Maine’s combined sewer systems released 745 million gallons of untreated sewage and storm water into the waterways last year. It’s an increase of 244 percent compared with 2022 according to the Maine Department of Environmental Protection report.

Many sewer systems in Maine use the same pipes to transport raw sewage and runoff collected from storm drains to water treatment facilities. Heavy rain events like the rainstorms seen last year can fill these combined sewers systems to capacity with wastewater. To prevent sewage from overflowing into homes, the pipes discharge the untreated wastewater into nearby waterways.

“[The] EPA and municipalities have tried really hard to reduce the number of times that that happens,” said professor Jean MacRae who studies wastewater systems at the University of Maine’s civil and environmental engineering department. “And they’ve done a great job over the years. But there’s still rain events that can flush the system and move some of that wastewater to the environment without any treatment.”

There are 31 communities in Maine that use combined sewer systems — including Portland, which accounted for more than half of the state’s total overflow. More modern sewer systems use separate pipes so that only stormwater gets released during storm-related overflows.

MacRae said these heavy rain events may be more frequent due to climate change.

“A lot of the design is based on climate conditions that are up to 50 years old, and we know that those conditions are changing,” MacRae said. “And probably we’ll need to be updating the design requirements to deal with our new realities of 20-year and 100-year flood events.”

MacRae said more focus should be put on flood control and on designing landscapes that better retain urban runoff.