What causes combined sewer overflows? Is it the capacity of the treatment plant or the interceptor pipes?
Depending on where the rain falls within the City, either treatment plant capacity, interceptor capacity, or the CSO regulating structure could be the limiting factor. If Philadelphia receives evenly spread, low-intensity rainfall, the treatment plants may be at their capacity without any interceptors at capacity. However, in the scenario that an inch of rain falls mostly within several sewersheds, the interceptor may be at capacity but not the treatment plant. Additionally, most regulating structures are designed with a static dam, meaning if the water level rises above the dam height, it overflows. Although the dam heights are designed to overflow only when the interceptor has reached its capacity, you could theoretically overflow before that time. Philadelphia has several more advanced regulators with computer controlled gates that can better regulate the contribution each sewershed makes to its respective interceptor. For regulators that are tidally influenced, there can be no over