Contractors for the city Of Alexandria scrape and flush a sewer at the corner of King and Union streets in Old Town in 2012. (Tracy A. Woodward/The Washington Post)

The Virginia Department of Environmental Quality has approved Alexandria’s plan for fixing its sewer overflows into the Potomac River, a major milestone that allows the city to move ahead on an expensive and controversial project.

The June 29 letter also certified that Alexandria is in compliance with state legislation that requires the city to finish the job by 2025.

“We have a very aggressive schedule, but this has been a long-standing issue so we’re putting in place a plan we can actually construct,” said Liliana Maldonado, the lead engineer and program director on the project for Alexandria Renew, the city’s sewer authority.

The 200-year-old combined sewers in Old Town for decades have overflowed into the river whenever it rains, because the underground system can’t handle normal effluent from tubs, toilets and sinks as well as rainwater runoff.

The overflow violates the federal Clean Water Act.

Alexandria began to address the issue several years ago, knowing that a state deadline was approaching for fixing three of the four outfalls. But the city’s initial plan left the largest outflow untouched, until residents, environmentalists and downstream state legislators raised an outcry. The General Assembly then passed a bill that required the city to fix the sewers by 2025, despite local objections that the deadline was unrealistic.

The project will replace old combined sewers with a deep storage tunnel that can hold sewage and runoff until the wastewater treatment plant is ready to clean it. When complete, the new system will reduce the average number of overflows each year from about 60 to fewer than four, Maldonado said.

While construction on the sewer authority’s property will begin sooner, the tunnel construction is to begin in the last quarter of 2020 and peak in 2022, Maldonado said.

Officials say the project will cost between $356 million and $534 million. Alexandria taxpayers are already seeing higher sewer bills to cover the cost of the project, and more increases are coming. By 2025, monthly bills will rise an additional $25 to $35 per month.