Drinking  Water  Treatment   /
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
        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
                                 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).

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
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

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
         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
          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)