combined sewer overflow

Combined sewer overflow (CSO) occurs when a sewer system designed to carry both domestic sewage and stormwater becomes overwhelmed during wet weather. These combined systems, common in older cities, use large pipes to manage the high volume of rainwater.12

Because wastewater treatment plants cannot process such large volumes, the excess mixture of sewage and stormwater bypasses treatment and is discharged directly into rivers, lakes, or oceans. This untreated discharge causes significant water pollution. To mitigate this, some cities store the initial, more polluted flow in basins or tunnels for later treatment.12