How Water Is Made Safe For Drinking

There are two critical elements in providing safe drinking water for human consumption, filtration and disinfection.

As the first step in the filtration process, the raw (untreated) water enters the plant through a series of screens designed to remove large debris such as twigs and leaves. Then, at most treatment plants, the water slowly moves through a presedimentation basin where much of the sand and silt settle to the bottom.

During the next phase of the process, the water is speeded up and a liquid chemical, known as a coagulant, is added through a flash-mix process. The coagulant contains positively charged atoms called ions.

The water then flows into a large basin where it is slowly stirred by large paddles. Suspended particles, which are negatively charged, are attracted to the positively charged coagulant. As the particles join together, they become heavier and start to settle. The process, called flocculation, takes about 25 minutes.

After flocculation, the water enters a final sedimentation basin where the floc that was formed earlier is allowed to settle to the bottom. It takes about four hours to complete this process.

Water at the very top of the basin then moves to a large gravity filter. The filtration media can be a combination of hard coal (anthracite), gravel and sand or, sometimes, just coal or just sand.

After the water is filtered, chlorine is added to kill any disease causing bacteria. Fluoride also is added to help prevent tooth decay. After water has completed the treatment process, it is referred to as finished or potable water. Most people simply call it drinking water.

The total treatment process takes five to eight hours.