In 1908, Jersey City, New Jersey, became the first municipality in the United States to institute chlorination of its water supply, followed that same year by the Bubbly Creek plant in Chicago. As had happened in European cities that had also introduced chlorination and other disinfecting techniques, death rates from waterborne diseases—typhoid in particular—began to plummet. By 1918 more than 1,000 American cities were chlorinating 3 billion gallons of water a day, and by 1923 the typhoid death rate had dropped by more than 90 percent from its level of only a decade before. By the beginning of World War II, typhoid, cholera, and dysentery were, for all practical purposes, nonexistent in the United States and the rest of the developed world.

As the benefits of treatment became apparent, the U.S. Public Health Service set standards for water purity that were continually revised as new contaminants were identified—among them industrial and agricultural chemicals as well as certain natural minerals such as lead, copper, and zinc that could be harmful at high levels. In modern systems, computerized detection devices now monitor water throughout the treatment process for traces of dangerous chemical pollutants and microbes; today's devices are so sophisticated that they can detect contaminants on the order of parts per trillion. More recently, the traditional process of coagulation, sedimentation, and filtration followed by chemical disinfection has been complemented by other disinfecting processes, including both ultraviolet radiation and the use of ozone gas (first employed in France in the early 1900s).

One important way to improve water quality, of course, is to reduce the amount of contamination in the first place. As early as 1900, engineers in Chicago accomplished just that with an achievement of biblical proportions: They reversed the flow of the Chicago River. Chicago had suffered more than its fair share of typhoid and cholera outbreaks, a result of the fact that raw sewage and industrial waste were dumped directly into the Chicago River, which flowed into Lake Michigan, the source of the city's drinking water. In a bold move, Rudolph Hering, chief engineer of the city's water supply system, developed a plan to dig a channel from the Chicago River to rivers that drained not into Lake Michigan but into the Mississippi. When the work was finished, the city's wastewater changed course with the river, and drinking water supplies almost immediately became cleaner.

City fathers in Chicago and elsewhere recognized that wastewater also would have to be treated, and soon engineers were developing procedures for handling wastewater that paralleled those being used for drinking water. It wasn't long before sewage treatment plants became an integrated part of what was fast becoming a complex water supply and distribution system, especially in major metropolitan centers. In addition to treatment facilities, dams, reservoirs, and storage tanks were being constructed to ensure supplies; mammoth tunnel-boring machines were leading the way in the building of major supply pipelines for cities such as New York; networks of water mains and smaller local distribution pipes were planned and laid throughout the country; and pumping stations and water towers were built to provide the needed pressure to support indoor plumbing. Seen in its entirety, it was a highly engineered piece of work.