Influent Pumping Station, Storage Tank and Treatment Capacity Expansion
Since 2000, when the original Black & Veatch (B&V)-designed Reclaimed Water Treatment Facility (RWTF) went online, the City of Westminster, Colo., has been a leading regional provider of high quality reclaimed wastewater for irrigation. Demand for the reclaimed water has steadily grown to the point where the city supplies the largest customer base in Colorado.
Project Goal: Boost the reclaimed system’s “supply side” storage and transfer capacity
In 2009, Black & Veatch was retained to study and design improvements to the RWTF system to better serve present and future customers. That work resulted in two sequentially implemented expansion projects. Phase 1 was focused on boosting the reclaimed system’s “supply side” storage and transfer capacity and was completed using a progressive design-build approach. The system is used for storage and transfer of secondary effluent from the city’s Big Dry Creek Wastewater Treatment Facility to its neighboring RWTF. Phase 2 was aimed at increasing the treatment capacity of the RWTF to 10 MGD and was completed under a conventional design/bid/build process.
The new 12-million-gallon-per-day pumping station incorporates three deep-canister,vertical turbine pumping units with adjustable-frequency drive controllers. Two of the three pumps were existing units that were factory refurbished and relocated to the new pumping station, resulting in reduced project costs. The pumps were installed in stages to allow the city’s existing pumping station to be decommissioned without adversely impacting the ability to operate the RWTF.
The new 2.2-million-gallon enclosed, pre-stressed concrete storage tank replaces open-air storage ponds and allows the city to store more secondary effluent for improved flow equalization. Covering the storage tank significantly reduced effluent quality degradation associated with the former open-air lagoons and simultaneously reduced the cost of producing high quality water from the RWTF. Gravity flow hydraulic constraints necessitated a below-grade tank configuration. A hydrodynamic tank volume mixing system was incorporated to maintain homogeneity and reduce the potential for solids deposition.
The tank foundation construction was challenging due to the 30-foot-deep excavation, high groundwater and expansive bedrock. The 1,100-cubic-yard concrete base slab was constructed in a single pour at night to minimize impact to nearby residents. All work was sequenced around the city’s irrigation season to minimize impact on the existing online facilities and to prevent any loss of RWTF production capacity and associated customer service interruptions.
EPC method supports project completion on-time and under budget
Phase 1 was delivered on time and under budget in December 2010. It featured the use of a progressive design-build/engineer-procure-construct (EPC) method to provide cost certainty and a single source of responsibility. The first part of the EPC process included design development to establish scope and schedule, and to develop a target price for EPC delivery negotiations. Included was the option for the city to have B&V complete the design and bid the work traditionally. Preliminary design and pricing were completed to allow development of the target price, which was then used as the basis for negotiating a lump sum EPC contract between B&V, Overland Contracting Inc. – a B&V company – and the city.