Water on Tap: A Consumer's Guide to the Nation's Drinking Water

Where Does My Drinking Water Come From?


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. Including the approximately 23 million Americans who use ground water as a private drinking water source, slightly more than half of the population receives its drinking water from ground water sources.

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 (50 feet or less) or deep (more than 1,000 feet). 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?

The Safe Drinking Water Act (SDWA) defines a public water system 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. EPA also regulates other kinds of public water systems--such as those at schools, factories, campgrounds, or restaurants--that have their own water supply. 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.

EPA is in the process of developing regulations limiting the amount of disinfection by-products (DBPs). DBPs are contaminants that form when disinfectants react with organic matter that is in treated drinking water. Long-term exposure to some DBPs may increase the risk of cancer or other adverse health effects.



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How Much Does It Cost To Treat And Deliver My Drinking Water?

On a per gallon basis, water is cheap. On average, water costs are slightly more than $2 per 1,000 gallons, 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.


Source: http://www.epa.gov/OGWDW/wot/introtap.html