Part of an occasional series answering readers’ questions about Maine

Q: What is the big ugly structure that is under construction next to I-295 and Back Cove?

A: Sheets of metal have been inserted into the soccer field next to Interstate 295 in Portland, some standing as tall as 90 feet above the ground.

But what exactly is going on? Another hotel, or condominiums, being built on Back Cove? Maybe it’s a cold storage building? Perhaps it’s a levee to protect Bayside from rising ocean levels? Or better yet a wall to keep off-peninsula residents in their place?

“We have a lot of people stopping by,” said Bradley Roland, a senior engineer for the city. “There’s been quite a few people asking about it.”

The truth, fortunately, is much less interesting but it is expected to produce tangible improvements for the environment and area residents. It’s all part of the city’s plan to reduce the amount of raw sewage and polluted stormwater runoff being discharged into the cove though antiquated combined sewer and stormwater overflows, or CSOs.

The overflows are safety valves built into the city’s original sewers and were designed release diluted sewage during storms so it didn’t flood the treatment plant, or back up into homes.

The $40 million project is the city’s largest sewer-stormwater project to date and it’s being funded through wastewater fees, which are based on water usage by homeowners and businesses, Roland said.

“None of this is tax base,” he said. “It’s paid for every time someone flushes the toilet.”

Construction taking place near Back Cove in Portland on Saturday. Shawn Patrick Ouellette/Staff Photographer

The sheet pilings are being driven deep into the ground to prepare for the excavation of a giant 25-foot-deep hole. The metal will prevent the loose dirt from filling in the 500-by-150-foot hole, as crews construct a series of four underground concrete storage tanks capable of holding 3.5 million gallons of combined sewer and stormwater. 

That underground storage tank will collect the first flush of stormwater – the equivalent of 1 inch of rainfall containing the highest concentration of pollution – during heavy rainfalls and raw sewage from three of the city’s CSOs and store it until it can be pumped to the East End Treatment Plant. There, the contaminants can be removed and treated before the water is discharged into the cove.

It’s part of a decades-long effort to eliminate or modify combined sewer overflows in Maine’s largest city, often by diverting and storing the diluted sewage until the rainstorm has passed.

Portland currently has 30 overflow discharge sites, down from 42 in 1988. Last year, the city discharged a total of 184 million gallons combined sewer and stormwater during 40 rainstorms. That’s by far the most of the state’s 31 communities with overflows, which range from Madawaska to Calais to Kittery. Bangor discharged the second highest volume of 96 million gallons from eight overflow sites during 34 storms, according to the Maine Department of Environmental Protection.

Statewide, more than 455 million gallons of combined sewer and stormwater were discharged into waterways such as Casco Bay and the Penobscot, Saco, Androscoggin and Kennebec rivers from 131 outfalls. That’s down significantly from the 6.2 billion gallons discharged from 338 outfalls in 1988.

Roland said the three overflow outfalls that will feed the storage tank generally account for 50 percent of the city’s total CSO discharges and that Portland generally accounts for half of the state’s entire CSO discharges.

“By reducing these by 50 percent, we’re looking to reduce the state’s CSO volumes by 25 percent, which would be huge,” he said. 

BayKeeper Ivy Frignoca, who advocates for improving water quality for the nonprofit Friends of Casco Bay, said the project will lead to less nitrogen pollution, which can feed destructive algae blooms and worsen ocean acidification, as well as less bacterial contamination from the raw sewage outfalls that happen when rainfall overwhelms the treatment plant.

Frignoca said the friends were instrumental in convincing state regulators and the city to move forward with the project after several delays.

Once complete, the project promises to reduce the polluted overflows into the cove by 88 percent, or 132 million gallons a year, from 150 million gallons to 18 million gallons.

“With the plan in place and the deadline imposed on these permits, there will be a real opportunity to substantially improve water quality in the Back Cove,” Frignoca said. “That’s really important to the health of the bay.”

When finished, likely sometime in 2022, the tank will be buried and the soccer field and adjacent to Bay Cove Trail will be restored.

The project has one other important feature prompted by environmental concerns.

When complete, the soccer field will be elevated by 3 feet to prevent flooding during high tides as the sea level continues rising because of climate change.

The project is also expected to improve storm capacity along Franklin Street and a back-flow prevention device could prevent areas of Bayside, including the area by Whole Foods, from flooding during high tides.

A small building for electrical and mechanical systems will be constructed near Preble Street, Roland said.

This is the third storage conduit to be installed along Back Cove.

In 2013, the city installed two storage conduits – one in Payson Park and another under Baxter Boulevard – each capable of capturing 1 million gallons of sewer and stormwater to prevent overflow from entering the cove.

And a fourth project for a 2.25 million storage conduit is also in the works.

“In the next five years, when all of these are implemented, we expect to see those discharges into the Back Cove reduced dramatically,” Roland, the city engineer, said.