East Lansing has completed an approximately $22 million project consisting of an in-line retention tunnel for several combined sewer areas and completion of a sewer separation program in areas that were already partially separated.

The in-line retention tunnel consists of 8,000 feet of 10-foot and 450 feet of 7-foot diameter tunnel built under the main business street in the City.

Diversion structures have been built on the existing combined sewers. These structures allow the dry weather flow to flow to the existing sanitary interceptor and will divert excess combined sewage flows to the tunnel. All CSOs are settled and skimmed in the weir-and-baffle structure, located at the downstream end of the tunnel, before discharging to the river. A 5 MGD pump station is used to dewater approximately 5.42 MG of combined sewage collected in the tunnel from the approximate 1,000-acre service area.

This system is designed to convey the flow from a 10-year, one-hour, rainfall event. Carrying this design flow, the hydraulic grade line in the tunnel collector sewer at the point of intersection of the existing combined sewers will be below the 0.8 point of the interceptor sewer. This avoids impacting the existing sewer system's hydraulic capacity. It is estimated that this phase will completely retain 49 storm events of an average of 61 storm events per year during the warm weather months (April-October, inclusive). The volume of CSO from the remaining storm events will be reduced and the quality improved.

During the construction of the 10-foot diameter tunnel, unforeseen petroleum related contamination in the soil and groundwater was encountered. As a result, an analysis of the soil and groundwater was conducted to determine proper treatment of the groundwater using activated carbon and proper disposal of the soil in a Type II landfill.