EPA 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 water systems (CWSs) drink water that originates
as ground water. Ground water is usually
Public Water Systems (PWSs) come in all shapes
pumped from wells ranging from shallow to deep
and sizes, and no two are exactly the same. They
(50 to 1,000 feet). The remaining 65 percent of
may be publicly or privately owned and main-
the population served by CWSs receive water
tained. While their design may vary, they all
taken primarily from surface water sources like
share the same goal providing safe, reliable
rivers, lakes, and reservoirs.
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 Treating Raw Water
PWS vary depending on the size of the system,
The amount and type of treatment applied by a
whether they use ground water or surface water,
PWS varies with the source type and quality.
and the quality of the source water.
Many ground water systems can satisfy all
Federal requirements without applying any
Tapping a Source of Water treatment, while others need to add chlorine or
additional treatment. USEPA is developing a
Large-scale water supply systems tend to rely on
ground water rule that will specify the appropri-
surface water sources, while smaller systems
ate use of disinfection and will address other
tend to rely on ground water. Around 35 per-
components of ground water systems to assure
cent of the population served by community
public health protection. Because surface water
systems are exposed to direct wet weather runoff
All public water systems must have at least 15 service and to the atmosphere and are therefore more
connections or serve at least 25 people per day for 60 easily contaminated, federal and state regulations
days of the year. require that these systems treat their water. Appli-
Drinking water standards apply to water systems cable Federal Regulations include the Surface
differently based on their type and size: Water Treatment Rule, the Interim Enhanced
Community Water System (there are approximately Surface Water Treatment Rule, and the Stage I
55,000) A public water system that serves the Disinfectants/Disinfection Byproducts Rule.
same people year-round. Most residences including
Water suppliers use a variety of treatment pro-
homes, apartments, and condominiums in cities,
small towns, and mobile home parks are served by cesses to remove contaminants from drinking
Community Water Systems. water. These individual processes may be
arranged in a treatment train (a series of pro-
Non-Community Water System A public water
system that serves the public but does not serve the cesses applied in sequence). The most commonly
same people year-round. There are two types of used processes include filtration, flocculation and
non-community systems: sedimentation, and disinfection for surface water.
Non-Transient Non-Community Water System Some treatment trains also include ion exchange
(there are approximately 20,000) A non- and adsorption. Water utilities select a combina-
community water system that serves the same tion of treatment processes most appropriate to
people more than six months per year, but not treat the contaminants found in the raw water
year-round, for example, a school with its own used by the system.
water supply is considered a non-transient system.
Transient non-community water system (there
are approximately 95,000) A non-community Types of Treatment
water system that serves the public but not the Flocculation/Sedimentation
same individuals for more than six months, for
Flocculation refers to water treatment processes
example, a rest area or campground may be
considered a transient water system. that combine or coagulate small particles into
larger particles, which settle out of the water as
sediment. Alum and iron salts or synthetic organic Transient non-community water systems may monitor
polymers (used alone or in combination with metal less frequently and for fewer contaminants than CWSs.
salts) are generally used to promote coagulation. Because these types of systems serve an ever-changing
Settling or sedimentation occurs naturally as floccu- population, it is most important for them to monitor for
lated particles settle out of the water. contaminants such as microbiologicals and nitrate that
Filtration can cause an immediate, acute public health effect.
Many water treatment facilities use filtration to remove Water systems also monitor for a number of contami-
all particles from the water. Those particles include nants that are currently not regulated. This monitoring
clays and silts, natural organic matter, precipitates from data provides the basis for identifying contaminants to
other treatment processes in the facility, iron and be regulated in the future.
manganese, and microorganisms. Filtration clarifies
water and enhances the effectiveness of disinfection.
Distribution to Customers
Ion Exchange
Ion exchange processes are used to remove inorganic An underground network of pipes typically delivers
contaminants if they cannot be removed adequately by drinking water to the homes and businesses served by
filtration or sedimentation. Ion exchange can be used the water system. Small systems serving just a handful
to treat hard water. It can also be used to remove of households may be relatively simple. Large metro-
arsenic, chromium, excess fluoride, nitrates, radium, politan water systems can be extremely complex
and uranium. sometimes with thousands of miles of piping serving
millions of people. Although water may be safe when
Adsorption leaving the water treatment plant it is important to
Organic contaminants, unwanted coloring, and taste ensure that this water does not become contaminated
and-odor-causing compounds can stick to the surface in the distribution system because of such things as
of granular or powder activated carbon and are thus water main breaks, pressure problems, or growth of
removed from the drinking water. microorganisms. Much of the existing drinking water
Disinfection (chlorination/ozonation) infrastructure was built many years ago. The USEPA
Water is often disinfected before it enters the distribu- Infrastructure Needs Survey, released in 1997, esti-
tion system to ensure that potentially dangerous mated that drinking water systems will need to invest
microbes are killed. Chlorine, chloramines, or chlo- $138.4 billion over a 20 year period to ensure the
rine dioxide are most often used because they are very continued source development, storage, treatment, and
effective disinfectants, not only at the treatment plant distribution of safe drinking water. Many agree this is
but also in the pipes that distribute water to our homes a very conservative low estimate.
and businesses. Ozone is a powerful disinfectant, and
ultraviolet radiation is an effective disinfectant and The Water Cycle
treatment for relatively clean source waters, but
neither of these are effective in controlling biological Drinking water can come from both surface water and
contaminants in the distribution pipes. ground water. The water cycle begins with rainwater
and snow melt that gathers in lakes and rivers which
interact with ground water.
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 The Hydrologic Cycle
state standards. Currently, the nations
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 (IOCs),
radionuclides, and microbial organisms
(including bacteria). Testing for these
contaminants takes place on varying sched-
ules and at different locations throughout
the water system.
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.
