Combined Sewer Overflows

Combined Sewer Overflows (CSOs), which are also called bypasses can occur in a combined sewer system. A combined sewer system consists of one sewer that collects and transfers both sanitary wastewater and storm water to the sewage treatment plant for treatment. These systems were generally installed in the turn of the 20th century (late 1800s to mid 1900s) and were the best technology of the time.

Around the 1950s, new technologies and philosophies emerged and more stringent regulations were implemented. The most significant change was to have separate sewer systems for sanitary wastewater and storm water in new developments. The sanitary wastewater was collected and sent to the sewage plants and the storm water was collected and sent to a nearby lake or river.

CSOs are the discharge of untreated storm water and sanitary wastewater runoff to nearby bodies of water. These bypass events occur during heavy rainfall and snow melt, or because of equipment failure. The CSOs are used to alleviate the overloading of the combined sewer system to prevent sewer backups in residential and commercial buildings and local streets.

One other contributing factor to overloading the combined sewer system is the loss of sewer capacity due to extraneous flow. Extraneous flow is basically any flow that is not household sanitary waste or storm water in a combined area. As shown in the illustration above, examples are ground water leaking into the sewer pipe from cracks, discharge from basement sump pumps and discharges from roof drains connected directly to the sanitary sewer.