Drinking water comes from surface water and ground water.
Large-scale water supply systems tend to rely on surface water
resources, and smaller water systems tend to use ground water.
Surface water includes rivers, lakes, and reservoirs. Ground
water is pumped from wells that are drilled into aquifers.
Aquifers are geologic formations that contain water. The quantity
of water in an aquifer and the water produced by a well depend on
the nature of the rock, sand, or soil in the aquifer where the
well withdraws water. Drinking water wells may be shallow (15
meters or less) or deep (more than 300 meters). Your water
utility or your public works department can tell you the source
of your public drinking water supply.
What is a Public Water System?
A public water system is often defined as one that serves
piped water to at least 25 persons or 15 service connections for
at least 60 days per year. Such systems may be owned by homeowner
associations, investor-owned water companies, local governments,
and others. Water that does not come from a public water supply,
and which serves one or only a few homes, is called a private
supply.
Community water systems are public systems that serve people
year-round in their homes. The data shown in this report cover
only community water systems because they are the source of most
drinking water.
How Does Water Get To My Faucet?
In a typical community water supply system, water is
transported under pressure through a distribution network of
buried pipes. Smaller pipes, called house service lines, are
attached to the main water lines to bring water from the
distribution network to your house. In many community water
supply systems, water pressure is provided by pumping water up
into storage tanks that store water at higher elevations than the
houses they serve. The force of gravity then "pushes"
the water into your home when you open your tap. Houses on a
private supply usually get their water from a private well. A
pump brings the water out of the ground and into a small tank
within the home, where the water is stored under pressure.
How Do Public Water Suppliers Treat My Water To Make It
Safe?
Water suppliers use a variety of treatment processes to remove
contaminants from drinking water. These individual processes may
be arranged in a "treatment train" to remove
undesirable contaminants from the water. The most commonly used
processes include filtration, flocculation and sedimentation, and
disinfection. Some treatment trains also include ion exchange and
adsorption. A typical water treatment plant would have only the
combination of processes needed to treat the contaminants in the
source water used by the facility. If you want to know what types
of treatment are used for your water supply, contact your local
water supplier or public works department.
Flocculation/Sedimentation
Flocculation refers to water treatment processes that combine
small particles into larger particles, which settle out of the
water as sediment. Alum and iron salts or synthetic organic
polymers (alone, or in combination with metal salts) are
generally used to promote coagulation. Settling or sedimentation
is simply a gravity process that removes flocculated particles
from the water.
Filtration
Many water treatment facilities use filtration to remove
remaining particles from the water supply. Those particles
include clays and silts, natural organic matter, precipitants
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
constituents 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, color, and taste- and odor-causing
compounds can stick to the surface of granular or powdered
activated carbon (GAC or PAC). GAC is generally more effective
than PAC in removing these contaminants. Adsorption is not
commonly used in public water supplies.
Disinfection (chlorination, ozonation)
Water is often disinfected before it enters the
distribution system to ensure that dangerous microbes are killed.
Chlorine, chloramines, or chlorine dioxide most often are used
because they are very effective disinfectants, and residual
concentrations can be maintained to guard against biological
contamination in the water distribution system. Ozone is a
powerful disinfectant, but it is not effective in controlling
biological contaminants in the distribution pipes.
How Much Does It Cost To Treat And Deliver My Drinking Water?
On a per liter basis, water is cheap. On average, water costs are slightly more than 3 lita per cubic meter or 1000 liters, although the costs tend to be lower for large water systems. Treatment accounts for about 15 percent of that cost. Other costs are for equipment (such as the treatment plant and distribution system) and labor for operation and maintenance of the system.
Adapted From:
EPA 815-K-97-002
Water On Tap
July 1997
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