Automatic Control of CSO's Using a Slide Gate

The City of Nashua is under a Consent Decree from the U.S. Environmental Protection Agency (EPA) to reduce Combined Sewer Overflows (CSOs) and has an ongoing program to meet the requirements of the Consent Decree. One of these program components was construction of an automated 108 inch wide by 96 inch high slide gate to throttle flow and induce in-system storage upstream of CSO 006. The gate was located on the face of an influent combined sewer pipe in an existing CSO regulator structure. A stainless steel slide gate was ultimately chosen as the recommended type of gate to serve the project goals. In addition, a new slide gate Hydraulic Power Unit (HPU) and a Nitrogen-charged Accumulator System were incorporated into the design to automatically open and close the gate based on upstream water levels. The water levels are measured by redundant depth sensors that can measure surcharge depths above the pipe crown.