AUBURN — City crews will have to undo some of the work they did last summer after storm and sanitary sewage overflowed a manhole cover below Festival Plaza during a Jan. 25 rainstorm.

Auburn Sewerage District crews capped the Miller Street outfall in December after a summer’s worth of work building a parallel system to drain storm water off several city streets.

But heavy rain on Jan. 25 overflowed the system, District Superintendent John Storer said.

Since the storm runoff was unable to flow directly into the river, it overflowed the Miller Street manhole, flooding the parking area there. Miller Street runs between Festival Plaza and the Androscoggin River.

“From there, it did drain into the river,” Storer said. “It’s disappointing that this happened, but we have to unseal that storm outflow now.”

The city is in the middle of a federally required 15-year, $19 million project to build a parallel storm-sewer system and seal 11 combined sewer outflows. The old system combines sanitary sewage and storm water, sending both to the water treatment plant. But during heavy storms, water powering into the combined sewers can overflow the system, sending raw sewage into the Androscoggin River.

So far, the city has spent $16 million connecting storm-sewer drains directly to the river and sealing eight of the 11 combined sewer outflows.

“It’s made a huge difference in how much flows into the river,” Storer said. “As far back as 10 years ago, we had 200 million gallons of water per year that overflowed into the river. For the last two years, that’s down to about 10 million gallons.”

The Miller Street outflow was the ninth one sealed. Crews worked last summer on the Perryville neighborhood east of Pettengill Park. That area includes Turner, Summer, Winter and Whitney streets from Center Street northwest to Mayfield Road.

Water from those streets flowed into the Miller Street outflow.

“We were a little optimistic, after we finished the Perryville work,” Storer said. “That was a very big one to finish, and we thought it should have taken care of most of the problem. But there are too many smaller, isolated pockets that still have combined sewers and those contributed storm water to the system during that Jan. 25 storm.”

The city received permission from the Maine Department of Environmental Protection on Feb. 10 to unseal that outlflow. Storer said the district hopes to do enough storm-sewer work next summer to seal it up again, for good this time.

“According to our estimates, we have thousands of storm-water grates in the city and about 70 of them still need to be separated out,” Storer said. “We’re going to do that, over the next couple of years. We are committed to building a completely separate storm system.”

[email protected]