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City staff recommend pursuing regional wastewater treatment while retaining Sandy River outfall as backup
Summary
After a year-long stress test, consultants recommended keeping two alternatives in the facility plan amendment: a regional pipeline to Gresham (preferred) and an expanded membrane bioreactor (MBR) with a Sandy River outfall as a secondary option. Staff warned both choices are costly and will require outside funding and rate increases.
City staff and consultants presented an amended wastewater facility plan to the City of Sandy council and recommended the city further develop a regional treatment option while retaining an expanded membrane bioreactor (MBR) with a Sandy River outfall as a backup.
The presentation, led by a Stantec consultant, said the city's 2023 stress testing and inflow-and-infiltration work reduced peak hour design flow by about 30 percent, from 17.1 million gallons per day (MGD) in the 2019 plan to 12.2 MGD in the amendment, which lowers the scale of some upgrades but does not eliminate the need for additional capacity through 2040. The consultant described five alternatives: conventional activated sludge with tertiary filtration, an MBR, a hybrid option at the current Yarrow Road plant, regional pumping to a larger plant (Gresham or Tri-Cities), and collection-system storage.
The two alternatives staff identified as most viable were the MBR expansion with a new pipeline/outfall to the Sandy River and a regional option to pump untreated flow to the Gresham Wastewater Treatment Plant. Staff said the MBR alternative's remaining program costs are substantial (consultants estimated roughly $166 million in phase-2 costs, plus earlier work, producing a program-level estimate of about $228 million including contingency). The regional option had a wide buy-in/SDC (system development charge) range in preliminary modeling, producing a total-program estimate that overlapped the MBR figure depending on assumptions (roughly $211 million to $245 million in the staff presentation).
Councilors pressed staff on affordability, asking how many additional equivalent residential units (ERUs) would be unlocked by near-term work (staff said about 190 conditionally approved ERUs could be unlocked by Phase 1b work already underway, in addition to 590 already approved). Staff emphasized these investments are partly to maintain compliance under an existing consent decree and that lenders and regulators typically require demonstration of coverage and compliance when evaluating grant/loan support.
Staff framed the regional option as more attractive to state and federal funders because it serves multiple jurisdictions and a much larger population, improving the political and financial case for outside funding. They also warned a full permitting and construction timeline for either permanent option will be measured in years to a decade and that bridging strategies and interim reliability investments will be needed while planning proceeds.
Next steps identified by staff include: asking council endorsement to pursue conceptual pipeline design to Gresham, conducting public outreach in 2025, further examination of conveyance alternatives and potential shared interceptors, and continuing legislative/funding outreach. Staff recommended keeping both alternatives in the facility plan amendment with the regional option as the preferred path to be developed further; the Sandy River outfall would remain a secondary fallback if regional negotiations or permits fail.
