The Allegheny County Sanitary Authority is moving ahead with a massive project aimed at curbing sewage overflow by creating a massive tunnel system under Pittsburgh and its rivers.

Alcosan officials said the new system — along with other elements of their Clean Water Plan — will reduce sewage overflow by about 7 billion gallons each year. Currently, about 9 billion gallons of sewage mixed with rainwater runs into the local water system each year.

“Our existing sewer system can become overloaded during rainstorms or snow bouts, and diluted, untreated wastewater then runs into our rivers and streams,” said Michelle Buys, Alcosan’s director of environmental compliance.

The proposed tunnel system is part of Alcosan’s $2 billion Clean Water Plan, approved in 2020. The goal is to make the system more efficient, reduce the amount of sewage entering waterways and improve water quality, Alcosan officials said during a public briefing Wednesday.

The new regional tunnel system will be constructed in bedrock about 150 feet deep — or about 50 feet below existing tunnels and roughly 120 feet below the city’s rivers. The tunnels will be up to 18 feet in diameter and lined in concrete, according to Michael Lichte, Alcosan’s director of regional conveyance.

In all, the tunnel system will be about 16.5 miles long with a storage capacity of approximately 160 million gallons, Lichte said.

“The newly constructed system will also stop sewage overflows to sensitive areas such as riverfront parks, marinas, recreational areas, boat launches and areas near drinking water intakes,” he said.

The project is divided into three parts. The first, the Ohio River Tunnel, is slated to begin construction in January 2025, said Kimberly Kennedy, the authority’s director of engineering and construction. Construction of that tunnel is expected to be finished by 2028, with operations starting the following year.

A new wet weather pump station will be following a similar timetable.

A graph displayed during Wednesday’s public meeting showed that the second piece, the Allegheny River Tunnel, is slated to begin construction in late 2028, with construction wrapping up by 2033. The third and final piece of the project, the Monongahela River Tunnel, is slated for construction from 2029 through 2036.

“This schedule is subject to change as design and permitting processes progress,” Kennedy said.

The massive project will be funded through a series of rate increases through 2036, she said. The cost this year is around $549 per year for a typical residential customer — or about $1.50 per day, she said.

As costs increase, she said, Alcosan also is increasing the Clean Water Assistance Fund to help offset the burden on low-income households.

Alcosan is accepting public comments regarding the project on its website.

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