Storage facility brings city closer to compliance
Last summer, Bangor, Maine, completed construction of a facility that, in four months, reduced the city’s combined sewer overflow (CSO) by 11 million gallons. Designed by the city, the $1.2 million Davis Brook storage tunnel is one of many projects undertaken by Bangor to bring the city into compliance with state and federal environmental regulations.
Thirteen years ago, Bangor came under the scrutiny of regulators because the city’s collection and treatment system was undersized for the area’s wet weather loads. During storm events, flow exceeded the system’s 43-mgd capacity and spilled untreated into the local waterways.
Agreeing to consent decrees with the Maine Department of Environmental Protection and the U.S. Environmental Protection Agency, Bangor developed a long-term plan for CSO control in 1992. Most of the subsequent projects have involved separating storm and sanitary sewers, but, in the Davis Brook drainage area, the city determined that a storage facility could be a more cost-effective method of controlling CSO than sewer separation. As the Davis Brook project got under way last year, the site’s proximity to the Penobscot River produced concerns for the city. Soil conditions, shoring and dewatering issues, and the probability of floatation made construction of a deeply buried cast-in-place tank unfeasible.
As an alternative, the city engineering department designed a 1.2 million gallon storage tunnel constructed of 470 box culvert sections (each measures 8 feet wide by 9 feet high by 5 feet long). The precast concrete sections, manufactured by Bangor-based American Concrete Industries, were made with a V-shaped bottom to maintain flow. They were installed in a 14-foot deep trench on a 12-inch base of crushed stone. The sections were bolted together at the corners and sealed with butyl sealant.
The tunnel extends 2,400 feet and includes access openings at 200-foot intervals. It has sluice gates at each end to accommodate SCADA control, and the city added a sidewall weir that handles overflow during extreme storms.
Today, the Davis Brook storage facility is used continuously as a portion of the Penobscot interceptor sewer, which handles approximately 80 percent of Bangor’s wastewater flow. During wet weather events, the water level in the tunnel rises, then returns to normal as the storm subsides. Wastewater flows through the structure by gravity (eliminating the need for pumps) and is ultimately treated at the Bangor Wastewater Treatment Plant.
Bangor has until 2009 to comply with the consent decrees for CSO abatement. Since 1987, the city has spent more than $26 million on CSO-related projects, and it has budgeted $2.5 million annually for the remaining work. Future projects include sewer separation; construction of monitoring and sampling facilities; and the addition of two more precast storage tunnels.
Bangor has funded most of its CSO improvements with money generated from sewer user fees. (The fees have risen from $1.09 in 1988 to $4.14 today.) However, in 1997, the city received a $6 million federal grant, which it is using to fund 55 percent of its work through 2002. For the remaining 45 percent, the city is issuing bonds annually, to be repaid from revenue generated by the user fees.