Understanding ALCOSAN's Wet Weather Plan and the Consent Decree
In This Article
When you flush the toilet in Pittsburgh, where does it go? If you are one of the roughly 800,000 people served by the Allegheny County Sanitary Authority, the answer is: to a plant beneath the McKees Rocks Bridge on the North Side, where ALCOSAN treats wastewater for 83 of the county's 130 municipalities.
But when it rains — and in Pittsburgh, it rains frequently — the answer becomes more complicated. And that complication is the reason our rivers still carry billions of gallons of sewage overflow every year.
The Combined Sewer Problem
Pittsburgh, like many older American cities, has a combined sewer system. This means stormwater from streets and rooftops and sewage from homes and businesses travel through the same pipes. In dry weather, the system works fine: everything flows to ALCOSAN's treatment plant, which can handle 250 million gallons per day.
But in wet weather, the volume far exceeds that capacity. When the pipes fill up, the system is designed to overflow — discharging a mixture of rainwater and raw sewage directly into the Allegheny, Monongahela, and Ohio Rivers. These are called combined sewer overflows, or CSOs. ALCOSAN estimates that approximately nine billion gallons of this diluted sewage overflow into the region's rivers and streams each year.
There are also sanitary sewer overflows (SSOs), which occur when separate sanitary sewer systems — those not combined with stormwater — overflow due to infiltration, blockages, or capacity issues. The consent decree requires elimination of SSOs and reduction of CSOs.
The 2008 Consent Decree
In 2008, the Pennsylvania Department of Environmental Protection and the Allegheny County Health Department issued a consent decree requiring ALCOSAN to bring its system into compliance with the federal Clean Water Act. The decree set a compliance deadline of September 2026.
In January 2018, ALCOSAN's Executive Director, Arletta Scott Williams, publicly acknowledged that the deadline would not be met. Negotiations with regulators were ongoing, and the new target was expected to be 2032 or 2036.
"We got to a point where we and the regulators recognized, 'You're not going to be able to accomplish this in the time period that we wanted you to.' I like to say you can't get blood from a turnip. There's only so much money and resource available." — Arletta Scott Williams, ALCOSAN Executive Director
ALCOSAN's Wet Weather Plan
To comply with the consent decree, ALCOSAN developed its Wet Weather Plan — also referred to as the Clean Water Plan. The plan has two major components:
- Gray infrastructure: Expanding the treatment plant from 250 million gallons per day to 600 million gallons per day by 2027, and constructing a network of deep tunnels to store and convey overflow during wet weather events.
- Green infrastructure: Projects such as rain gardens, tree trenches, permeable pavement, and green roofs that capture stormwater before it enters the sewer system, reducing the volume of water that reaches the pipes.
The total investment is approximately $2 billion. Of that, roughly $100 million is currently allocated for green infrastructure through the Green Revitalization of Our Waterways (GROW) program.
Why the Timeline Matters
ALCOSAN expects to begin designing the plant expansion in spring, with construction starting in late 2019. Design for the first tunnel section was expected to begin within a few years of the 2019 plan presentation.
Once tunnel design and construction commitments are made, the financial and engineering momentum shifts toward gray infrastructure. Green infrastructure, which requires building many small distributed projects rather than one large one, needs time to demonstrate performance at scale. The narrow window between plan approval and tunnel construction is the critical period for advocates to push for a higher green allocation.
"It's really incumbent on those of us who favor green solutions to, as quickly as possible, get the evidence that they will perform." — Brenda Smith, Nine Mile Run Watershed Association
What Happens If Nothing Changes
If ALCOSAN proceeds with a gray-dominant plan, the tunnels and plant expansion will eventually reduce overflows — but the timeline stretches into the 2030s and beyond. Meanwhile, every rainstorm continues to send sewage into the rivers. Communities along the waterways — particularly low-lying and lower-income neighborhoods — continue to bear the health and environmental consequences.
The rivers are not just scenery. They are the source of drinking water for the region. They are where children learn to fish, where families boat and swim, and where the city's identity lives. Nine billion gallons of sewage overflow per year is not a statistic. It is a public health issue, an environmental justice issue, and a quality of life issue.
There is a better way. The 3RiversProud Clean Water Plan describes it in detail. The question is not whether the technology works — Philadelphia has proven that it does. The question is whether we have the will to demand it.
Related Reading
The 3RiversProud Clean Water Plan: A Green-First Proposal for Pittsburgh
Nine billion gallons of diluted sewage overflow into Pittsburgh's rivers every year. Our plan charts a different course — one that puts green infrastructure first, invests in neighborhoods, and follows Philadelphia's proven model.
Philadelphia's Green Infrastructure Model: Lessons for Pittsburgh
In 2011, Philadelphia won federal approval for a green-first approach to stormwater management. Eight years in, the data shows it works — and the benefits go far beyond clean water.