Design and Construction Considerations for the State’s Largest Wastewater Pump Station
- Meredith Bridwell, PE, Tony Greiner, PE, CCM, Jim Struve, PE - Hazen and Sawyer
- Bill Davis - City of Columbia
This presentation will discuss the challenges faced during the design and construction of the City of Columbia’s new 150 mgd Influent Pump Station (IPS) and Preliminary Treatment Facility (PTF) at the Metropolitan WastewaterTreatment Plant (Metro WWTP).
The Metro WWTP has a permitted capacity of 60 mgd and consists of two parallel liquid treatment trains (Train 1 and 2) capable of treating 40 mgd and 20 mgd, respectively. A previously prepared preliminary engineering report recommended a new consolidated IPS and a new PTF to resolve operational issues, address aging infrastructure concerns, and better prepare the plant for future upgrade and expansion. Specifically, the existing facilities consisted of multiple pump stations that severely limited influent flow control to the two liquid treatment trains which impacted biological treatment; site constraints limited the expansion of the existing facilities; the existing screenings and grit removal facilities had limited capacity at peak flow conditions; and, diversion of raw sewage into and out of the existing 160 MG flow equalization basin could not be adequately controlled to take advantage of the basin’s full volume.
Design considerations associated with the new consolidated IPS included: evaluating various pump station configurations and alternatives (selected alternative was a two-stage screw lift station, 4 pumps per stage, each rated at 50 mgd); providing sufficient capacity, hydraulic grade and functionality to replace four on-site existing pump stations; incorporating a 5th pump bay to easily accommodate future expansions to a firm pumping capacity of 200 mgd; and, providing means for odor control if required in the future.
Design considerations associated with the new PTF included: evaluating various screening and grit removal technologies (selected alternative was a filter screen and stirred vortex grit tank); incorporating a bypass channel with manual screen for high flow and maintenance conditions; accommodating flow measurement and distribution to the two liquid treatment trains through fixed weirs and isolation gates; allowing capability to divert up to 80 mgd of screened and degritted wastewater to the existing flow equalization basin; and, providing automatic control to convey equalized flow back into the treatment process.
Construction of the new facilities posed many challenges, of utmost importance was maintaining plant operations in continuous service during the 30-month construction period. Other challenges included: installation of deep pile foundations for all new structures due to liquefaction concerns during seismic events; extensive dewatering operations due to high groundwater conditions; coordination of multiple concurrent on-site construction projects performed by different general contractors (i.e., disinfection project, secondary clarifier project, aeration basin modification project, and temporary dewatering project); installation of an 84” process pipe line through the middle of the plant site; and, sequencing the startup of the new facilities while demolishing existing facilities.
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