Insystem Storage Capacity Enhances Sewer System Operation

Bangor, Maine has been addressing control of Combined Sewer Overflows since 1987. In 1992, a Combined Sewer Overflow Facilities Plan was prepared, which included a variety of recommendations for CSO Control. For three locations, insystem storage with subsequent treatment was the recommended method of controlling combined sewer overflows.Although the specific intent of these projects was reduction of CSO discharges, operators have discovered that the additional collection system capacity has also provided significant benefits regarding management, operations, and maintenance. The experiences of the system operators should be of interest to all communities looking at options of complying with the proposed cMOM requirements.In 1998, Bangor conceived, designed, and constructed the 1.2 million-gallon Davis Brook Storage Facility, a unique and innovative in-line tunnel-like structure constructed of V-bottom pre-cast concrete box sections. The project was a radical departure from the conventional storage tank approach and was extremely successful from a construction and operational perspective, and very cost-effective with a total cost of only 1.3 million.In 2000, Bangor constructed the City's second storage project, the 1.2 million-gallon Kenduskeag East Storage Facility. Bangor again utilized pre-cast concrete box sections to construct this offline structure underneath an existing public parking lot. Constructed at a total cost of 2.4 million, this project has contributed to a significant reduction of CSO discharges in the downtown area of Bangor.In 2002, Bangor constructed the City's third storage project, the 1.4 million-gallon Barkersville Storage Facility. This 2 million project is another in-line tunnel-like facility constructed of Vbottom precast box sections, controlled by a SCADA-operated gate system.Bangor now has 3.8 million gallons of extra capacity within its sewer system, provided at a total cost of only 5.7 million, compared to projected costs of 18.8 million for constructed-in-place facilities. There has been considerable interest to date regarding Bangor's approach to CSO storage, especially the use of precast V-bottom box sections, and the significant time and cost savings of this method of insystem storage.Now that the operators have gained some experience with these new facilities, they have found several benefits beyond significant reduction of CSO discharges. Some of these benefits are:Real-time flow management within the system,Leveling peak flows into the treatment plant,Avoiding operation of the plant's CSO by-pass function,Stopping flows to undertake downstream maintenance,Computer graphic displays of system flow levels,Interceptor flushing, andControl of peak electrical demand at the Treatment Plant.