By Steve Cooper
PROJECT:8 Combined sewer separation
OWNER:8City of Portland, Maine
ENGINEERING AND DESIGN:8Woodard & Curran, Portland
LOGISTICS AND SYSTEM DESIGN SUPPORT:8E.J. Prescott Inc.; Gardiner, Maine
CONSTRUCTION MONITORING:8Gorrill-Palmer Consulting Engineers Inc.; Gray, Maine
CONTRACTOR:8R.J. Grondin & Sons Inc.; Gorham, Maine
INSTALLATION: June through October 2010
COST: $4 million
In June, sewer managers for Maine's largest city received a regional American Public Works Association award for a project that involved one of the nation's first and largest installations of polypropylene pipe for combined sewer overflow remediation.
In 2007, when Portland began exploring how to eliminate overflows and keep untreated wastewater from flowing into an estuary, such a product wasn't commercially available. But the next year, a pipe that provided a watertight joint and met the city's required stiffness standard of 46 pii was introduced, and added to the list of options — which included epoxy-coated, reinforced concrete and centrifugally-cast glass fiber reinforced polymer — contractors could choose from to meet an extremely tight installation deadline.
Developed specifically for the gravity-flow sanitary sewer market, SaniTite HP (for “high-performance”) pipe is made from an enhanced grade of polypropylene resin: an engineered, impact-modified, co-polymer compound that's chemically resistant to the hydrogen sulfide gas and sulfuric acid concentrations typical of sanitary sewers. Developer and manufacturer Advanced Drainage Systems Inc. is the world's largest producer of corrugated high-density polyethylene (HDPE) pipe.
SaniTite HP 30- to 60-inch-diameters meet ASTM F2764.
The 60-inch pipe has a triple-wall construction that provides a smooth interior and exterior wall design, supported by a corrugated structural core for improved stiffness and greater beam strength to minimize deflection and enhance long-term performance. It's available in 16.3- and 20-foot lengths.
The 30-inch pipe is available with either dual- or triple-wall construction and, like the 48-inch pipe, comes in 13- and 20-foot lengths to accommodate various trench box dimensions.
Pipe 12 to 30 inches in diameter meets ASTM F2736 and is made with dual-wall construction to provide performance ratings that exceed many industry standards for gravity-flow sanitary sewers.
With dual gaskets and banded reinforced bell and spigots, all are watertight, exceeding the requirements of ASTM D3212.
Consulting firm Woodard & Curran and city engineers had spent several years exploring design alternatives for replacing hundred-year-old 24- and 30-inch vitrified clay pipelines with a single, larger-diameter pipe and separating a parallel, 10-foot-diameter pipe to convey only stormwater.
They wanted a system designed around data from actual storms, not a predicted 25- or 100-year-event model. Monitoring equipment installed in the existing line collected flow data from September 2007 to September 2008, supplementing data the city had collected from monitoring sites within the project area in 2004 and 2005. The largest storm occurred Sept. 6-7, 2008, when 5.46 inches of rain fell with a maximum intensity of 0.94 inches in one hour.
“Our analysis determined the main line would require a 60-inch pipe to handle the peak flow rate anticipated for the system of 70 million gallons a day along its 1-mile length,” says Woodard & Curran Project Manager Dave Senus, PE. “The ADS pipe with its Manning's ‘n' value of 0.012 met this requirement.”
So it was added to the city's request for proposals.