Milwaukee completes tunnel extension work

Milwaukee Metropolitan Sewerage District (MMSD) has completed construction work on a 3.2km extension of the deep tunnels adding 123M litres of wastewater storage capacity to the underground system.

The cavern, which is 6.4m in diameter and 91m beneath North 27th Street, is the last tunnel segment in district plans. With the completion of extension, total capacity of the deep tunnel system beneath Milwaukee County is 2369M litres - large enough to contain 11 minutes' worth of the flow over Niagara Falls. Californian based joint venture Shea/Kenny completed the construction of the USD65.4M tunnel

In a June 2008 storm, combined sewerage and storm sewers in central Milwaukee and eastern Shorewood spilled 13 billion litres of untreated sewage and storm water into rivers and Lake Michigan. The district would need a tunnel system nearly six times larger than its current capacity to hold all of the overflows that month.

The extension was built to comply with a 2002 state court order requiring USD1 billion worth of upgrades to MMSD facilities to reduce sewer overflows. The tunnels temporarily store sewage and storm water after heavy rains until the Jones Island and South Shore treatment plants have space to treat the flow.

In a deluge, combined sewerage and storm sewers in central Milwaukee and eastern Shorewood fill quickly. Leaks of storm water into separate sanitary sewers in other communities boost flows in those pipes up to several times normal daily volumes. Excess wastewater drops into the tunnels until those caverns fill to capacity. When tunnels are closed, overflow from combined sewers is discharged to waterways to prevent sewage backups into basements. Most of the wastewater in sewers continues to flow to treatment plants during overflows.