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At the turn of the 20th century, drinking water in many American cities carried a hidden danger. Outbreaks of typhoid fever and cholera were common, often spreading through municipal pipes. In 1908, a physician named John L. Leal took a radical and controversial step in Jersey City: he added a chemical to the public water supply to kill the bacteria. It was the first continuous city water chlorination system in the United States, and it changed public health forever.
Why was Jersey City facing a water crisis?
In the early 1900s, Jersey City contracted the Jersey City Water Supply Company to build a new system. They opened the Boonton Reservoir in 1904, but soon discovered a problem. After heavy rains, raw waste washed into the reservoir, contaminating the water with dangerous bacteria. (You can read more about how waste is measured today in our guide to sewage treatment and BOD).
The city sued the company, arguing that the contract required them to deliver "pure and wholesome" water. In 1908, a judge agreed, ordering the company to either build an expensive system of sewers to divert the waste or find another way to purify the water.
Why was the chlorination project a secret?
Dr. John L. Leal, the company's sanitary adviser, had a deeply personal reason for fighting waterborne diseases: his own father, also a physician, had died from chronic amoebic dysentery contracted from contaminated water.
Leal believed that traditional filtration would be too slow and expensive to build in time for the court's deadline. Instead, he proposed something that was considered unthinkable: treating the entire water supply with a chemical. Knowing that the public would likely panic at the idea of "poison" being added to their drinking water, Leal and sanitary engineer George W. Fuller worked in secret.
They designed and built a chlorination plant at the Boonton Reservoir in just 99 days. On 26 September 1908, without a public announcement, they began feeding dilute solutions of chloride of lime (calcium hypochlorite) into the 40 million gallons of water flowing to Jersey City every day.
How did chlorination reduce typhoid fever?
When the trial resumed a few days later, Leal revealed what he had done. He presented evidence showing that the trace amounts of chlorine had eradicated the harmful bacteria without altering the water's safety or taste. The judge ruled in the company's favour, legally validating chemical disinfection.
The impact on public health was profound. According to a landmark 2005 study by economists David Cutler and Grant Miller, published in *Demography*, clean water technologies like chlorination were responsible for nearly half the total mortality reduction in major U.S. cities during the early 20th century. Their research concluded that it also drove three quarters of the infant mortality reduction.
If you are studying how chemicals like chlorine disrupt bacterial cell walls and halt the growth of pathogens, you can use the Fermi Concept Library to explore the biology of microbes and the mechanisms of disinfectants.
How did chlorination change public health?
Leal's success in Jersey City triggered a rapid shift in water treatment. By 1918, over 1,000 North American cities were chlorinating their water. The practice virtually eliminated waterborne diseases like typhoid in the developed world, building on earlier foundations laid by scientists mapping clean water sources (such as Ellen Swallow Richards and the chlorine map of clean water).
Despite saving millions of lives, Leal died in relative obscurity just six years later in 1914. Today, chlorination remains a standard step in municipal water treatment worldwide.
