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.
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 percent
of the population served by community 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. 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.
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 refers to water treatment processes that combine or coagulate small particles into
larger particles, which 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.
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 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.
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.
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 organic compounds (SOCs), inorganic compounds (IOCs),
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-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 contaminants
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 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.
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.
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 requiresless treatment than water from lakes, rivers, and streams.