How do you slake the thirst of 5.5 million Chicagoans spread throughout the United States' third largest city and 118 surrounding suburbs? You treat 3.7 million litres of water a minute at the world's largest water treatment plant.
Opened in 1968, the James W. Jardine Water Purification Plant provides virtually all of Chicago's drinkable water, though the smaller South Water Purification Plant also helps provide for the south end of the city. The process begins 4km off-shore at massive in-flow points called "cribs". Each crib pulls water from 6m below the surface down a 51m vertical shaft to tubes cut from the bedrock which flow back to the shore. Once it reaches the plant, fish and other large debris are filtered out using a rotating screen and the water is pumped 7.6m up to begin the gravity-powered treatment process.
The water is first chemically treated with chlorine to kill microbes and activated carbon to remove objectionable tastes and odours, then fluoride is added for healthier teeth, and finally aluminium sulfate about an hour into the process to increase the stickiness of microscopic solids which then adhere to each other, creating "floc". Large paddles — floculators — assist with the mixing of aluminium and water. One of the very last chemicals added, polyphosphate, is used to coat the inside of Chicago's pipes, preventing the lead in old plumbing from leaching into the water supply. The water is then pumped into settling tanks where the floc sinks to the bottom. This sedimentation phase eliminates roughly 90 per cent of the particulate matter from the water.
The water is then filtered in one of 96 swimming pool-sized tanks filled with a layer of coarse gravel under an upper layer of fine sand — together these layers effectively filter much of the remaining floc and debris from the water. The entire process from crib to filtration takes eight hours. From here, the water is pumped through 6437km of pipe for consumption by the Chicago area's 5.5 million residents.