combined sewer

Combined sewers are a type of wastewater collection system that carries both domestic sewage and storm runoff in the same pipes. These systems, common in older cities, use large-diameter pipes to accommodate the significant volume of rainwater during storms.12

However, combined sewers are no longer built because wastewater treatment plants cannot handle the large volumes of stormwater. During wet weather, untreated sewage bypasses treatment plants and is discharged directly into waterways, causing pollution. To mitigate this, some cities store the initial flow of combined sewage for treatment, or use devices like swirl concentrators to manage the volume. Modern systems use separate pipes for sanitary sewage and storm runoff.12