The popular East End Beach in Portland was closed Monday after a malfunction at the East End Wastewater Treatment Plant caused a wastewater discharge into Casco Bay, officials said.

It was not immediately clear on Monday how much wastewater flowed into the bay from Sunday’s spill.

About 1.7 million gallons of partially treated wastewater went into the bay during a similar breakdown on July 28, 2018, enough to force the closure of the beach for 36 hours. An investigation revealed that the 2018 spill came because of a worker’s failure to fully open a release valve after the routine cleaning of a tank at the plant.

The Portland Water District operates and maintains four wastewater treatment plants, including the East End facility, and provides wastewater services to Cape Elizabeth, Cumberland, Gorham, Portland, Westbrook and Windham, according to its website.