THE EVOLUTION OF THE MILWAUKEE TUNNEL SYSTEM: A HISTORICAL PERSPECTIVE

The Milwaukee Metropolitan Sewerage District's (MMSD or the District) Inline Storage System, also called the deep tunnel system, began operating in 1994. At a cost of 1 billion, this tunnel system was a key element of a 2.3 billion water pollution abatement effort which included upgrades and improvements to the District's two treatment plants, rehabilitation and expansion of the near surface interceptor sewer system and the construction of the deep tunnel system. Although initially contemplated in the early 1970's as a single purpose tunnel which would abate pollution from combined sewer overflows, the District's deep tunnel system evolved over the decade-long planning effort into one which receives and holds overflows from both the District's combined and separated sewer areas. Construction of the system occurred during the 1980's, with start-up in the early 1990's. The District is now in its tenth year of tunnel system operation. The system continues to evolve today, as operational experience is gained with each new large storm; as increasing demands are placed on the system as the result of ongoing land development and changing weather patterns; as additional storage facilities tributary to it are brought on line; and as treatment plant capacity improvements are planned.