Pedestrians pass the spot, shown in September, where city sewage enters the Potomac River at the Oronoco outfall in Alexandria. (Bill O’Leary/The Washington Post)

The cost of significantly reducing sewage overflows into the Potomac River watershed is expected to rise at least another $25 to $35 per month each year for the next five years for Alexandria homeowners as the city scrambles to figure out how to design and build a system that will meet state demands.

City officials told about 40 residents Tuesday night at the Lee Center that they are mulling how to build three store-and-treat systems in Old Town by 2025, as the General Assembly demanded this past session.

A mixture of sewage and rainwater overflows the sewer system in the oldest part of the city every time it rains, and that pours 11 million gallons of raw sewage into the Potomac each year. While that’s a small percentage of the pollution that fouls the river, it is an issue that the city has known about for decades but did not begin to address until last year.

Mayor Allison Silberberg (D) repeated Tuesday that the city is committed to properly cleaning up the overflow. But she and others warned it will not be easy.

The latest construction costs are estimated at $300 million to $400 million over the next 10 years, said Bill Skrabak, deputy director of the city’s Transportation and Environmental Services department.

Digging up the 200-year-old sewers, and replacing them with a set of storage tunnels or tanks that will hold the overflow until the city’s treatment plant can accommodate the mixture, is expected to cause major disruptions for several years in a heavily populated portion of town. In addition, much of this excavation will occur in areas where historic artifacts and industrial contamination are common.

The additional cost of the project to residential property owners, which Skrabak estimated at $264 to $432 per year, caught the attention of the audience. That price hike is just on the city’s portion of the sewer bill; residents also pay a wastewater treatment bill to the independent entity called Alexandria Renew, which collects and treats wastewater. The rate for that portion of the bill has not yet been set; Skrabak said it will be next year before cost estimates are firmed up, after engineering plans are further along.

Some people, including representatives of the Potomac Riverkeeper Network, an environmental group, wanted to know where the city plans to locate its storage tanks or tunnels, particularly for the Oronoco Bay outfall. Skrabak said several locations are under consideration, including beneath an existing park and beneath the river.

“If you put a tank in the river, we’re going to challenge that,” said Dean Naujoks, the Potomac riverkeeper. Phillip Musegaas, vice president of litigation for the group, agreed.

“We would be very concerned,” he said. “It depends, but it’s not considered a good environmental practice to put something like that in the river.”

The deadline of 2025 set by the legislature to complete construction of the giant tunnels and tanks is generally regarded among city leaders as unrealistic, and they complained that no such deadline has been placed on Richmond or Lynchburg, cities that have been working on similar problems since the 1970s and 1980s.

State Sen. Adam P. Ebbin (D-Alexandria), Sen. George Barker (D-Fairfax) and Del. Mark H. Levine (D-Alexandria) said the city had been singled out by the Republican-controlled legislature for political reasons. But Levine pointed out that the draconian penalties originally proposed, which would have withheld all state funding from the city if the deadline was not met, had been removed.

Barker and Ebbin added that if the city cannot meet that deadline, because of problems beyond its control, they were confident that the deadline could be extended.