The Safe Drinking Water Act (SDWA) defines public water systems (PWS) as one that serves piped water to at least 25 persons or 15 services connections for at least 60 days each year. There are approximately 161,000 public water systems in the United States. Such systems may be publicly or privately owned. Specifically, public water system can be separated into two categories:
- Community Water System (54,000 systems)—A public water system that serves the same people year- round. Most residences are served by Community Water Systems.
- Non-Community Water System (approximately 108,000 systems)- A public water system that does not serve the same people year-round. There are two types of non-community systems:
- Non-Transient Non-Community Water System (almost 19,000 systems) —A non-community water system that serves the same people more than six months of the year, but not year-round. For example, a school with its own water supply is considered a non-transient system.
- Transient Non-Community Water System (more than 89,000 systems)— A non-community water system that serves the public but not the same individuals for more than six months. For example, a rest area or a campground may be considered a transient system.
Large-scale water supply systems tend to rely on surface water resources, while smaller water systems tend to use ground water. 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.
All sources of drinking water contain some naturally occurring contaminants. 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 system to identify and remedy any problems such as water main breaks, pressure variations, or growth of microorganisms. Nevertheless, problems with local drinking water can, and do, occur. You can contact your water utility or public works department to find out the source of your public water supply. If your drinking water comes from a community water system, the system will deliver to its customers annual drinking water quality reports that shows what contaminants have been detected in their drinking water, how these detection levels compare to drinking water standards.
How Drinking Water is Treated in Public Water Utility
Water utilities treat nearly 34 billion gallons of water every day. The amount and type of treatment applied varies with the source and quality of the water. Generally, ground water typically requires less treatment than water from lakes, rivers, and streams. In contrast, 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 depending on the quality of the water that enters the plant. 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. Some water systems also use 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.
- Sedimentation: The flocculated particles then settle naturally out of the water.
- 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, chlorinates, or chlorine dioxides are most often used because they are very effective disinfectants, and residual concentrations can be maintained in the water system. However, disinfectant practices can be problematic which may pose health risks because:
- Certain microbial pathogens, such as Cryptosporidium, are highly resistant to traditional disinfection practices.
- Disinfectants themselves can react with naturally-occurring materials in the water to form byproducts, such as trihalomethanes and haloacetic acids, which may pose health risks.
It is still a challenge for water suppliers to balance the risks from microbial pathogens and disinfection byproducts.