Drinking Water—Safety on Tap - Modern Water Treatment

Although the Greek physician Hippocrates is credited with emphasizing the importance of clean water to good health as early as 400 B. C. (he recommended boiling and straining rainwater), the first recorded observation of the connection between drinking water and the spread of disease came from Dr. John Snow, a London physician, in 1849. Snow noted that his patients who were getting their drinking water from one particular well were contracting cholera, while patients getting drinking water from other wells were not. His solution to the problem was to remove the handle from the contaminated well's pump so that no one could get water, thereby stopping a cholera epidemic. This event is generally credited as the beginning of modern water treatment.

The most significant water treatment event in the United States was the introduction of the use of chlorine as a disinfectant in water supplies. The introduction of chlorine in water supplies began in the early 1900s. As towns and cities began to introduce this practice, epidemics and incidence of typhoid, cholera, and dysentery were dramatically reduced. From this humble beginning evolved the complex drinking water treatment technology available today.

The multiple barrier approach is the basis for modern water treatment. This approach recognizes that contaminants reach drinking water through many pathways. Working together, water suppliers and health professionals try to erect as many "barriers" as possible to prevent contaminants from reaching consumers. These barriers include:

• Protecting the water source from contamination by eliminating or limiting the waste discharges to the water source through a variety of protection programs
• Improved contaminant detection methods
• New and ongoing research into contaminants and their effects
• Removing contaminants or reducing contaminant levels through various treatments
• Disinfection
• Elimination of cross-connections and breaks in the distribution lines
• Safe plumbing in residences and businesses

The water treatment process begins with choosing the highest quality groundwater or surface water source available, and ensuring its continued protection. (See Figure 5.3 for an illustration of the drinking water treatment process.) Groundwater is usually pumped directly into the treatment plant. In many cases, however, because groundwater is naturally filtered as it seeps through layers of rock and soil, disinfection is the only treatment needed before the water is distributed to consumers.

Surface water is transported to the water treatment plant through aqueducts or pipes. A screen at the intake pipe removes debris such as tree branches and trash.

Water suppliers use a variety of treatments to remove contaminants. These treatments are usually arranged in a sequential series of processes called a "treatment train." In the plant, the water is aerated to eliminate gases and add oxygen. Chemicals may be added to remove undesirable contaminants or to improve the taste. If the water is "hard," lime or soda is added to remove the calcium and magnesium. Hard water can clog pipes, stain fixtures, and interfere with soap lathering.

Coagulation or flocculation is frequently the next step. Alum, iron salts, or synthetic polymers are added to the water to combine smaller particles into larger particles (floc) to remove contaminants. In the sedimentation basins, the floc settles to the bottom and is removed. Additional treatment may be required if the raw water shows signs of high levels of toxic chemicals. The water is then sent to sand filtration beds to remove the remaining small particles and clarify the water, and to enhance the effectiveness of disinfection. Chlorine, ozone, or ultraviolet light may be used as disinfectants.

At various points in the treatment process, the water is monitored, sampled, and tested using various physical, chemical, and microbial testing procedures. As the water leaves the treatment plant and enters the distribution system, chlorine is added as a disinfectant, particularly where ozone or ultraviolet light were used as disinfectants, to keep it free of microorganisms.

The water then goes to holding units where it is stored until needed. These may be water towers, which use gravity to bring the water to the consumer without extra energy expense, or ground-level containers that require pumps to move the water. The water that ultimately flows from the tap should be clear, tasteless, and safe to drink.

Chlorination

The most extensively used disinfectant in the United States is chlorine, which is used to kill infectious microorganisms and parasites in water. Disinfection with chlorine or other similar chemicals prevents waterborne disease outbreaks. The practice of chlorination first began in the early 1900s to eliminate the cholera and typhoid outbreaks that were widespread in the United States.

In the early 1970s some scientific researchers became concerned by the possible health effects of total trihalomethanes (TTHMs), a byproduct of chlorination. Chlorine reacts with naturally occurring organic substances in water to form TTHMs. The level of TTHMs formed varies widely across water supplies and is dependent on the amount of organic material in drinking water and the amount of chlorine applied. TTHMs are removed by passing the water through activated carbon filters.

The health effects of TTHMs are unclear. Some studies of human populations have indicated a slightly higher incidence of bladder and colon cancer in areas where the water is chlorinated. Other studies, however, have not shown an increased cancer risk. Although animal studies have shown the carcinogenic and mutagenic potential of TTHMs, some scientists and public health officials have suggested that these studies may be unreliable because the animals had been subjected to TTHM levels 10,000 times greater than the levels experienced by humans. Currently, the available data show that the risk of getting cancer from TTHMs is extremely low.

Because the public health benefits of the practice of chlorination far outweigh the risk associated with TTHMs, an extensive research effort was conducted to better understand the potential risks of exposure to disinfection byproducts. While this research was being completed, an agreement was reached among regulatory agencies, water suppliers, consumer groups, and environmental groups. A maximum contaminant level was established at 0.1 ppm until January 2002, when the level was reduced to 0.08 ppm.

FIGURE 5.3
Water treatment process
SOURCE: "Water Treatment Plant," in Drinking Water Treatment, U.S. Environmental Protection Agency, 1999

Fluoridation

Fluoride is nature's cavity fighter, occurring naturally in combination with other minerals in rocks and soils. Water fluoridation is the process of adjusting the naturally occurring level of fluoride in most water systems to a concentration (a range of 0.7 to 1.2 ppm) sufficient to protect against tooth decay. The decision to add fluoride to drinking water is left to each community. If the community elects to use fluoride, the water must meet the EPA maximum concentration limit.

In 1945 Grand Rapids, Michigan, became the first city in the world to add fluoride to its drinking water to prevent tooth decay. Since that time, most community water systems in the United States have introduced water fluoridation. Fluoridation of drinking water proved so effective in reducing dental cavities that researchers also developed other methods to deliver fluoride to the public (toothpastes, rinses, dietary supplements). The widespread use of these products has assured that virtually all persons have been exposed to fluoride. The American Dental Association reported in 2000 that, thanks in large part to community fluoridation, half of all children ages five to seventeen have never had a cavity in their permanent teeth. The CDC has recognized fluoridation as one of the ten great public health achievements of the twentieth century.