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Drinking Water Treatment /
SAFE DRINKING WATER ACT • 1974-2004 • PROTECT OUR HEALTH FROM SOURCE TO TAP
Public Water Systems
Public Water Systems (PWSs) come in all shapes
and sizes, and no two are exactly the same. They
may be publicly or privately owned and maintained.
While their design may vary, they all share the same
goal: providing safe, reliable drinking water to
the communities they serve. To do this, most water
systems must treat their water. The types of treatment
provided by a specific PWS vary depending on the
size of the system, whether they use ground water or
surface water, and the quality of the source water.
All public water systems must have at least 1 5
day for 60 days of the year.
Drinking water standards apply to water systems
differently based on their type and size:
11 ere are approximately
em that serves
le same people year-round. Most residences
including homes, apartments, and condominiums
in cities, small towns, and mobile home parks are
Non-Community Water System
ublic water
but does not serve
There are two types
of non-community systems:
Non-Transient Non-Community Water
System
mmunity water system that serves the same
ile more than six months per year, but not
year-round, for example, a school with its own
" snt system.
sient non-community water system
in
there are
ity water
approximately 89,000) - A non-community water
system that serves the public but not the same
individuals for more than six months, for example,
a rest area or campground may be considered a
transient water system.
Tapping a Source of Water
Large-scale water supply systems tend to rely on
surface water sources, while smaller systems tend
to rely on ground water. Around 32 percent of the
population served by community water systems
(CWSs) drink
water that
originates as
grou nd water.
Ground water is
usually pumped
from wells
ranging from
shal low to deep
(50 to 1,000
feet). The remaining 68 percent of the population
served by CWSs receive water taken primarily from
surface water sources like rivers, lakes, and reservoirs.
Treating Raw Water
The amount and type of treatment applied by a PWS
varies with the source type and quality. Many ground
water systems can satisfy all federal requirements
without applying any treatment, while others need
to add chlorine or additional treatment. US EPA
is developing a ground water rule that will specify
the appropriate use of disinfection and will address
other components of ground water systems to assure
public health protection. Because surface water
systems are exposed to direct wet weather runoff
and to the atmosphere and are therefore more easily
contaminated, federal and state regulations require
that these systems treat their water. Disinfection
of drinking water is one of the major public
health advances of the 20th century. However, the
disinfectants themselves can react with naturally
occurring materials in the water to form unintended
byproducts which may pose health risks. A major
challenge for water suppliers is balancing the risks
from microbial pathogens and disinfection byproducts.
The Stage 1 Disinfectants and Disinfection Byproducts
Rule and the Interim Enhanced Surface Water
Treatment Rule together address these risks.
Water suppliers use a variety of treatment processes
to remove contaminants from drinking water. These
individual processes may be arranged in a "treatment
train" (a series of processes applied in sequence).
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The most commonly used processes include filtration,
flocculation and sedimentation, and disinfection for
surface water. Some treatment trains also include
ion exchange and adsorption. Water utilities select a
combination of treatment processes most appropriate
to treat the contaminants found in the raw water used
by the system .
Types of Treatment
Flocculation 'Sedimentation: Flocculation
refers to water treatment processes that combine or
coagu late small
particles into larger
particles, wh ich
settle out of the
water as sediment.
Alum and iron
salts or synthetic
organic polymers
(used alone or
in combination
with metal salts)
are generally
used to promote
coagulation. Settling
or sedimentation
occurs naturally as
flocculated particles
settle out of the
water.
The Water Cycle - Drinking water can come from both surface water
and ground water. The water cycle begins with rainwater and snow melt
that gathers in lakes and rivers which interact with ground water.
Filtration: Many
water treatment
facilities use
filtration to remove all particles from the water. Those
particles include clays and silts, natural organic
matter, precipitates from other treatment processes in
the facility, iron and manganese, and microorganisms.
Filtration clarifies water and enhances the
effectiveness of disinfection.
