Sign warns passersby in Alexandria about sewage where it enters the Potomac River at the Oronoco outfall on in Alexandria, Va. The sewer system in Alexandria, Virginia, gets overwhelmed during almost any type of wet weather, sending untreated waste into nearby waterways and on to the Potomac River scores of times each year. (Bill O'Leary/Washington Post)

The Alexandria City Council is transferring ownership of its troublesome combined sewer outlets to the local sanitation agency, Alexandria Renew.

Officials said the ownership change, unanimously approved by the council Tuesday night, will not let the city escape a state-imposed 2025 deadline to eliminate the 200-year-old system’s frequent overflows, which send both sewage and rainwater into the Potomac nearly every time it rains.

But it gives responsibility for the massive effort to an agency that has experience doing big infrastructure projects, more bonding authority than the city government and a stake in making sure the replacement works well with the rest of the system.

Calling the repair a “Herculean task” that will cause disruption and significantly raise sewer bills, Alexandria Renew chief executive Karen Pallansch pledged to work closely with the city and public.

“We know communication is important. We know we have to work collaboratively with the city staff,” Pallansch said, before alluding to a dispute between government officials and the agency more than a decade ago over the location of a sewage treatment facility. “Many, many years ago, we made some missteps ... We’re open to any suggestions or ideas you offer.”

Alexandria, like many old cities, in the 1800s built a single sewer for both waste from toilets, sinks and tubs, and rainwater runoff. The sewer led straight to the river. Over the years, many cities, including Alexandria, separated sewage and rainwater runoff as they built new sewers, and sent the sewage to a treatment plant which processes and cleans the effluent until it meets federal water quality standards before discharging it into the river.

But the city’s combined sewer system, which still serves a portion of Old Town, allows sewage to escape into the river along with rainwater whenever there is precipitation.

The project, which will replace old combined sewers with a storage tunnel that can hold sewage and runoff until the existing wastewater treatment plant is ready to clean it, is subject to approval from the Virginia Department of Environmental Quality, which acts on behalf of the federal government to enforce the Clean Water Act.

Alexandria officials knew they would be required to address the overflows that caused high bacteria levels in Hunting Creek, a tributary of the Potomac. But the city hadn’t planned to tackle other overflows for two decades. Then local citizens and state legislators who represent downstream districts got involved, prompting the General Assembly to demand a faster timetable.

Local taxpayers are already seeing higher sewer bills to cover the cost of the project, and more increases are coming, Pallansch warned: By 2025, monthly bills will rise another $25 to $35 per month. Officials estimate the capital cost of the project as between $356 million and $534 million.

The ownership transfer has been in the works for months. In November, Alexandria Renew hired an engineering consultant to help plan the work. The city will retain responsibility for granting land-use permits to Alexandria Renew as the project goes forward, so it retains oversight of the construction.

Correction: An earlier version of this story misstated the monthly bill increase. It has been corrected.