How is tap water made safe to drink, and why do we add chlorine?

Tap water is made safe in steps: it is settled and filtered to remove particles, and then it is disinfected to kill the germs that are left, most often with chlorine. Water chlorination is the process of adding chlorine or chlorine compounds to water, and it is used to kill bacteria, viruses and other microbes. In particular, it is used to prevent the spread of waterborne diseases such as cholera, dysentery and typhoid.

The idea came out of hard lessons. In 1854, a cholera outbreak around Broad Street in Soho, London, killed 420 people. The physician John Snow argued that germ-contaminated water, not foul air, was the cause, and by talking to local residents he identified the public water pump on Broad Street as the source. In 1897 the town of Maidstone, England, became the first to have its entire water supply treated with chlorine.

Chlorine's reputation was made in Lincoln in 1905, when a faulty slow sand filter and a contaminated water supply caused a serious typhoid epidemic. Alexander Cruickshank Houston used chlorination to stop it, and permanent water chlorination began that year.

Chlorine works because it is a highly efficient disinfectant. It is not perfect: it can react with organic compounds in the water to make by-products, and ozone and ultraviolet light can be used for disinfection too. But before routine disinfection, the germs that cause cholera, typhoid and dysentery killed countless people every year.