Somerton Wastewater Treatment Plant Phase II Expansion

Somerton, Arizona

The Somerton Wastewater Treatment Plant (SWTP) Phase III Expansion increases the existing treatment capacity from 0.8 MGD to a total of 1.8 MGD. To increase the treatment capacity, the existing four-tank Sequential Batch Reactor (SBR) secondary process was converted into a four-stage continuous flow Biological Nutrient Removal (BNR) process.  For secondary clarification, two new clarifiers and a new RAS/WAS pump station were added. From the secondary clarifiers, treated effluent will be disinfected, prior to gravity discharge to the Yuma Main Drain, through the existing chlorine contact tank. In addition, a new mechanical solids dewatering building was constructed to process bio-solids from the facility. No modifications were made to the existing influent lift station or headworks of the facility as these process units already have sufficient capacity to meet expansion needs.

Each of the SBR tanks is used to provide either Aerobic or Anoxic conditions within each in-service reactor.  The reactors are connected through a new mixed-liquor conveyance channel and a series of pumps and pipes which are valved to allow any number of reactors to be taken out of service while maintaining an operating process.  The valves also allow the completed facility to operate in several well-known activated sludge process modes, based on operator selection of configuration.  This provides ultimate flexibility with respect to current and potential future permit requirements.

Expansion Design Approach Benefits:

  • Reduced land use by utilizing existing SBR basins as alternating anoxic and aerobic reactor tanks rather than constructing four new SBR tanks.
  • Expansion to 1.8 MGD achieved with 4-stageContinuous Flow BNR for the same budget of an expansion to 1.2 MGD with SBR process.
  • Added 1.0 MGD of treatment capacity at under $7/gallon with an SBR expansion.

Additionally, an evaluation comparing the energy costs of the existing Sutorbult rotary lobe blowers compared to high-speed single-stage turbine blowers (similar technology to jet engines) was performed to determine if a long-term cost savings could be yielded. Compared to the rotary lobe blowers with a capital cost of $55,000, the turbine blowers (K-Turbo USA Inc.) had a relatively high capital cost at $104,500. However, the turbine blowers operate with 15-20% less energy, with few moving parts, making maintenance easier and more cost effective. A payback period and 20 year lifecycle cost analysis was performed, demonstrating a significant cost savings from the turbine blower system.

Advanced Elements:

  • Value engineering alternative to modify the existing SBR process to a 4-Stage Continuous Flow BNR process provided a 1 MGD increase in capacity and smaller footprint for the same budget as the original 0.8 MGD SBR expansion design.
  • The use of performance-based specifications reduces capital cost and construction change order by providing a market environment to bid for equipment prior to design
  • PACE experience with funding agencies, including the US Department of Agriculture (USDA) and Water Infrastructure and Finance Authority (WIFA), helped to secure funding for the construction of the expansion.