Rudolph Hering, a noted consulting sanitary engineer who began his career with the Philadelphia Bureau of Surveys, headed two city-sponsored water commissions; the second, in 1899, recommended filtering the supply.

Scientists had known for years that contaminated water, when filtered through soil, would be cleansed of some contaminants and rendered more drinkable. Using this knowledge, engineers in the 19th century developed sand filters to remove impurities from drinking water. They discovered that a properly designed and maintained "slow sand" filter, through which water flows continuously by gravity, will trap disease-causing bacteria and other contaminants. European cities were the first to install sand filters for their water supplies, and generally suffered lower rates of typhoid fever and other water-borne diseases than cities with unfiltered supplies. By the time Philadelphia began building its water filtration plants in the first years of the 20th century, the benefits of such systems had long been proven elsewhere.

For an overview of Hering's career, see this obituary from the Journal of the American Water Works Association. The National Museum of American History at the Smithsonian Institution has a Rudolph Hering Collection; perusing the finding aid at this link will give you some idea of the scope of his work throughout the United States and in other countries.