Combined Sewer System The combined flow of wastewater and stormwater can overwhelm the system. Permitted outfalls are located throughout the system to act as relief points during wet weather. These outfalls discharge untreated or partially treated stormwater and wastewater into nearby waterbodies.

combined sewer systems: human and industrial waste mixed in with urban runoff from storm seqwers before flowing into the sewage treatment plant.... combined sewer overflow is when too much water enters the system, the EXCESS flows into nearby streams and waterways without being treated.

During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess water directly to nearby streams, rivers, or other water bodies.

This is a problem because a cocktail of contaminants such as faecal pathogens, personal care products, microplastics and pharmaceuticals in untreated sewage is discharged into our waterways when you get these sewage spills.