Why do sewers overflow?

SAWS is working to reduce all sewer overflows. SAWS has undertaken a comprehensive Capital Improvement Program to address all causes of sanitary sewer overflows.

Heavy Rain

During heavy storms, rainwater can enter the sanitary sewer system through cracks in pipes and through spaces between manhole covers and frames. When too much rainwater enters the system capacity is exceeded and sewer can overflow.​


When objects are intentionally introduced into the sanitary sewer system or it is damaged, flows are impeded and overflows can occur.​

Sewer system defects

Sewer pipes, manholes, and connections are subject to breaks or deterioration caused by a multitude of factors including age, shifting ground and pipe defects. When the system is broken, overflows can occur.​

Grease, wipes, debris blockages​

Cooking oil, grease, and wipes are wastes that the city’s sewer system is not designed to handle. These are a major cause of SSOs and should not be discarded down the drain.

Power failures at lift stations​

Much of the sewer system uses gravity to move sewage to SAWS wastewater treatment plants. Where that is not possible, lift stations pump sewage through the lines. When power fails and a lift station is not operational, overflows can occur.​