Methods for improving the taste and smell of drinking water have been around since at least 4000 B.C., according to the U.S. Environmental Protection Agency (EPA). During the 1800s, a greater understanding of water-borne diseases shifted the focus onto purifying water in order to remove disease-causing microbes. Many of the steps in the modern water purification process have been in use for centuries, but ongoing research and technological advancements have also contributed to the process, helping to ensure we have safe drinking water.
Screening
If the water comes from a surface water source, such as a lake or river, it probably passes through screens as it enters the water treatment facility. The screens prevent fish, plants, pieces of wood and other large objects from entering the water tank.
Coagulation
The water, now in large holding tanks, is mixed with alum. The alum causes sticky globs to form, and these are called floc. The floc attract dirt particles and bacteria, making them heavy.
Sedimentation
The water with floc in it flows into a sedimentation basin, where the floc settles to the bottom. The floc remains in the sedimentation basin while the rest of the water moves on to filtration.
Filtration
Borrowing nature’s techniques, the treatment plant has layers of sand and gravel that serve as filters for removing any remaining particles in the water. A gravel filter is typically about one foot deep, while a sand filter is about two and a half feet deep. A charcoal layer may also be used to filter the water further. As the water passes through all of the treatment plant’s filters, smaller and smaller particles get left behind each time.
Disinfection
Chlorine or other disinfecting chemicals, such as ozone or chloramine, are added to the water to kill any remaining microorganisms. These chemicals also help the water remain clean during storage, until it flows through pipes for distribution to homes and businesses. If a treatment plant gets water from the ground, it is possible to eliminate some or all of the steps above and simply disinfect the water prior to distribution.
References
- Duke University: How is Water Purified?
- U.S. Environmental Protection Agency: Water Treatment Process
- U.S. Environmental Protection Agency: History of Drinking Water Treatment
- U.S. Environmental Protection Agency: Drinking Water and Health
- World Health Organization: How Does Safe Water Impact Global Health?
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