Westminster Reclaimed Water Treatment Facility (CO)

Challenge

Since 2000, when the original Black & Veatch-designed reclaimed water treatment facility (RWTF) went online, the city of Westminster, CO, had been a leading regional provider of high-quality reclaimed wastewater for irrigation. Demand for the reclaimed water had steadily grown to the point where the city supplies the largest customer base in Colorado. To better serve present and future customers, the facility needed to boost the supply side storage and transfer capacity of its reclaimed water system.

Approach

Black & Veatch was retained to study and design improvements to the RWTF system, work that 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 to provide cost certainty and a single source of responsibility. The first portion of the process included design development to establish scope and schedule and to develop a target price for the design-build delivery negotiations. The project included the option for the city to have Black & Veatch complete the design and then 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 Black & Veatch, Overland Contracting, Inc.—a Black & Veatch company—and the City. The system developed in Phase 1 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 work involved the development of a new 12 mgd pumping station, which incorporates three deep cannister vertical turbine pumping units with adjustable frequency drive controllers. It also involved development of a new 2.2-million-gallon storage tank. 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 presented a challenge due to the 30-foot-deep excavation, high groundwater, and expansive bedrock.

Results

Phase 1 was delivered on time and under budget and resulted in several efficiencies for the city. For the new pumping station, two of the three pumps used were existing factory refurbished units that were 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 its ability to operate the RWTF.

The new storage tank was an enclosed, pre-stressed concrete structure that replaced open-air storage ponds and allows the city to store more secondary effluent for improved flow equalization. Covering the storage tank also 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. The 1,100-cubic-yard concrete base slab for the storage tank 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.