McHenry Sanitary Sewer System Improvements

HR Green evaluated the entire wastewater collection system for the City of McHenry, Illinois. With two separate wastewater treatment plants, the sanitary sewer system consisted of two main catchments and 21 sub-catchments, with a total of 3,000 manholes, 606,000 feet of gravity sewers, 31,000 feet of force mains, and 19 pump stations.

HR Green employed a subcontractor to provide flow monitoring services at 21 manhole locations for a six-month duration to establish dry weather and wet weather flow conditions. Two separate sewer models, one for each main catchment, were constructed from available GIS data. The Haesteads Methods modeling software SewerGEMS was used to create the sewer system models. Once the gravity sewers, manholes, pump stations, and force mains were input to the model, calibration runs were completed using the flow monitoring data. Both static and extended period simulations were completed for IEPA design loading conditions, dry weather measured flows and wet weather measured flows. The measured storm events were used to predict wet weather flows for several larger storm events including a 100-year storm.

The sanitary sewer system design flows resulting from these modeling efforts were then used to size a pump station and force main to eventually replace one of the City’s existing wastewater treatment plants.