Private Property Inflow Control Makes Hydraulic and Financial Sense. A Case Study in Cambridge, MA

The City of Cambridge is pursing sewer separation in the Concord and Huron Ave neighborhoods. Hydraulic modeling of the separated system indicated that the proposed network would not achieve the desired level of protection. This is mostly caused by limited downstream capacity of the regional interceptor that backs up into the municipal sewer system impeding free discharge conditions as well as significant upstream inflow contributions generated from private properties. Since addressing regional, downstream conditions is not cost effective, the hydraulic and financial impact of removing these private inflow sources was evaluated. An extensive building inspection campaign was carried out and, based on the field information, private properties with inflow sources were ranked based on the cost-to-inflow removed ratio. Then, the hydraulic model was run assuming the top 25, 50, 75, or 100% of the private inflow sources had been eliminated. The model indicated that a 75% level of control is necessary to achieve adequate level of protection for the desired 10-year, 24-hour storm. This option would save the City of Cambridge close to five million dollars in the next twenty years due to reduced inflow treatment fees and would reduce Cambridge’s Combined Sewer Overflows (CSO) to the Alewife Brook by approximately 4000 cubic meters per year.