Flagstaffs water-services director and staff told council Friday that the citys aging Wildcat Hill wastewater treatment plant and existing Rio De Flag facility are approaching operational limits and that long-term growth will require a new treatment strategy.

Lee Williams, director of Water Services, said the two existing plants have a combined nominal liquid capacity of about 10 million gallons per day (MGD) but are constrained by solids-processing capacity because per-capita water use has declined. "Because the toilets are using less water...there's still the same amount of waste in that stream," Williams said. "We are not near the liquid capacity, but we are at the solids capacity. That is what's driving the need for a new plant."

Wildcat Hill, built in 1979, has a design liquid capacity of about 6 MGD and is currently treating roughly 4.5 MGD; the Rio De Flag plant (1996) serves the east side of town and is underutilized because of collection-system flows. Williams presented cost estimates for a replacement or new advanced-technology plant: current construction-price assumptions range from about $25 to $30 per gallon of capacity. That yields a rough sticker price of $225–$300 million for a 10 MGD plant and $350–$420 million to reach an estimated 14 MGD needed at full buildout.

Williams described two basic construction approaches: 1) build a single new plant at Wildcat Hill and then decommission the old facilities once the new plant is operational; or 2) use a phased approach of smaller modular membranes or cells (for example, 2.5 MGD increments) that can be added over time. He noted membrane-biological-reactor (MBR) technology provides very high effluent quality and is frequently used when municipalities plan for advanced water treatment or indirect potable reuse. Williams described MBR membrane pore sizes (0.02–0.4 microns) as much smaller than a human hair (~50 microns), which is why MBR produces high-quality effluent suitable for additional treatment steps.

Design for a new plant is already programmed in the citys 10-year capital-improvement plan at roughly $25 million (about 10 percent of construction cost), and the city is on an approved project list for a Water Resources Development Act appropriation (WERDA) that could yield $5 million for design if funded. Williams and staff emphasized that the WERDA appropriation has not been funded and that construction dollars are not in place.

Council members asked about options to prolong the life of existing facilities. Williams said staff is designing solids-removal improvements at the Rio plant (for example, dryers) that could reduce solids routed to Wildcat Hill and gain operational time. The city is also pursuing a biochar pilot and has sought appropriations for equipment that would convert dried biosolids into char for carbon sequestration and potential reuse.

Councilors asked about timing. Williams said design is in the CIP and that a realistic design timeline would be about two years; construction would follow and could be staged to match growth, but exact schedules depend on funding and permitting timelines.

Ending: Staff asked council for direction to continue planning and to pursue funding opportunities. Council members generally agreed the city should explore funding options for a major wastewater-project program, recognizing the scale of projected costs and the need to combine grants, low-interest loans, bonds and potentially other mechanisms with rate and fee strategies.