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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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