264 CHAPTER 10 Freshwater Resources and Water PollutionTurbid water is treated with a chemical coagulant that
causes the suspended particles to clump together and
settle out. The water is then filtered through sand to
remove remaining suspended materials as well as many
microorganisms.
In the final purification step before distribution
in the water system, the water is disinfected to kill any
remaining disease-causing agents. The most common
way to disinfect water is to add chlorine. A small amount
of chlorine is left in the water to provide protection
d uring its distribution through many kilometers of pipes.
Other disinfection systems use ozone or ultraviolet (UV)
ra diation in place of chlorine.
Although adding chlorine to drinking water has un-
doubtedly saved millions of lives, chlorine by-products
are tentatively linked to several kinds of cancer, increased- Describe how most drinking water is purified in
the United States. - Distinguish among primary, secondary, and
tertiary treatments for wastewater. - Compare the goals of the Safe Drinking Water
Act and the Clean Water Act.
W
ater quality is improved by removing con-
taminants from the water supply before and
after it is used. Technology assists in both
processes.Purification of Drinking Water
Most U.S. municipal water supplies are treated before
the water is used so it is safe to drink (Figure 10.20).Improving Water Quality
LEARNING OBJECTIVES
PROCESS DIAGRAM
264 CHAPTER 10 Freshwater Resources and Water Pollution1 The water supply for a town may be stored in a reservoir, as
shown, or obtained from groundwater.2 The water is treated before use so it is
safe to drink.4 The quality of the wastewater is fully or partially 3 After use, municipal sewer lines collect the wastewater.
restored by sewage treatment before the treated
effluent is dispersed into a nearby body of water.ReservoirDamWastewater
Treated effluentInput to
sewage
treatment
system
Sewer linesWater supply
system1 24 3/Ài>ÌiÌÊvÊÜ>ÌiÀÊvÀÊÕV«>ÊÕÃiÊUÊ}ÕÀiÊ£ä°Óä
✓✓THE PLANNER