Combined sewers are pipes that catch both sewage and stormwater and route it to a waste water treatment plant. In dry weather, it’s all sewage in the pipes. In small rain storms, the pipes carry sewage mixed with stormwater and it all goes to the wastewater treatment plant to get cleaned up and returned to a stream or lake. The origins of combined sewers predate waste water treatment, when there was little distinction between stormwater and sewage and stream and city dwellers just wanted the foul-smelling, disease-festering stuff out of their way as soon as possible. Later, engineers and public health folks added the crucial waste water treatment plant step to the system but the sewers remained combined. Combined sewers were common until the early 20th century, so over 772 communities in the US, mostly in the Northeast and Great Lakes regions have combined sewers, as shown on this map from the US EPA:
US EPA map of Combined Sewers. Click for source.
Most of the time, combined sewers route all of the water to the waste water treatment plant, and all is relatively well. But in large storms, the volume of stormwater and sewage can overwhelm the waste water treatment capacity. If the volume of water was too much to treat, you can imagine the pipes starting to fill up with sewage. If there were no “pressure release valve” on the system, urban dwellers in combined sewer cities would see the sewage/stormwater cocktail start to back up into their basements, sinks, … and, you get the picture. Fortunately for those city residents, there is a “pressure release valve in the system,” but it’s a solution that creates more problems downstream, literally. When flows in the combined sewers are too great to be treated, the sewage/stormwater cocktail overflows out of the pipe network and into local streams. Then you’ve got raw sewage in your stream and that’s not pretty, or healthy, or environmentally friendly. This is the infamous combined sewer overflow or “CSO.”
US EPA diagram of a combined sewer in dry and wet weather. From U.S. Environmental Protection Agency, Washington, D.C. “Report to Congress: Impacts and Control of CSOs and SSOs.” Document No. EPA 833-R-04-001 found on Wikimedia commons. Click for source.
Here’s a Northeast Ohio Regional Sewer District video explaining combined sewers and touting their treatment system:
Under the Clean Water Act, cities and sewer districts can be required to bring their raw sewage discharges down to acceptable levels by reducing the frequency and magnitude of combined sewer overflows (CSOs). Right now, Cleveland, the District of Columbia, Philadelphia, and other cities are under mandate to reduce their CSO discharges. This is a big, expensive undertaking because we’re talking about billions of gallons of overflows each year and thousands of miles of combined pipe network underneath the city. Big problems require big solutions, so how are the cities dealing with their CSO problem? It turns out that they are taking a range of different approaches.
In Cleveland, waste water treatment and stormwater are managed by the Northeast Ohio Regional Sewer District (NEORSD). Their “consent decree” with the EPA was filed in July 2011, and according to that decree, they have 25 years to reduce CSO volumes by 90%. That’s taking the CSOs from 4.5 billion gallons per year to the still non-trivial 494 million gallons per year. If they meet that goal, 98% of all wet weather flows will be treated before being released to a stream. The price tag for this ambitious project is $3 billion, and it has been termed “Project Clean Lake” in homage to Cleveland’s Lake Erie shoreline. a source of regional pride.
How is NEORSD planning to reduce CSOs? With a lot of digging. Most of the money and effort is being spent on “gray infrastructure” – big engineering projects. BIG engineering projects. NEORSD is boring 7 tunnels, each 2-5 miles long, up to 24 feet in diameter, and up to 300 feet below the ground or lake bottom. These tunnels will intercept the combine sewers before they overflow and store the water until the treatment plants have capacity to treat it.
The 7 future storage tunnels of Cleveland’s combined sewers. Image courtesy NEORSD. Click for larger.
This is a massive undertaking, and it’s just getting started. The videos below show the first tunnel boring machine arriving in Cleveland and a tour of the tunnel first tunnel to begin construction. You can follow the progress of the tunnel boring on the NEORSD blog.
But it’s not just tunnels, NEORSD is also enhancing their wastewater treatment capacity and spending $42 million on green infrastructure. Green infrastructure is defined as “a range of stormwater control measures that use plant/soil systems, permeable pavement, or stormwater harvest and reuse, to store, infiltrate, or evapotranspirate stormwater.” These can include things like green roofs, green streets, bioretention swales, and other projects. The goal is control 44 million gallons of would-be stormwater using green infrastructure, with projects completed in the next 8 years. Those numbers are nothing to sneer at it, but it’s 1% of the current combined sewer overflow volume and 1.5% of the budget. The fact that the budget % is bigger than the volume percent may hint at why green infrastructure isn’t being used more broadly in Cleveland.
Washington DC is taking a somewhat different approach than Cleveland. One-third of DC is served by combined sewers, and they are spending $2.6 billion over 25 years to reduce their overflow problem, which is currently about 2.5 billion gallons per year. DC Water has nicknamed their CSO program the “Clean Rivers Project.” Like Cleveland, they are also building large storage tunnels, improving their waste water treatment plants, and rehabilitating pumping stations. Unlike Cleveland, DC will actually be separating the sewers in some areas, sending sewage and stormwater down different pipes from each other. In DC, green infrastructure seems to get only a rhetorical nod, rather than a significant component of the budget. Their plan says they will “advocate implementation of Low Impact Development,” but they’ve only budgeted $3 million for it, a mere 0.1% of their overall project cost. However, they do have the world’s best explainer video.
Philadelphia is taking a radically different approach. Like Cleveland and DC, their price tag comes out to about $3 billion over 25 years. However, in Philadelphia it’s a “Green City, Clean Waters” program and green infrastructure steals the show. Philadelphia’s goal is to “reduce reliance on construction of additional underground infrastructure” by pushing extensive green infrastructure throughout the city. In other words, they don’t want to dig tunnels. Instead, they want to green acres:
Each Greened Acre represents an acre of impervious cover within the combined sewer service area that has at least the first inch of runoff managed by stormwater infrastructure. This includes the area of the stormwater management feature itself and the area that drains to it. One acre receives one million gallons of rainfall each year. Today, if the land is impervious, it all runs off into the sewer and becomes polluted. A Greened Acre will stop 80–90% of this pollution from occurring.
Philadelphia’s rationale for making green infrastructure their big push centers around social and economic benefits to come and their historic heritage as a park city. Their video is all about people, not all about pipes:
Philadelphia’s vision is the most radical departure from a traditional “grey infrastructure” approach like that pursued in Cleveland, DC and other cities. There’s certainly an aesthetic and emotional appeal behind greening a city and its stormwater. This is the way many people want to move urban hydrology in the 21st century, integrating the built and natural environment more closely than we’ve done in the past. But it will be interesting to watch where Philadelphia succeeds and if and where it fails, as the fully green infrastructure approach could be seen as much riskier than a traditional engineering-driven approach. Fortunately, EPA is devoting some funding to research on the effectiveness of Philadelphia’s project. I won’t be doing that work directly, but I will be following it closely and think it would be fascinating to put together a more rigorous multi-city analysis of approaches and outcomes.
More broadly, the combined sewer overflow problem is a fantastic example of how our environmental and societal choices are constrained by decisions made in the past. No one today would build a combined sewer, but yet millions of people live in cities served by them, thousands of engineers, scientists, and sewer district workers work with them, and billions of dollars are being spent trying to mitigate the problems they cause. We can’t just rebuild cities from the underground up, so we have to work with what we’ve inherited and try to make decisions that won’t cause consternation for future generations.
Note: This blog post is adapted from the lecture I gave today in Urban Hydrology. If I’ve gotten anything wrong or missed an important point, please let me know and I’ll try to make it better for current and future students.
