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During a heavy rain the pipes may get too full and start to overflow into the Missouri River. When this happens, it's called a Combined Sewer Overflow (CSO). This provides a "safety valve" that prevents back-ups of untreated wastewater into homes and businesses, flooding in city streets, or bursting underground pipes.
The Combined Sewer System was built as St. Joseph grew during the early 1970's, as an economical way to handle wastewater and stormwater. One advantage of this system is that most of the time, when rainfall is low to moderate, both the stormwater and wastewater go to the treatment plant.
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. In fact, there are far fewer overflows now than in the past due to construction projects to control the overflows.
One way to eliminate CSOs would be to add a separate system of pipes to drain stormwater directly to the rivers. Again, this would be very expensive and would also release untreated stormwater in all storms, which would add chemicals from surface runoff to the water bodies.