Treating Raw Water
The amount and type of treatment applied by a public water system (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. The U.S. Environmental Protection Agency (EPA) is developing a ground water rule that will specify the appropriate use of disinfection and will address other components of ground water systems to assure public health protection. Because surface water systems are exposed to and fed by direct land runoff and to the atmosphere, they are therefore more easily subjected to contamination. Federal and state regulations require that these systems treat this water to meet health-based standards.
Disinfection of drinking water is one of the major public health advances of the 20th century. However, the disinfectants themselves can react with naturally occurring materials in the water to form unintended byproducts which may pose health risks. A major challenge for water suppliers is balancing the risks from microbial pathogens and disinfection byproducts. The EPA’s Stage 1 Disinfectants and Disinfection Byproducts Rule and the Interim Enhanced Surface Water Treatment Rule together address these risks.
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 treatment 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 public water system.
Types of Treatment
Flocculation/Sedimentation Flocculation refers to water treatment processes that combine or coagulates 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.
Filtration Many water treatment facilities use filtration to remove or reduce suspended solids and 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 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.
Absorption 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 or reduced from the drinking water.
Disinfection (chlorination/ozonation) Tap 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 a cost-effective approach to disinfection where public distribution pipelines provide water to our homes and businesses. Ozone is a powerful and very effective disinfectant, while ultraviolet radiation is an effective disinfectant and treatment for relatively clean source or processed waters that are typical in bottled water processing. Neither of these is effective in controlling biological contaminants in the distribution pipes of public water systems.
Bottled Water Treatment
Natural water (bottled water derived from protected ground water sources) is minimally treated via filtration and sanitization (ozonation or UV) only, if necessary. The addition of any chemicals or the application of any other techniques that would alter the natural balance of minerals is not allowed.
For processed or purified waters, a purification process is applied, in addition to the filtration and sanitation. This purification process typically consists of distillation or reverse osmosis. Both techniques are designed to further purify the water in order to provide the desired high-value beverage.