According to Westchester County health officials, the treatment plant lost power around midnight.
The facility, located in Yonkers, processes sewage from 22 municipalities across lower Westchester and handles up to 145 million gallons per day.
The resulting health advisory covers a stretch of the Hudson River from New York City to Briarcliff Manor. Officials said people should avoid recreational activities that could result in river water coming into contact with skin, eyes or open cuts.
"New Yorkers should refrain from swimming or any other activity that involves the full immersion of the body under water at closed beaches. It is safe, however, to engage in other recreational activities which involve partial contact with the water such as boating, kayaking and fishing," the Department of Health and Department of Environmental Conservation said in a statement.
On Friday evening, officials gave an update, saying that as of 1:45 p.m., the Yonkers Wastewater Recovery Facility was fully operational after suffering a complete power failure due to the power reductions by Con Edison from the extreme heat.
"It's really unfortunate that this has happened, on such an important weekend for our communities. I'm sure that everyone's very disappointed. But we want everyone to stay safe," said Dr. Sherlita Amler, Westchester County health commissioner.
The boat launch at JFK Marina in Yonkers was closed Friday, cutting off a popular access point for jet skiers. The water near the marina appeared visibly discolored.
"I didn't notice it at first, but once you get close to the water, you can. You can smell like a very nasty smell. Definitely, something spilled. You can tell," said Andrew Bellantoni of the Yonkers Parks Department.
Despite the advisory, some people were still on the water Friday morning in Sleepy Hollow, including kayakers using sail-equipped vessels.
The Philipse Manor Beach Club in Sleepy Hollow was closed for swimming under the health department order.
Officials said the closure will remain in effect throughout the weekend, even if the treatment plant returns to operation.
"It will be the first part of the next week before we can sample. And even when we sample, it takes time for the test to be run, for us to get the results and for us to clear it," Amler said.
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