Ion Exchange: Ion exchange processes are used
to remove inorganic contaminants if they cannot be
removed adequately by filtration or sedimentation. Ion
exchange can be used to treat hard water. It can also
be used to remove arsenic, chromium, excess fluoride,
nitrates, radium, and uranium.
Absorption: Organic contaminants, unwanted
coloring, and taste-and-odor-causing compounds can
stick to the surface of granular or powder activated
carbon and are thus removed from the drinking water.
Disinfection (chlorination/ozonation): Water
is often disinfected before it enters the distribution
system to ensure that potentially dangerous microbes
are killed. Chlorine, chloramines, or chlorine dioxide
are most often used because they are very effective
disinfectants, not only at the treatment plant but
also in the pipes that distribute water to our homes
and businesses. Ozone is a powerful disinfectant,
and ultraviolet radiation is an effective disinfectant
and treatment for relatively clean source waters, but
neither of these are effective in controlling biological
contaminants in the distribution pipes.
Monitoring Water Quality
Water systems monitor
for a wide variety of
contaminants to verify
that the water they
provide to the public
meets all federal
and state standards.
Currently, the nation's
community water
systems (CWSs) and
nontransient non-
community water
systems (NTNCWSs)
must monitor for more
than 83 contaminants.
The major classes of
contaminants include
volatile organic
compounds (VOCs),
synthetic organ ic
compounds (SOCs),
inorganic compounds
(lOCs), radionuclides,
and microbial organisms (including bacteria). Testing
for these contaminants takes place on varying
schedules and at different locations throughout the
water system.
Transient non-commu n ity water systems may monitor
less frequently and for fewer contaminants than CWSs.
Because these types of systems serve an ever-changing
population, it is most important for them to monitor
for contaminants such as microbiologicals and nitrate
that can cause an immediate, acute public health
effect. Water systems also monitor for a number of
contaminants that are currently not regulated. These
monitoring data provides the basis for identifying
contaminants to be regulated in the future.
Distribution to Customers
An underground network of pipes typically delivers
drinking water to the homes and businesses served
by the water system. Small systems serving just a
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handful of households may be relatively simple. Large
metropolitan water systems can be extremely complex
. sometimes with thousands of miles of piping serving
millions of people. Although water may be safe when
leaving the water treatment plant it is important to
ensure that this water does not become contaminated
in the distribution system because of such things as
water main breaks, pressure problems, or growth of
microorganisms. Much of the existing drinking water
infrastructure was built many years ago. The US
EPA Infrastructure Needs Survey, released in 2001,
estimated that drinking water systems will need to
invest $150.9 billion over a 20 year period to ensure
the continued source development, storage, treatment,
and distribution of safe drinking water. Many agree
this is a very conservative low estimate.
For More Information
To learn more about drinking water treatment
and treatment techniques, call the Safe Drinking
Water Hotline at 1-800-426-4791 or visit the
safewater web site at www.epa.gov/safewater.
Water Treatment Plant
Follow a drop of water from the source through the treatment process. Water may be treated differently in
different communities depending on the quality of the water which enters the plant. Groundwater is located
underground and typically requires less treatment than water from lakes, rivers, and streams.
Lake or Reservoir
Coagulation removes dirt and other particles suspended in
water. Alum and other chemicals are added to water to form
tiny sticky particles called "floe" which attract the dirt
particles. The combined weight of the dirt and the alum
(floe) become heavy enough to sink to the bottom during
sedimentation.
Sedimentation:
The heavy particles
(floe) settle to the
bottom and the
clear water moves
to filtration.
Disinfection: A small amount of chlorine is added or
some other disinfection method is used to kill any
bacteria or microorganisms that may be in the water.
Storage: Water is placed in
a closed tank or reservoir
for disinfection to take
place. The water then flows
through pipes to homes
and businesses in the
community.
Filtration: The water passes
through filters, some made
of layers of sand, gravel, and
charcoal that help remove
even smaller particles.
Source: AWWA Drinking Water Week Blue Thumb Kit
Office of Water (4606)
www.epa.gov/safewater
EPA816-F-04-034
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