ABSTRACT
Drinking water treatment began in response to high levels of waterborne diseases-such as dysentery, typhoid, and cholera-transmitted through fecal contamination of food and water in urban populations. Cities arose close to freshwater sources, often rivers, which became progressively polluted. One of the earliest and still most effective treatments for river water is slow sand filtration. In this process, incoming raw water is passed slowly through a bed of sand, which builds up a layer of microorganisms on the surface. As the water passes down through this surface layer, pathogens are removed and organic molecules oxidized. For over 150 years, this process has been in use in treating water for the city of London. About 100 years ago, this water filtration process was reinforced by the addition of chlorine as a sterilizing agent to destroy pathogens that might escape removal by filtration and also pathogens that enter the drinking water supply after treatment.
