Once heavily favored because they transported sanitary sewage and stormwater runoff in a single pipeline, combined sewer systems became the bane of wastewater managers across the nation upon enactment of the Clean Water Act of 1972. More than 1000 cities are expected to spend $100 billion on sewer reconstruction to separate their sanitary sewage and stormwater flows.
While this has meant huge expenditures for most affected cities, Avon Lake Municipal Utilities in Ohio found a way to save $1 million on one portion of its $20 million sewer-separation project, which the EPA approved in 2004 and is expected to be fully completed in 2020. The utility's creative solution can be applied to any combined sewer system that has pipes large enough for a man or woman to work comfortably in.
Plan Eliminates Open-Cut Trenching
Avon Lakes is a relatively small (about 27,000 residents) western suburb of Cleveland. Established in the 1920s on the shores of Lake Erie, the city's sewer system had been discharging into the freshwater lake—a source of drinking water from Avon lake as well as 185,000 people in a 600-square-mile area of northern Ohio—since then.
Reacting quickly to the federal mandate to eliminate combined sewer overflows (CSOs), in the mid-70s Avon Lake Municipal Utilities developed a master plan that divided reconstruction on 21.4 miles of combined sewer into 15 separate projects spanning three decades.
One of these projects involved an 84-inch-diameter combined sewer that periodically overflowed into a park and beach area. After considering such options as building a retention basin or other storage facility for later treatment, the utility decided to build separate stormwater and sanitary sewage lines, with the latter routed directly to the treatment plant.
There was a major drawback to the plan, however: This particular stretch of combined sewer ran along the city's main street, a heavily traveled state route that provides access to the city's core. Conventional open-cut trenching requires very deep excavations to connect laterals to a new sanitary sewer line, an expensive proposition that would greatly disrupt traffic flow.
But then studies revealed that the line rarely flowed full because of a reduction in industrial loads. That's when the idea of placing the new sanitary sewer line inside the existing pipeline surfaced.
Plastic Complements Concrete
Partnering with the Ohio office of PB (formerly Parsons Brinckerhoff), Avon Lake Municipal Utilities completed a design based on installing a 16-inch PVC sanitary sewer pipeline inside 8800 linear feet of the existing 84-inch concrete combined sewer pipe.
Upon receiving the necessary EPA approvals, installation began in 2003 and was completed the following year. A long-term user of PVC pipe, the utility chose plastic over concrete or steel for its:
- Longitudinal rigidity (beam strength) to minimize sag between supports
- Light weight for easy lifting and maneuvering inside the host pipe and easy field cutting and modification when needed
- Leak-free gasket joints
- Ability to adjust to thermal movements within the joints
- Chemical inertness
- Resistance to the aggressive gases that pervade sanitary sewage lines
- Hydraulic efficiency, which permits installation on shallow grades.
During construction, residential connections were bypassed around the work area. New sewer lateral connections from the concrete pipe wall to the new PVC sanitary sewer line were completed as the new line was installed.
Since completion, the installation has been inspected annually. It's leak-free, has no sediment buildup, and is functioning as designed. When Avon Lake Municipal Utilities finishes its conversion, all of the city's sanitary sewage will be treated before being discharged into Lake Erie, widely regarded as Ohio's most important waterway.
— Beck is a freelance writer based in Middletown, Ohio.
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WEB EXCLUSIVE: Pipe-within-a-Pipe: To see a schematic of how existing laterals in the concrete pipe wall were connected to a new sanitary sewer pipe, click here
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