When the rain began to fall in southeast New Hampshire, the concern of Nashua city officials would grow as the threat of combined sewer overflows (CSO) increased with every raindrop.
Sewer overflows in the area had become so common, Nashua, N.H., city officials turned to the engineering firm of Metcalf & Eddy to help solve the CSO problems that plagued their city in recent years.
After monitoring the city's sewer system before, during, and after rainstorms, Metcalf & Eddy proposed a CSO plan that would not only make better use of the city's existing infrastructure, but also help manage stormwater at the wastewater treatment facility in a controlled fashion, to reduce the risk of CSOs. As a result, Metcalf & Eddy designed a 16 million gallon per day (mgd) CSO treatment facility, which currently is being constructed. This facility incorporates a series of concrete structures: a sedimentation facility, a pump station, and an extension to the existing area that houses the chlorine tank used to treat the plant effluent.
When completed in 2009, the Nashua Wastewater Treatment Plant will immediately increase its overall treatment capacity to 110 mgd, and the threat of CSOs in the area will be resolved.
What causes a CSO?
The premise behind a combined sewer system is to handle both domestic sewage and stormwater in the same piping system.
A CSO occurs when stormwater fills the combined sewer system so quickly that the infrastructure cannot handle the volume. Because the combined sewer system is unable to manage the excess flow, the untreated stormwater often ends up flowing directly into local streams and rivers. Included in this overflow is not only stormwater, but also sewage.
The CSO retrofit project constructed at the Nashua Wastewater Treatment Plant is a necessary measure that has become fairly common for municipal wastewater plants around the country. Many of the nation's original combined sewer systems are still in place and simply inadequate when it comes to handling excessive stormwater.
For the concrete structures built as part of the city of Nashua's CSO plan, a 4000-psi mix with 1½-inch aggregate was chosen for the larger sections of the project, including the base slabs and walls. The large aggregate was selected in order to reduce the shrinkage effects of the concrete paste. Smaller aggregate sizes were used for thinner members or where rebar or piping congestion was an issue. The mix parameters were predetermined by Metcalf & Eddy, including water-cement ratio, maximum slumps, and cement type.
According to Gregory Galbraith, project manager for general contractor Methuen Construction Co. Inc., Salem, N.H., “The water-cement ratio is specified as 0.44, which required the use of a water-reducing admixture for the 4000-psi concrete.”
For the construction of the concrete facilities, Aggregate Industries, Saugus, Mass., is providing just over 10,000 cubic yards of concrete and Baker Steel is fabricating approximately 1300 tons of reinforcing steel.
Specifically, the sedimentation tank will contain 4300 cubic yards; the pump station 4800 cubic yards; and the chlorine contact tank 1300 cubic yards of concrete. The cast-in-place concrete work began earlier this year with an expected completion date in early 2008.
As for the construction that currently is underway, the Northeast Concrete Pumping Corp., Portland, Maine, is placing the concrete using a Putzmeister concrete pump. Approximately 20% of the project has been completed.
Every load of concrete provided by Aggregates Industries is being tested thoroughly onsite. “Aggregate Industries has been very consistent with the concrete they have delivered to the jobsite,” says F. Douglas Newman, president of Newman Concrete Services, Richmond, Maine, the contractor responsible for performing all of the formwork for the project.
Testing is done on the concrete prior to placement in order to determine slump and air content, and cylinder samples are taken on a regular basis.
“Every truck is tested by an independent agency prior to placement,” says Rick Seymour, director of public works for the city of Nashua, N.H.