Big Sky, MT WWTF and Effluent disposal improvements

Need for Upgrades and Expansion

The AE2S team was selected by the Big Sky Water and Sewer District (District) for preliminary phase engineering to upgrade and expand the WWTF and for an Effluent Reuse and Disposal Analysis to diversify and expand effluent reuse and disposal options. The District currently operates a Sequencing Batch Reactor wastewater treatment system followed by a multimedia filtration system to produce effluent for irrigation. The District utilizes aerobic digestion and composting for solids treatment. The existing wastewater treatment facility was constructed in 2004. The District has recycled 100 percent of their treated effluent for utilization as irrigation on three golf courses, but significant growth of the mountain ski area is pushing the limits of the existing system.

The District selected the AE2S team to evaluate effluent reuse for snow-making, groundwater recharge, and surface water discharge to the blue-ribbon trout fishery of the upper Gallatin River. All these disposal alternatives require a higher quality of effluent than the existing treatment facility can provide.

Two secondary-tertiary treatment alternatives were evaluated which included continued use of SBRs and dual media filtration, aerobic granular sludge followed by tertiary membrane ultrafiltration (UF), and membrane bioreactors (MBR). With the MBR alternative, the City’s existing SBRs would be retrofitted to comprise a new Biological Nutrient Removal configuration consisting of anaerobic, anoxic, and aerobic basins. A Kepner-Tregoe (KT) decision making process tool was used to evaluate the secondary-tertiary treatment alternatives. Twenty-five criteria were developed and organized across four categories, including stakeholder acceptance; cost; performance; and operations, management, and construction considerations. MBR was selected based on treatment performance, small footprint and site constraints, and its proven track record. The future WWTF capacity expansion will utilize membrane bioreactor (MBR) technology with a focus on achieving limit-of-technology nutrient removal and a good start on removing compounds of emerging concern (CECs).

Advanced treatment (advanced oxidation process (AOP) and biological activated carbon (BAC) filtration), which would further improve the District’s effluent reuse options to include potential indirect potable reuse (IPR) was evaluated. These processes would be retrofitted into the District’s existing Filtration Building and this project may be pursued in a second phase treatment upgrade.

The concurrent effluent reuse and disposal study evaluated four effluent reuse and disposal alternatives (continued irrigation development, subsurface disposal/groundwater recharge, snowmaking, and surface water discharge to the Gallatin River). The effluent reuse and disposal study included a Public Communications plan for education and consensus building with a highly educated and engaged group of stakeholders. All effluent reuse and disposal alternatives were proven viable and will be pursued by the District in the near future.

The WWTF Improvements project is currently transitioning to the Design Phase.

Key Project Elements

  • WWTF Expansion with Limits-of-Technology Treatment
  • Very Low Nutrient Limits
  • Discharge Permitting
  • Public Communications

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