Combined Sewer Overflow (CSO)

In several areas throughout the Wyoming Valley, mixed stormwater and wasterwater flow together in a single pipe—referred to as a Combined Sewer System. In heavy rains, however, the pipes may become too full, causing overflows into the Susquehanna River or local creeks. A Combined Sewer Overflow (CSO) is used to stem backups of untreated wastewater flowing into homes and businesses, street flooding, and underground pipes bursting. 

When the combined sewer system was designed more than 100 years ago, it was deemed to be more cost effective, as compared to building two entirely separate systems of pipes and tunnels to carry stormwater and wastewater. Before the sewage treatment plants were built, all sewage and stormwater were discharged into the nearest body of water.

The CSO locations were left in place when the present sewer system was updated to act as safety valves when the pipes get too full to handle the high volume of water during heavy rains. The advantage of a combined sewer system is that both stormwater and wastewater are treated most of the time. The disadvantage is that during heavy rains, untreated stormwater and wastewater may be discharged at CSO locations. However, there are far fewer overflows now than in the past due to construction projects to control the overflows.

Learn more about CSO at 3 Rivers Wet Weather Regulations.