The Detroit Water & Sewerage Department (DWSD) has opened a $26 million, three-mile-long interceptor sewer tunnel in Macomb County, Mich. The seven-foot-wide tunnel, which is about 40 feet below the ground, helps transfer wastewater from communities in the county to the Detroit Wastewater Treatment Plant. The tunnel replaced infrastructure that included a two-foot-wide force main sewer line located 5 feet below the ground and a pump station.

As the Macomb County population grew during the 1990s, the previous sewer line and pump station were on a pace to be overburdened soon, says Awni Qaqish, assistant director of engineering services for Detroit. Therefore, the city and Macomb County decided to replace the infrastructure, which was built in the early 1970s.

DWSD contracted with Rochester Hills, Mich.-based Spalding DeDecker Associates to design the new tunnel and Livonia, Mich.-based Jay Dee Contractors to build it. Construction of the tunnel began in July 2000, and the contractors quickly encountered some challenges. First, boulders were uncovered in the soil and had to be blasted to gravel for removal. The soil in the area also featured natural gas (methane) deposits, which had to be vented to avoid the risk of fire or explosion. Traffic presented another issue, as construction took place underneath and next to area roads. The neighborhood traffic consists of about 50,000 vehicles per day, which limited construction during rush hours.

The tunnel relies on gravity instead of a pump to maintain the flow of wastewater; the tunnel was constructed with a slight downward angle to keep sewage moving. Because the tunnel does not feature a pump station, it should be less expensive to operate than the previous infrastructure, Qaqish says.

The elimination of a pump station also means the new tunnel will not be as susceptible to the whims of Mother Nature. “Electrical power feeds the pumps, and sometimes you have no control over an electricity outage,” Qaqish says. “If a storm hits the transformers, and that does happen, it will put the [pump] out of service.”

The project included the construction of two metering facilities, one for Shelby Township and another for Macomb and Washington townships, that replaced meters that were part of the pump station/force main infrastructure. (DWSD charges Macomb County for the wastewater that it treats from the communities in the county. The department uses the meters to measure flow so that it can compute the county's bill.)

Construction of the tunnel was completed in November. DWSD financed the project by issuing bonds, which will be paid off through the bills that the department collects from Macomb County.