the higher trophic levels in the community. Each stream, due
to a limited amount of dissolved oxygen (DO), has a limited capacity for aerobic decomposition
of organic matter without becoming anaerobic. If the organic load received is above that capacity,
the stream becomes unfi t for normal aquatic life and it is not able to support organisms sensitive to
oxygen depletion (Mason, 1991).
Effl uent from a sewage treatment plant is most commonly disposed of in a nearby waterway.
At the point of entry of the discharge, there is a sharp decline in the concentration of DO in the
stream. This phenomenon is known as the oxygen sag. Unfortunately (for the organisms that normally
occupy a clean, healthy stream), when the DO is decreased, there is a concurrent massive
increase in BOD as microorganisms utilize the DO as they break down the organic matter. When
the organic matter is depleted, the microbial population and BOD decline, while the DO concentration
increases, assisted by stream fl ow (in the form of turbulence) and by the photosynthesis of
aquatic plants. This self-purifi cation process is very effi cient, and the stream will suffer no permanent
damage as long as the quantity of waste is not too high. Obviously, an understanding of this
self-purifi cation process is important to prevent overloading the stream ecosystem.
As urban and industrial centers continue to grow, waste disposal problems also grow. Because
wastes have increased in volume and are much more concentrated than earlier, natural waterways
must have help in the purifi cation process. This help is provided by wastewater treatment plants. A
wastewater treatment plant functions to reduce the organic loading that raw sewage would impose
on discharge into streams. Wastewater treatment plants utilize three stages of treatment: primary,
secondary, and tertiary treatment. In breaking down the wastes, a secondary wastewater treatment
plant uses the same type of self-purifi cation process found in any stream ecosystem. Small
bacteria and protozoans (one-celled organisms) begin breaking down the organic material. Aquatic
insects and rotifers are then able to continue the purifi cation process. Eventually, the stream will
recover and show little or no effects of the sewage discharge. Again, this phenomenon is known as
natural stream purifi cation (Spellman and Whiting, 1999).