Combined Sewer Overflows (CSO)
What are combined sewer overflows?
Combined sewer overflows (CSOs) are the result of historic infrastructure practices in which stormwater and wastewater were directed to the same sewer system. In the event of large rainfall or snowmelt events, these sewers may overflow, releasing a mixture of rainwater and untreated sewage directly into surface waters. In Ottawa, our combined sewers overflow into the Fox and Illinois Rivers.