Water from the Croton watershed has been kept out of NYC since 2008, thanks to the seemingly never-ending construction of the Croton Water Filtration Plant in the Bronx. But on Thursday, the city finally turned the taps on, pumping in filtered water from Westchester, Putnam and Dutchess Counties.

The plan, located in Van Cortlandt Park, cost $3.2 billion to build (the initial estimate was $800 million) and is expected to treat about 290 million gallons of water a day. Water coming from the watershed farther upstate goes through a complicated filtering process. Per the Times:

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.

Floc is about 98 percent water, said James Roberts, the deputy commissioner for water and sewer operations. It is sent through the sewer system to the Wards Island Wastewater Treatment Plant.

What the filters largely miss are microscopic parasites like cryptosporidium, which can cause diarrhea in otherwise healthy people and threaten the lives of those with compromised immune systems, and giardia, which can cause abdominal cramps, nausea and diarrhea.

These micro-organisms are inactivated by exposure to ultraviolet light, said Michael Keating, the director of Croton operations. And that is the next step in the process.

After ultraviolet treatment, sodium hypochlorite is added as a secondary disinfectant (it makes part of the plant smell like a swimming pool), as is phosphoric acid, to inhibit corrosion within the distribution pipes. The water is also fluoridated at this stage.

The treated water can be fed by gravity to some areas of the city, but must be pumped elsewhere, since the Jerome Park Reservoir is about 170 feet lower than the Hillview Reservoir in Yonkers, through which Catskill and Delaware water flows down into the city.

You can see the progress of the construction of the plant in the photos above.