A Tale of Two Tunnels

In late 2005, the Milwaukee Metropolitan Sewerage District (MMSD) began operation of the Northwest Side Relief Sewer. This is a 20 foot diameter bored tunnel, 38,000 feet long, and was designed to provide hydraulic grade line relief to upstream sewers and to provide storage for excess sanitary sewage flows to benefit the entire service area.MMSD has been operating a 405 million gallon capacity deep tunnel system (called the Inline Storage System, or ISS) for more than 12 years. This network of 19 miles of underground tunnel storageprovides relief for both the excess combined sewage and excess sanitary sewage that enter the collection system during rain events.These two facilities operate together during large rain events to protect regional water resources. The strategies of operation for both facilities must be designed to meet the objectives of improved water quality and the reduction of flooding occurrences within the district's service area.