Nearly 4 million gallons of partially treated wastewater was discharged into Casco Bay last weekend during a power failure at the East End Wastewater Treatment Plant, the Portland Water District said Friday.

The discharge prompted the city to close the popular East End Beach from Sunday to Tuesday while tests were done to determine that contamination levels were not a threat to public health. The amount of wastewater discharged was not immediately known because the power failure interfered with the plant’s computer system.

The amount of partially treated wastewater discharged into the bay this week was more than twice the amount that spilled from the treatment plant two years ago when a disinfection tank overflowed and forced a two-week closure of East End Beach.

The water district estimates 3.97 million gallons of partially treated wastewater was discharged Sunday when power to the plant was interrupted at 8:15 a.m., according to an incident report filed Friday with the Maine Department of Environmental Protection.

A backup generator failed and a generator service company later determined several bolts inside the generator had come loose.

The effluent that went into the bay had gone through some initial stages of treatment but had not been disinfected with chlorine, the water district said.

The DEP will now decide what penalties to impose for the discharge, which is a violation of the water district’s license. The water district was fined $16,800 last summer for unlicensed discharges and other violations, for example. That penalty was used to pay for environmental restoration work at Evergreen Cemetery.

The power outage happened when an electrical splice in a Central Maine Power feeder line to the plant failed, Scott Firmin, director of wastewater services for the water district, wrote in the report. Power was restored to the plant at 2:30 p.m., about six hours after the outage began.

Firmin said the backup generator that failed had operated successfully on June 26 and during weekly and monthly tests. The generator had been rebuilt in May 2019 and regularly serviced by a contractor, he said.

“There was no indication of any issue, and we had every reason to expect the generator would operate if required,” Firmin said in the report.

A rented generator was brought into the plant Sunday evening and will remain there while the backup generator is repaired. Firmin said the repairs will cost $3,500 and will be completed next week.

Firmin said screenings, grit removal and primary sedimentation of the wastewater all occurred. Flow continued through aeration tanks and the secondary clarifiers, but aeration equipment also was not active due to the loss of power. The flow continued through the disinfection process, but without chemical addition, he said.

“My primary concern was the lack of disinfectant in the effluent. Based on the duration, detention time in the plant and my familiarity with the plant, in my opinion, the effluent for the entire day would have met permit limits other than limits for bacteria,” Firmin said.

After the spill, the water district notified the city of Portland, the DEP and the Maine Healthy Beaches program to suggest they review their procedures for beach closures, according to the report.

David Madore, a spokesperson for the Maine DEP, confirmed that the water district filed an incident report in response to the July 19 discharge. Department staff have been in contact with treatment facility staff throughout the week and are currently reviewing the report, he said.

Test results showed the beach water contained 10 MPN/100 ml, a level of bacterial pollution that is “well below Maine’s safety threshold of 104 MPN/100 ml,” said Jessica Grondin, spokesperson for the city. MPN stands for most probable number, a statistical method for testing water samples for harmful bacteria and other pathogens that could cause people to become sick.

“It’s always regrettable when things like this happen,” said Ivy Frignoca, Casco Baykeeper with the Friends of Casco Bay. “We are concerned when 4 million gallons of partially treated wastewater reaches the bay.”

Frignoca said the biggest threat to the bay from the discharge is bacteria because the wastewater wasn’t disinfected, but it appears that bacteria levels have gone back down quickly.

While 4 million gallons of partially treated wastewater is “not good for the bay,” there are combined sewer overflows that dump more than that into Casco Bay after major rainstorms, Frignoca said. In 2016, nearly 69 million gallons of stormwater mixed with raw sewage, debris and polluted runoff flowed into Back Cove, Portland Harbor and other local waterways following a 4-inch rainstorm.

Some older sewer lines in Portland collect stormwater in addition to sewage, and heavy rains routinely overwhelm those pipes, causing the sewage and stormwater to overflow into the bay with no treatment.

East End Beach was closed for almost a week two years ago when 1.6 million gallons of wastewater spilled from the treatment plant, washing away a section of the Eastern Promenade Trail, and flowing into Casco Bay. Following the accident, water tests showed elevated levels of bacteria.

The East End Wastewater Treatment plant treats an average of 6.4 billion gallons each year.