Sources of Drinking Water Water Distribution Systems Drinking Water Treatment Limiting the Presence of Disinfection Byproducts Rising Costs of Drinking Water
Where Does It Come From?
Your drinking water travels a complicated path before arriving at your home or business. Learn about the processes involved with your drinking water below.
Sources of Drinking Water
In most cases, drinking water comes from two sources: surface water or ground water. Surface water sources are bodies of water on the earth's surface, such as rivers, lakes, and reservoirs. Ground water sources are underground aquifers which are geologic formations containing water underground.
Water is collected from surface water systems by pumps and piped to a drinking water treatment plant before distributed to homes and businesses. Groundwater is accessed by drilling a well into the underground water source and then pumping the well water up to the surface.
Large scale water supply systems, such as for a town or city, typically rely on surface water resources and smaller water systems, such as for an individual home, tend to rely on ground water as the drinking water source.
The majority of the drinking water in Raleigh comes from the Falls Lake Reservoir located in northern Wake County and is treated at the E.M. Johnson Water Treatment Plant. Raleigh's second water plant is the Dempsey E. Benton Water Treatment Plant in southwest Wake County.
Water Distribution Systems
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, while large metropolitan systems can be extremely complex-sometimes consisting of thousands of miles of pipes serving millions of people.
The City of Raleigh manages approximately 2,500 miles of water distribution lines that provide water service to over 450,000 people. Drinking water must meet required health standards when it leaves the treatment plant.
After treated water leaves the plant, it is monitored within the distribution lines to identify and remedy any problems such as water main breaks, pressure variations, or growth of microorganisms.
Drinking Water Treatment
Water utilities throughout the country treat nearly 34 billion gallons of water every day. In fact, during 2006, the City of Raleigh's E.M. Johnson water treatment plant processed an average of 48 million gallons per day (MGD).
The amount and type of treatment applied to the water varies with the water source and quality of the water. Generally, surface water systems require more treatment than ground water systems because they are directly exposed to the atmosphere and runoff from rain and melting snow.
Water suppliers use a variety of treatment processes to remove contaminants from drinking water. These individual processes can be arranged in a "treatment train" (a series of processes applied in a sequence).
The most commonly used processes include coagulation (flocculation and sedimentation), filtration, and disinfection. Typically the water is stored in a separate large tank for each of these different processes. Some water systems also use oxidation, ion exchange, and adsorption. Water utilities select the treatment combination most appropriate to treat the contaminants found in the source water of that particular system.
- Coagulation (Flocculation & Sedimentation)
- Flocculation: This step removes dirt and other particles suspended in the water. Alum and iron salts or synthetic organic polymers are added to the water to form tiny sticky particles called "floc," which attract the dirt particles and clump them together so they will sink to the bottom.
- Sedimentation: The flocculated particles then sink down to the bottom of the tank with the cleaner water remaining above the settled particles.
- 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 the water and enhances the effectiveness of disinfection.
- Disinfection: Disinfection of drinking water is considered to be one of the major public health advances of the 20th century. Water is often disinfected before it enters the distribution system to ensure that dangerous microbial contaminants are killed. Chlorine, chloramines, chlorinates, or chlorine dioxides are most often used because they are very effective disinfectants, and residual concentrations can be maintained in the water distribution system.
Limiting the Presence of Disinfection Byproducts
Disinfection of drinking water has eliminated the concern for waterborne diseases such as dysentery and giardia. However, sometimes the disinfectants themselves can react with naturally occurring materials in the water to form unintended byproducts, which may pose long term health risks. The U.S. Environmental Protection Agency recognizes the importance of removing microbial contaminants while simultaneously protecting the public from disinfection byproducts and has developed regulations to limit the presence of these byproducts. This is why the City of Raleigh began using ferric sulfate, ozone, and chloramines in its treatment process and flushing the entire water distribution system once per year in March with free chlorine.
All sources of drinking water contain some naturally occurring contaminants. At low levels, these residual contaminants are not currently considered harmful in our drinking water. Removing all contaminants would be extremely expensive, and in most cases, would not provide increased protection of public health. A few naturally occurring minerals may actually improve the taste of drinking water and may provide nutritional value at low levels.
Rising Costs of Drinking Water
The cost of drinking water is rising as suppliers meet the needs of aging infrastructure, comply with public health standards, and provide additional water capacity as water usage per capita increases in most areas and the number of customers served increases.
Public water systems have already constructed their least expensive water supply sources and building future supply sources increases construction and operation costs. In most cases, these increasing costs have caused water suppliers to raise their rates.
Despite rate increases, water is generally still a bargain compared to other utilities, such as electricity, cable TV, gas, and telephone service. In the United States, combined water and sewer bills average only about 0.5 percent of household income.