A maze of pipes snakes under 83 municipalities in Allegheny County. Since the mid-20th century, their wastewater has flowed to Allegheny County Sanitary Authority (ALCOSAN) for treatment. But it has never been able to collect all of it. Rainwater pours into the pipes along with the sewage. As little as one-tenth of an inch of rain can overload the system, sending about 9 billion gallons of sewage into Pittsburgh’s rivers each year.
“Everything that comes from businesses and houses and toilets and sinks gets mixed with all the water off of roofs and roads,” said Jonathan Burgess, director of the Pittsburgh Water Collaboratory at the University of Pittsburgh. “And it eventually ends up directly in the river without actually being treated.”
ALCOSAN has been under a federal consent decree since 2008 that requires the agency to fix their combined sewer overflow problem. Their $2 billion solution includes expanding their treatment plant, building large underground storage tunnels under the three rivers, and taking ownership of some of the overflowing municipal pipes. The goal is to stop about 7 billion gallons of sewage from pouring into the Monongahela, Allegheny and Ohio.
The project is still in its early stages with a deadline to finish by 2036. So far, the agency has made progress expanding its wastewater treatment plant to take on some of the extra flow that's going to the rivers and streams. It has also taken over ownership of some municipal pipes that contribute to overflow and doled out grants to local governments to reduce the amount of excess water from entering the system, including finding nature-based solutions to absorb rainfall.
Later this year, ALCOSAN will take the first steps toward building a giant tunnel under the Ohio River. In October, they’ll start to take on more water at the treatment plant – the first step in seeing a “significant reduction” in the amount of sewage overflow in the rivers, according to Kim Kennedy, director of engineering and construction at ALCOSAN.
“That's just the nature of these long projects and how long they take to build,” Kennedy said. “But they're getting there and we're getting to the point where we're really gonna start to see some improvements.”
More water, more infrastructure
Pittsburgh isn’t the only place struggling to manage its overflow. From Paris, France to Cleveland, Ohio, cities all over the world with aging infrastructure near rivers and streams are at work overhauling their sewer systems to deal with excess stormwater and untreated sewage.
Pittsburgh is “not unique, but there are unique challenges here,” Burgess said. “Increasing rainfall, very hilly topography, a lot of old infrastructure. Some of our pipes are made out of terracotta and cast iron. I mean, it's just old.”
Climate models project that the average amount of rain will increase in southwestern Pennsylvania in the coming decades, taxing this aging infrastructure.
ALCOSAN’s plan is centered on increasing the capacity of the plant located on the North Shore of the Ohio River, as well as adding massive tunnel systems under the three rivers to handle the excess water. This January, ALCOSAN added a new disinfection tank — the last step in the treatment process — to handle more wastewater at the treatment plant.
“When you increase that treatment, that's now treated water that gets discharged as opposed to untreated water,” Michelle Buys, director of environmental compliance at ALCOSAN.
The plant currently treats 250 million gallons of wastewater a day. In October, ALCOSAN plans to treat 295 million gallons each day. And by October 2026, they want to treat 480 million gallons. “That'll be our first realization of overflow reduction other than a dry year,” Buys said.
Outside of the treatment plant, more than 50 miles of municipal sewer lines have been transferred to ALCOSAN ownership to try to untangle a complicated regional system. These are lines that have flows from more than one municipality and contribute to overflows. And they’re looking to acquire more to streamline the region’s system.
Later this year, ALCOSAN will look for contractors to build the wet weather pump station — that will take the water from the forthcoming big underground tunnels and transfer it to the plant for treatment — as well the Ohio River tunnel. It’ll be the first of the three tunnels. Construction on the pump station is slated to start at the end of the year and tunnel work is expected to begin in early 2026, according to Kennedy.
Like any massive multi-year project, the plan faces uncertain headwinds particularly around construction timelines and changes in costs for materials, according to Burgess. “With inflation and then pending tariffs, there's a chance that the initial estimates for what this will cost ALCOSAN, and by extension, rate payers, are undervalued.” Challenges could include potential delays in acquiring construction materials and increases in subcontracting costs for the people ALCOSAN hires to do the work. “There's a lot of uncertainty around what it will end up costing people at the end of the day and how easy it will be to get the materials and labor necessary to do the work,” Burgess said. “That's probably the single biggest unknown.”
Beyond the gray
Even if ALCOSAN’s tunnels and plant are successful, it won’t eliminate all the overflows. Around the region, local governments and nonprofits are at work on other solutions to keep excess stormwater from entering the system in the first place. The city of Pittsburgh already updated its stormwater code in 2022 to account for increased rainfall due to climate change.
For its part, ALCOSAN distributes grants under their GROW program, many of which are given for so-called green infrastructure projects — nature-based solutions like bioswales that could catch road runoff and let it sort of filter down naturally into the soil and rain gardens.
Other local work includes environmental nonprofit, Tree Pittsburgh’s, $8 million project to expand Pittsburgh’s tree canopy. More trees mean more roots soaking up heavy rains. And Allegheny County Conservation District recently received a Pennsylvania Department Environmental Protection grant for green infrastructure projects to manage stormwater, including the Plum Stormwater Project.
ALCOSAN’s $2 billion project is “a big piece of the puzzle”, according to Heather Manzo, executive director of the Allegheny County Conservation District. It’ll take more than one agency to solve the region’s stormwater challenges long-term, but Manzo is optimistic. “I think it's super exciting that there's really strong collaborations and public-private collaborations that are happening.”
