How is tap water produced?

A city’s water supply plant is like a deserted spa with pumps humming, water tinkling from one basin to another and a humid smell in the air. This is where tap water is produced, but how?

Raw water for large cities usually comes from a lake or river and requires more thorough treatment than groundwater. Harmful substances, those that alter the taste of water, and microorganisms must be removed.

In the first phase, iron or aluminium sulphate is added in to bind the organic matter contained in the water, including humus from soil. The water becomes clarified as particles of solids settle at the bottom of the basin.

Next, the water is filtered, perhaps several times, by leading it through a bed of sand and active carbon filters to remove any remaining impurities.

To remove bacteria and other pathogens, the water is disinfected with ozone, ultraviolet radiation or chlorine, often using all three methods in succession. While the down side of chlorine use is that it may affect the taste of water, it is essential in order to prevent the establishment of bacterial colonies in the water distribution network.

If the water has already been disinfected with other methods, only very small quantities of chlorine are needed. In this case, nearly all chlorine is removed from the water by natural oxidation before it reaches the tap, and the taste is no longer noticeable. Finally, the acidity of the water is adjusted with lime or carbon dioxide, for instance.

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