CSOs (Combined Sewer Overflows)

Combined Sewer Overflows (CSOs) occur when heavy rainfall or snowmelt causes a combined sewer system—designed to carry both wastewater and stormwater—to exceed its capacity. When this happens, excess untreated sewage and stormwater are discharged directly into nearby water bodies, such as rivers or lakes. CSOs are a way to prevent sewer backups in urban areas, but they can introduce pollutants, including human waste, chemicals, and debris, into water ecosystems. Managing CSOs involves infrastructure improvements and stormwater management practices to reduce overflows and protect water quality.