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EPA 810-F-99-013
December 1999
Drinking Water Treatment
SAFE DRINKING WATER ACT • CELEBRATING 25 YEARS • 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 main-
tained. 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.
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 35 per-
cent of the population served by community
All public water systems must have at least 15 service
connections or serve at least 25 people per day for 60
days of the year,
Drinking water standards apply to water systems
differently based on their type and size:
• Community Water System (there are approximately
55,000) - A public water system that serves the
same people year-round. Most residences including
homes, apartments, and condominiums in cities,
small-towns, and mobile home parks WQ served by
Community Water Systems.
• IVon-Community Water System - A public water
system that serves the public but does not serve the
same people year-round. There are two types of
non-community systems:
- Non-Transient Non-Community Water System
(there are approximately 20,000) - A non-
community water system that serves the same
people more than six months per year, but not
year-round, for example, a school with its own
water supplyls considered a non-transient system.
- Transient non-community water system (there
are approximately 95,000) - 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.
water systems (CWSs) drink water that originates
as ground water. Ground water is usually
pumped from wells ranging from shallow to deep
(50 to 1,000 feet). The remaining 65 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. USEPA is developing a
ground water rule that will specify the appropri-
ate 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. Appli-
cable Federal Regulations include the Surface
Water Treatment Rule, the Interim Enhanced
Surface Water Treatment Rule, and the Stage I
Disinfectants/Disinfection Byproducts Rule.
Water suppliers use a variety of treatment pro-
cesses to remove contaminants from drinking
water. These individual processes may be
arranged in a "treatment train" (a series of pro-
cesses applied in sequence). 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 combina-
tion 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 "coagulate" small particles into
larger particles, which settle out of the water as
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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 floccu-
lated particles settle out of the water.
Eiitration
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
rMriganeSe, and microorganisms. Filtration clarifies
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.
Adsorption
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.
DJsinfectio_D^(cMQrination/ozonation)
Water is often disinfected before it enters the distribu-
tion system to ensure that potentially dangerous
microbes are killed. Chlorine, chloramines, or chlo-
rine 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 non-
transient non-community water systems
(NTNCWSs) must monitor for more than 83
contaminants. The major classes of contami-
nants include volatile organic compounds
(VOCs), synthetic organic compounds
(SOCs), inorganic compounds (lOCs),
radionuclides, and microbial organisms
(including bacteria). Testing for these
contaminants takes place on varying sched-
ules and at different locations throughout
the water system.
Transient non-community 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 contami-
nants that are currently not regulated. This 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 handful
of households may be relatively simple. Large metro-
politan 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 USEPA
Infrastructure Needs Survey, released in 1997, esti-
mated that drinking water systems will need to invest
$138.4 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.
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.
The Hydrologic Cycle
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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.
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
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