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City utility study: water treatment capacity and South-side sewers limit growth under aggressive scenario
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Summary
City staff told the council that modeled scenarios show Ann Arbor’s water supply and treatment plant can support aggressive growth for roughly two decades, but sanitary collection in the southern district and treatment plant wet-weather capacity are constraints that will require targeted capital projects.
Skye Stewart, chief of staff for public services, told the council the city’s recently completed water‑distribution and sanitary‑sewer studies modeled an aggressive growth scenario to test where the system would fail. “We modeled an…scenario of 1,800 additional REUs per year for 20 years,” Stewart said, using residential equivalent units (REUs) as the study’s common planning metric.
The modeling found the city’s source water — primarily the Huron River with supplemental airport wellfields — and the water treatment plant’s rated capacity (about 47,000,000 gallons per day) would be sufficient to sustain that scenario for roughly 19–20 years, the consultants reported. Troy Baughman, the senior utility engineer leading the comp‑plan effort, said that while average day demand leaves ample headroom, the city must plan for regulatory “max day” conditions (hot weather, irrigation and fire flow) and for pipeline upsizing from airport wells to the plant.
The studies identified the sanitary collection system as more vulnerable to storm‑related inflow: engineers estimate that, after a major rain event, up to 50% of the flow in sanitary pipes can be clean rainwater, and the South District in particular has limited capacity. “The most pressing constraint in the sanitary collections is the sanitary collection system in the South Southeast part of the city,” Stewart said, adding that the final report will recommend a major capacity improvement project for that district.
The presenters emphasized the difference between modeling and policy: the triple‑rate growth scenario was chosen as an engineering stress test, not as an adopted growth target. The final reports — expected later this spring — will list specific capital projects and timing, distinguishing projects that will be needed regardless of development from those triggered only if certain growth intensities occur.
Councilors pressed staff on how University of Michigan growth factored into the models; Stewart said the modeled “new units” did not include U‑M projections directly but the team used the university’s 2050 campus plan and ongoing coordination to make assumptions. The council also raised demand‑management options; Maile Makieski, water treatment services unit manager (remote), said conservation programs can help, though the city has not historically needed mandatory water‑use restrictions.
Next steps identified by staff include programming near‑term distribution and sewer projects into the city’s Capital Improvements Plan and using the final report to refine financing approaches — including whether adjustments to the capital cost recovery charge are warranted to ensure new development contributes its fair share for system upgrades.
The council opened the item to questions and thanked staff; the final reports will return with project lists and financing recommendations.
