Running Waters - Mathematics and Statistics for the Environment - page 559

Of particular interest to environmental practitioners are agricultural effluents. As a case in point,
take, for example, farm silage effluent, which has been estimated to be more than 200 times as
potent (in terms of biochemical oxygen demand, or BOD) as treated sewage (Mason, 1990).
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 unfit for normal aquatic life, and it is not able to support organisms sensitive to
oxygen depletion (Smith, 1974).
Effluent 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 oxygen sag . Unfortunately (for the organisms that normally
occupy a clean, healthy stream), when the DO decreases, there is a concurrent massive increase in
BOD because 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 flow (in the form of turbulence) and by the photosynthesis of aquatic
plants. This self-purification process is very efficient, and the stream will suffer no permanent dam-
age as long as the quantity of waste is not too high. Obviously, an understanding of this self-purifi-
cation process is important in preventing stream ecosystem overload.
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 purification process. Wastewater treatment plants provide this help. Wastewater
treatment plants function to reduce the organic loading that raw sewage would impose when dis-
charged into streams. Wastewater treatment plants utilize three stages of treatment: pr ima r y, sec-
ondary, and tertiary treatment. In breaking down the wastes, a secondary wastewater treatment
plant uses the same type of self-purification 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 purification process. Eventually, the stream will recover
and show little or no effects of the sewage discharge. This phenomenon is known as natural stream
purification (Spellman and Whiting, 1999).
19.2 IS DILUTION THE SOLUTION?
In the early 1900s, wastewater disposal practices were based on the premise that “the solution is
dilution.” The most economical means of dealing with wastewater was to dispose of it into run-
ning waters (primarily rivers), and doing so was considered good engineering practice (Clark et al.,
19977; Velz, 1970). Early practices in the field evolved around mixing-zone concepts based on the
lateral, vertical, and longitudinal dispersion characteristics of the receiving waters (Peavy et al.,
1975). Various formulas predicting space and dispersion characteristics for diluting certain pollut-
ants to preselected concentrations were developed. Highly polluted discharged water was viewed
as acceptable because the theory prevailed that the stream or river would eventually purify itself.
Actually, a more accurate perception was that discharged wastewater out of sight was out of mind—
leave it to the running water body to dilute the wastestream.
Although dilution is a powerful factor in the self-cleansing mechanisms of surface waters, it has
its limitations. Dilution is a viable tool that takes advantage of the ability of running water to self-
purify, but only if the discharges are limited to relatively small quantities of waste into relatively
large bodies of water. One factor that impedes the ability of running water to self-purify is growth
in the number of dumpers. “Growth in population and industrial activity, with attendant increases
in water demand and wastewater quantities, precludes the use of many streams for dilution of raw
or poorly treated wastewaters” (Peavy et al, 1975).