Monitoring CSO Frequency While Saving Money and Reducing Regulatory Risk

A small Upstate New York city was spending more than 2,500 per month to monitor combined sewer overflow (CSO) frequency. In general the city's procedure was to observe the presence or absence of debris on the CSO weir after each significant rainfall event. The presence of debris indicated CSO activation. During the month, the city would visit each CSO location, record observations and then report findings to the New York State Department of Environmental Conservation (NYS DEC). The city's discharge permit required the city to report annual CSO frequency to the NYS DEC.The NYS DEC questioned the city's approach to monitoring CSO frequency because the absence of debris on the CSO weir was not an absolute indication that a CSO event did not occur. The NYS DEC requested that the city develop a new approach to monitoring CSO frequency that provided a more robust indication of a positive or negative CSO occurrence.Stearns & Wheler developed a simple motion detector system that will continuously monitor and record motion. When this system is installed at the proper elevation in a CSO diversion structure, it can monitor and record when an overflow is occurring. As the combined sewer overflow occurs, the flowing water sets the motion detector into motion. This approach was accepted by the NYS DEC and the motion detector systems were installed on each of the twenty CSO diversion structures.Per acceptance by the NYS DEC, the installation of the motion detectors eliminated the need to inspect the CSOs after every significant rainfall event. The city now downloads data once per month instead of performing inspections after every significant rainfall event. The crews were routinely performing six or seven inspections per month. This simple approach to monitoring CSOs costs the city less money and provides a more conclusive indication of CSO frequency, which is acceptable to the NYS DEC. The cost of the equipment and software was approximately 200 (2008) for each of the twenty CSO diversion structures.