A section of the Eastern Prom Trail was washed away Thursday after 1.7 million gallons of partially treated wastewater spilled out of the city's wastewater treatment plant and into Casco Bay, according to the Portland Water District. Credit: Courtesy of The City of Portland

The large spill of partially treated sewage into Casco Bay last week was caused by a worker’s failure to fully open a release valve after the routine cleaning of a tank at a Portland water treatment plant.

The role of human error in 1.7 million-gallon deluge is among the findings in a eight-page incident report that the Portland Water District sent to the state’s environmental regulator Tuesday afternoon.

The Maine Department of Environmental Protection will now use the document in determining whether to sanction the utility company for the spill, which the water district estimates caused between $30,000 and $50,000 of damage. The department’s decision is expected next month.

“We take full responsibility for the incident,” water district spokeswoman Michelle Clements said Tuesday evening. “Through the investigation we determined several improvements that will be made to ensure an incident like this does not happen again.”

Last Wednesday, a water district employee was cleaning a chlorination tank at the treatment plant on Portland’s East End and failed to fully open an inlet valve, according to the report. When a heavy rain began to fall around 4:25 a.m. Thursday, water could not flow out of the tank as quickly as it was entering and “was forced out of the front section,” it reads.

The outpouring ripped up parts of an Eastern Promenade trail, and forced the closure of the city’s East End Beach for a day and a half.

Clements said that the water district is continuing its personnel investigation and had not taken any disciplinary action as of Tuesday.

Brian Kavanah, the DEP’s director for the division of water quality management, said Tuesday morning that he’s not yet sure what, if any, action the department will take in response to the spill. The department sent its own investigator to the site last week. Staff will review their findings and the water district’s report at an Aug. 8 meeting and make decision from there, Kavanah said.

“It’s on the high end of a spill in terms of volume,” he said. “I don’t want to speculate on what our actions will be until we have all the facts.”

The report describes the spill as measuring at nearly 1.7 million gallons of wastewater, larger than was initially reported. Last Thursday, the water district estimated its volume at 1 million gallons. When it spilled, the water had been cleared of solid waste and was near the end of the treatment process.

The report also lays out the water district’s efforts to restore areas damaged by the flow. The East End Beach reopened Friday after a water test found no elevated levels of bacteria in the water. The damaged portion of the Eastern Prom Trail was repaved by Tuesday evening and is set to reopen as scheduled Wednesday.

Finally, the report details steps the water district will take to avoid future spills. The utility will add checking the chlorine tank system to workers’ daily checklist, make other procedural changes and “explore the need” for a spill alarm system, according to the report.

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