With Croton water available again, city officials will have more flexibility in supplying the city if and when there are problems in the Catskill-Delaware system.

Image Emily Lloyd, the commissioner of the Department of Environmental Protection. Credit... Ángel Franco/The New York Times

For instance, the Croton plant is intended to help the city get through a period, probably from 2022 to 2023, when no Delaware water will be available at all, while a bypass tunnel under the Hudson River is connected to the Delaware Aqueduct, overriding an irreparably damaged portion that leaks up to 35 million gallons a day.

The Croton watershed is largely in Westchester, Putnam and Dutchess Counties, where development has been more intense than in the Catskill-Delaware watershed, 125 miles away. Storm runoff that reaches the 12 Croton reservoirs has more contaminants.

Croton water enters the city through the Jerome Park Reservoir in the Bronx. Before 2008, it moved straight into distribution. Now, it is diverted northward to the filtration plant, an echoing 830-by-550-foot underground structure that could do double duty as a science-fiction movie set. There, it undergoes several treatments.

In the first treatment, organic particles in the water are lifted to the surface. Very simply said, alum is introduced to encourage particles to coagulate. Water saturated with air is added. This creates bubbles that attach to the coagulated particles so they float upward.

What results is an unappetizing green-brown foamy substance known as floc, which is skimmed mechanically from the surface of the water in 48 concrete troughs, 25 feet deep, 22 feet wide and 33 feet long. Within the troughs are three-foot filters, with a layer of sand atop a layer of anthracite, through which the water passes after it has been skimmed.