Swan Island CSO Pump Station Phase 1 and Phase 2 Projects

Carollo designed the 220-mgd Swan Island CSO Pump Station (SIPS) as part of the City of Portland’s Combined Sewer Overflow (CSO) Program—the largest capital construction effort in the City’s history. SIPS served as the terminus of two major CSO conveyance and storage tunnels, located more than 120 feet beneath the city, and was designed to collect and store over 100 million gallons of combined sewage. The pump station featured a 135-foot diameter, 150-foot deep cast-in-place concrete structure adjacent to the Willamette River. At the time of construction, it was considered the deepest circular pump station of its kind in the United States.

The wet well included several unique features that improved pump station performance and safety. Most notably, it incorporated a trench-style, self-cleaning design, the largest of its kind in the Pacific Northwest, which reduced maintenance needs and limited personnel exposure by minimizing confined space entry. To optimize performance and address potential hydraulic issues such as vortices and flow inefficiencies, Carollo conducted physical hydraulic modeling using a 1:7 scale model.

Designed in two phases, the primary function of the SIPS was to provide a balance between tunnel conveyance, storage, and treatment plant capacity, requiring a wide-ranging suite of operational modes. The pump station’s performance and reliability was the linchpin to the success of the City’s CSO Control Program.

The Phase 1 pumping system, placed in service in December 2006, included two 42.7-million-gallons-per-day (mgd) vertical extended shaft, 2,000-horsepower (hp) wet weather pumps, and two 16-mgd horizontal, 800-hp dry weather pumps. Together, these pumps provided a total capacity of 100 mgd through a parallel 30-inch and 48-inch interconnected forcemain system. The design included an alternatives analysis, preliminary and final design of the mechanical, electrical, and structural infrastructure to accommodate an ultimate capacity of 220 mgd. During early phases of construction, Carollo led a value engineering effort that cut three months from the critical path, resulting in an estimated savings of $3 million.

The Phase 2 pumping system, placed in service in December 2011, included three 40-mgd vertical extended shaft, 2,250-hp wet weather pumps. This phase added 120 mgd of capacity through a 66-inch forcemain. Combined with Phase 1, the pump station reached its ultimate capacity of over 220 mgd with a total connected motor load of 12,350 hp.

The design effort included an alternatives analysis, as well as preliminary and final design for the full buildout of the pump station. This work covered civil, mechanical, structural, electrical, and instrumentation engineering. It also involved selecting major components and evaluating existing infrastructure to optimize functionality and compatibility between the Phase 1 and Phase 2 pumping systems.

Carollo’s innovative pump station design leveraged the wide range of static head variation caused by surcharging in the wet well. As water levels within the wet well rise, the required pumping head decreases, boosting pump output. By capitalizing on this behavior, the design eliminated the need for a standby pump, which reduced the overall footprint of the 150-foot-deep station and saved an estimated $14 million in construction costs.