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A Closer Look

Water Quality and Wastewater Treatment
ne of the most common forms of pol- Primary Treatment
Olution control in the United States is
wastewater treatment. The United States
has a vast system of sewer pipes, pumping
stations, and treatment plants, and about
74 percent of all Americans are served by
such wastewater systems. Sewer pipelines
collect the wastewater from homes, busi-
nesses, and many industries and deliver it to
treatment plants. Most of these plants were
designed to make wastewater fit for discharge
Screens Grit chamber
into streams or other receiving waters.
The basic function of a waste treat- Sedimentation tank
ment plant is to speed up the natural process
BOX FIGURE 24.1 The primary stage of the wastewater treatment process physically
of purifying the water. There are two basic
removes solids from the sewage.
stages in the treatment, the primary stage and
the secondary stage. The primary stage physi-
cally removes solids from the wastewater. The can be removed in a sedimentation tank. treatment: (1) trickling filters or (2) activated
secondary stage uses biological processes to The speed of the flow through the larger sludge. A trickling filter is simply a bed of
further purify wastewater. Sometimes, these sedimentation tank is slower, and suspended stones from 1 to 2 m (3 to 6 ft) deep through
stages are combined into one operation. solids gradually sink to the bottom of the which the effluent from the sedimentation
As raw sewage enters a treatment plant, tank. They form a mass of solids called raw tank flows. Interlocking pieces of corru-
it first flows through a screen to remove large primary sludge, which is usually removed gated plastic or other synthetic media have
floating objects such as rags and sticks that from the tank by pumping. The sludge may also been used in trickling beds, but the
might cause clogs. After this initial screen- be further treated for use as a fertilizer or dis- important part is that it provides a place for
ing, it passes into a grit chamber where posed of through incineration, if necessary. bacteria to live and grow. Bacteria grow on
cinders, sand, and small stones settle to the Once the effluent has passed through the stones or synthetic media and consume
bottom (Box Figure 24.1). A grit chamber the primary treatment process, which is most of the organic matter flowing by in the
is particularly important in communities a physical filtering and settling, it enters effluent. The now cleaner water trickles out
with combined sewer systems where sand or a secondary stage that involves biologi- through pipes to another sedimentation tank
gravel may wash with storm water into the cal activities of microorganisms. The sec- to remove excess bacteria. Disinfection of the
system along with rain, mud, and other stuff. ondary stage of treatment removes about effluent with chlorine generally completes
After screening and grit removal, the 85 percent of the organic matter in sew- this secondary stage of basic treatment.
sewage is basically a mixture of organic age by making use of the bacteria that are The trend today is toward the use of an
and inorganic matter with other suspended naturally a part of the sewage. Two principal activated sludge process instead of trickling
solids. The solids are minute particles that techniques are used to provide secondary filters. The activated sludge process speeds

40 percent), and for industrial purposes (about 10 percent). ing to remove suspended particles, treatment to kill bacteria, and
These overall percentages of use vary from one region of the sometimes processing to remove pollution. In spite of the addi-
country to another, depending on (1) the relative proportions tional processing and treatment costs, surface water is less costly
of industry, agriculture, and population; (2) the climate of the as a resource than groundwater. Ground water is naturally filtered
region; (3) the nature of the industrial or agricultural use; and as it moves through the pore spaces of an aquifer, so it is usu-
(4) other variables. In an arid climate with a high proportion of ally relatively free of suspended particles and bacteria. Thus, the
farming and fruit growing, for example, up to two-thirds of the processing or treatment of groundwater is usually not necessary
available water might be used for agriculture. (Figure 24.9). Groundwater, on the other hand, will cost more to
Most of the water supply is obtained from the surface water use as a resource because it must be pumped to the surface. The
resources of streams, lakes, and reservoirs, and 37 percent of the energy required for this pumping can be very expensive. In ad-
municipal water supply comes from groundwater. If you then dition, groundwater generally contains more dissolved minerals
add farms, villages, and many suburban areas, the percentage of (hard water), which may require additional processing or chemi-
groundwater used by humans is well above 40 percent. Surface cal treatment to remove the troublesome minerals.
water contains more sediments, bacteria, and possible pollutants The use of surface water as a source of freshwater means
than groundwater because it is more active and is directly exposed that the supply depends on precipitation. When a drought occurs,
to the atmosphere. This means that surface water requires filter- low river and lake resources may require curtailing water

604 CHAPTER 24 Earth’s Waters 24-8