The City operates a wastewater plant at 300 Simpson Street that is staffed 24 hours per day by State of Illinois licensed operators. The plant's design capacity is 4.41 million gallons per day, and an average daily demand is 4.45 million gallons per day. Originally constructed in 1972, the plant was extensively remodeled in 1989, and a tertiary filter was added in 2000. It remains a state-of-the-art facility, ensuring compliance with the Environmental Protection Agency (EPA) regulations and protecting the public health and environment. Through intergovernmental agreements, the wastewater from the Village of Maryville and Mounds Public Water District is also treated at our plant. The City is currently undergoing a treatment plant expansion to increase the plant's capacity to 5.85 million gallons per day to ensure safe treatment for the future years. The City's treatment plant was the recipient of the “Illinois Wastewater Treatment Plant of The Year” in 1997. 

The City provides sewer service to its residents through a gravity flow system. The City is responsible for maintenance of the mains and lift stations; the customer is responsible for the service lines from the residence to the main. Collinsville's terrain requires the City to own and operate eight lift stations that pump the sewage to a higher level in order to continue the gravity flow onward toward the treatment plant.