MARLBOROUGH — Despite plans to re-open yesterday, a local water treatment plant will remain closed until authorities determine what caused a valve to malfunction, dumping dangerous amounts of fluoride into some of the city’s drinking water.

No injuries or illnesses were reported following Friday’s mishap.

The Department of Environmental Protection shut down the Millham Water Treatment Facility Friday afternoon after plant workers discovered that a concentrated amount of fluoride had contaminated the water. At its worst, water readings showed fluoride levels at six times the normal amount. The plant was supposed to be back on line yesterday but DEP Central Massachusetts Regional Director Martin Suuberg said state officials weren’t satisfied that the problem was entirely fixed.

“Our folks did go out to the plant (yesterday) and the plant is not up yet,” Suuberg said. “We’re still waiting for a written submission that we need as part of the process.”

Plant Manager Stephen Turner said the fluoride leak resulted from a faulty valve. Workers, who were monitoring water readings for a separate study, noticed that the pH readings were off, which alerted them of the problem.

While the cause has been identified, Suuberg said the DEP wants a written explanation describing what happened, why it happened and what will be done to prevent such episodes in the future.

“There was a seal in the valve that wasn’t working properly,” Suuberg said. “The seal didn’t work and, therefore, the concentrated quantity of fluoride got into the system.

“When you have a problem, particularly when we’re talking about a mechanical malfunction, we want to know what the factors were surrounding the malfunction.”

Suuberg said once the written statement is submitted to the DEP, the plant should be back up and running by tomorrow.

“When the plant goes back on line, they have to either have the problem fixed or have the fluoride dispensing system off line until it is properly fixed.”

The normal amount of fluoride in water should be at or under 4 parts per million, Suuberg said. Water readings Friday showed 24 parts per million – six times the normal amount. Too much fluoride can cause nausea if ingested and can irritate and burn the skin.

Fluoride is used to help fight tooth decay but is not a required additive according to state regulatory guidelines.

“It’s something a community chooses to do to promote dental health,” Suuberg said. “One can operate a water treatment plant without fluoride.”

Since Marlborough gets most of its water from another main facility, Suuberg said residents aren’t in any danger of being without water due to the problems at the Millham facility.

Those affected by the fluoride leak were within a 1-mile radius of the plant. It affected both businesses and residents on Millham and Boundary streets as well as those living on the lower end of Robin Hill Road and some small adjoining streets.

Plant workers went door-to-door Friday night to warn residents of the problem and to advise them to flush their hot and cold taps and dispose of any food made with tap water.

Suuberg said, to his knowledge, this was the first concentrated release of fluoride in Central Massachusetts.