Large Diameter Sewer Cleaning in City of Baltimore's Outfall Sewershed: It's Not As Simple As It Appears

The Outfall Sewershed (OS) is the most downstream sewershed in the City of Baltimore (City) that is tributary to the Back River Wastewater Treatment Plant. As such, all of the City’s sewersheds in the Back River WWTP service area are tributary to the OS. In all, the OS conveys wastewater from five other sewersheds within the City. The main sewer system components in the OS are the three large diameter trunk sewers: the 99-inch sewer, the Outfall Interceptor (arch sewer 129” x 144” and 132” x 147”), and the Outfall Relief Sewer (96” and 114”). As a key component of the sewershed study, Sonar imaging techniques were adopted to determine the pipe conditions and sediment and debris accumulations in the trunk sewers. It was determined the accumulation of sediment and debris in the large diameter trunk sewers were significant. The depth of sediment typically ranged from 30-inches in the Outfall Interceptor to 45- to 50-inches in the 99-inch sewer and the Outfall Relief sewer. These excessive sediment and debris accumulations occurring since the inception of these sewers were found to significantly impact the conveyance capacity of the sewers. The capacities of the large trunk sewers are not sufficient to convey the peak flows. Various alternatives developed in the study to eliminate SSOs in the OS showed that sediment removal from the large trunk sewers increased the conveyance capacity by restoring the full cross section area and reducing the hydraulic roughness of pipes. It was determined that the major hydraulic improvement to be implemented in the OS for the 2 year return period event is heavy sediment cleaning in the large diameter trunk sewers. This paper will highlight some of the challenges faced and lessons learned in attempting to clean a large diameter trunk sewer that is over 100 years old and one that has never been cleaned since its inception.