Implementation of Automated Sewer and Drainage Flushing Systems in Cambridge, Mass.

This paper summarizes the design of passive automatic flushing systems and grit control structures within the north east portion of the City of Cambridge tributary to the Alewife Brook. This work is under construction as part of a $60 M sewer separation program sponsored by the Massachusetts Water Resources Authority (MWRA) and the City of Cambridge. Grit deposition and debris accumulations are severe in the existing system of combined sewers, storm drains, and sanitary trunk sewers due to the flatness of the area. This condition is exacerbated by the outlet hydraulic constraints imposed by the Alewife Brook (shallow stream) and downstream constraining sanitary siphons (again because of the Alewife Brook). Complete separation of the area is envisioned, ultimately to be served by separate sanitary sewers and storm drains. The use of pumping systems to lift flows from sewers and drains to permit self-scouring velocities is prohibitively expensive. To overcome this problem automated flushing systems using quick opening (hydraulic operated) gates discharging collected stormwater are being constructed together with downstream collector grit pits at four locations covering a distance of 1220 meters for storm pipes ranging in diameter from 1.4 meters to 1.2 meter x 2.7 meter box culvert (storm trunk drain). New 450 millimeter and 600 millimeter sanitary trunk sewers 1000 meter in length will be flushed daily using spent filtrate water from the City of Cambridge's new water treatment plant (CWTP) currently being constructed in the same area. Construction of all work is underway and will be completed by the end of 2000.