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up the work of the bacteria by bringing Secondary Treatment
air and sludge heavily laden with bacteria Activated sludge process
into close contact with the effluent (Box
Aeration tank
Figure 24.2). After the effluent leaves the
sedimen tation tank in the primary stage, it
is pumped into an aeration tank, where it is
mixed with air and sludge loaded with bac-
teria and allowed to remain for several hours.
During this time, the bacteria break down the
organic matter into harmless by-products.
The sludge, now activated with addi-
tional millions of bacteria, can be used again
by returning it to the aeration tank for mixing
Air
with new effluent and ample amounts of air.
As with trickling, the final step is generally
the addition of chlorine to the effluent, which
kills more than 99 percent of the harmful
bacteria. Some municipalities are now manu-
Activated sludge
facturing chlorine solution on site to avoid
having to transport and store large amounts
of chlorine, sometimes in a gaseous form. BOX FIGURE 24.2 The activated sludge process removes about 85 percent of the
Alternatives to chlorine disinfection, such as organic matter in sewage.
ultraviolet light or ozone, are being used in
situations where chlorine in sewage effluents many troublesome pollutants at the begin- phosphorus to physical-chemical separation
can be harmful to fish and other aquatic life. ning, rather than at the end, of the pipeline. techniques such as filtration, carbon adsorp-
New pollution problems have placed The increasing need to reuse water calls tion, distillation, and reverse osmosis. These
additional burdens on wastewater treatment for better and better wastewater treatment. activities typically follow secondary treat-
systems. Today’s pollutants may be more Every use of water—whether at home, in ment and are known as tertiary treatment.
difficult to remove from water. Increased the factory, or on the farm—results in some These wastewater treatment processes,
demands on the water supply only ag- change in its quality. New methods for re- alone or in combination, can achieve almost
gravate the problem. These challenges are moving pollutants are being developed any degree of pollution control desired. As
being met through better and more com- to return water of more usable quality to waste effluents are purified to higher degrees
plete methods of removing pollutants at receiving lakes and streams. Advanced by such treatment, the effluent water can be
treatment plants or through prevention waste treatment techniques in use or under used for industrial, agricultural, or recre-
of pollution at the source. Pretreatment development range from biological treat- ational purposes, or even as drinking water
of industrial waste, for example, removes ment capable of removing nitrogen and supplies.

Source: Drawings and some text, from “How Wastewater Treatment Works . . . The Basics,” U.S. Environmental Protection Agency, Office of Water, www.epa.gov/owm/featinfo.html

consumption. The curtailing of consumption occurs more often the food and fibers that people must have. Industrial interests
when a drought lasts for a longer period of time and when claim they should have the water because they create the jobs
smaller lakes and reservoirs make the supply sensitive to rainfall and the products that people must have. Cities, on the other
amounts. In some parts of the western United States, such as the hand, claim that domestic consumption is the most important
Colorado River watershed, all of the surface water is already be- because people cannot survive without water. Yet others claim
ing used, with certain percentages allotted for domestic, indus- that no group has a right to use water when it is needed to main-
trial, and irrigation uses. Groundwater is also used in this tain habitats. Who should have the first priority for water use?
watershed, and in some locations, it is being pumped from the Some have suggested that people should not try to live and
ground faster than it is being replenished by precipitation (Fig- grow food in areas that have a short water supply, that plenty
ure 24.10). As the population grows and new industries develop, of freshwater is available elsewhere. Others have suggested that
more and more demands are placed on the surface water supply, humans have historically moved rivers and reshaped the land
which has already been committed to other uses, and on the to obtain water, so perhaps one answer to the problem is to find
diminishing supply of groundwater. This raises some very con- new sources of freshwater. Possible sources include the recy-
troversial issues about how freshwater should be divided among cling of wastewater and turning to the largest supply of water
agriculture, industries, and city domestic use. Agricultural in- in the world, the ocean. About 90 percent of the water used
terests claim they should have the water because they produce by industries is presently dumped as a waste product. In some

24-9 CHAPTER 24 Earth’s Waters 605