The City of Monticello Wastewater Treatment Plant, located on Railroad Street and adjacent to the Sangamon River, was constructed in 1938 with major additions/upgrades made in 1965, 1972 and 1988.  The city owns and maintains over 22.5 miles of sanitary of wastewater collection lines.

The waste flow receives secondary and tertiatry treatment prior to discharge to the Sangamon River and an unnamed tributary to the Sangamon River.  The plant, having dry weather flow and excess flow treatment facilities, is rated for a design average flow of 1.0 million gallons per day, and a design maximum flow of 2.0 million gallons per day.