FROM PARKING LOT TO PUMP STATION USE OF LARGE SUBMERSIBLE PUMPS FOR CSO SERVICE

Homes and businesses in the North 35th Street area of the Milwaukee Metropolitan Interceptor Sewer (MIS) system have experienced recurring basement flooding during major rainfalls. Although a system of deep tunnels provides in-line storage of peak flows, the storage capacity is sometimes not enough. On July 21, 1999, a heavy rainfall caused severe basement flooding in the 35th Street service area. As a result, the Milwaukee Metropolitan Sewerage District (MMSD) authorized an extreme event hydraulic analysis to evaluate the system capacity and recommend corrective action. The recommended alternative included increasing the capacity of an existing combined sewer overflow (CSO) pump station at 32nd and Hampton from 54 MGD to 125 MGD. The MMSD authorized an emergency project to design and construct a new CSO pump station at 32nd and Hampton on August 12, 1999. The MMSD mandated that the new pump station be operational by May 1, 2000.Because of the short time frame scheduled for design and construction, a fast-track approach was implemented. Major items of equipment such as pumps, generator, electrical switchgear, and sluice gates were procured by the MMSD separately from the construction contract. The MMSD solicited bids from two contractors, and the final selection was made not only on the basis of price, but also on overall construction approach, experience, and ability to meet the scheduled dates of completion and initiation of operation. Construction started while the detailed design was still in progress. The pump station itself is located in the parking lot of an existing MMSD maintenance building.This paper will present a discussion of the use of large submersible pumps for CSO service. Design considerations, procurement methods, and the challenges of fast-track design and construction are described.