Flagstaff examines lower-cost solids-handling at Rio de Flag; dryer option may cut costs and hauling

City wastewater staff presented alternatives on May 15 for handling solids at the Rio de Flag Reclamation Facility, including a lower-footprint dewatering-and-drying technology intended to reduce the solids delivered to the Wildcat Hill plant and to produce a dried, Class A biosolid product.

Troy Dagonard, wastewater treatment manager, summarized the city’s multi-year design effort to add solids handling at Rio. The initial design—two digesters and a dewatering facility—produced cost estimates approaching $60 million. Staff pared scope to one digester plus dewatering with a roughly $36 million estimate, but the rescope remained over budget. "Cost models came in at roughly $36,000,000," Dagonard said, and staff then evaluated alternative technologies.

Why it matters: Wildcat Hill’s solids loading has increased with changing flows and conservation trends; removing roughly 11,000 pounds per day of solids from the collection system would return Wildcat to earlier design‑basis loadings and provide relief for maintenance and future rehabilitation.

Drying alternative and pilot results Staff described a two-step mechanical approach piloted previously: a screw press dewatering stage followed by a low-temperature dehumidifying dryer. The pilot produced dewatered material that passed landfill paint-filter tests and a dried product meeting reduction targets for total solids and total coliforms.

Dagonard summarized estimated capital and operating tradeoffs: preliminary cost estimates for a dryer-based facility at Rio were in the $18–24 million range (PCL reported an initial estimate of about $18 million plus contingencies that pushed the total to roughly $22–24 million); staff said the dryer option is materially less costly than the earlier digester alternatives. Operating cost for a dryer was estimated at roughly $700–$1,000 per day in electricity under current assumptions. Dagonard said the dryer route would reduce hauling to the landfill to about 1–2 roll-off bins per week versus 4–5 roll-offs per week with dewatered-only material moved from a digester scenario.

Benefits and risks Staff said the dryer approach can produce EPA Class A biosolids, reduce solids transport costs and create a dried feedstock compatible with future biochar processes. The dryer’s footprint also fits the Rio site better than large digester tanks, and a fully enclosed building with odor control was part of the 60% design package. Dagonard said the finished dried product could be landfilled or used as a feedstock for biochar if the city pursues that sustainability pathway.

Commissioners pressed staff on lifecycle costs, energy consumption and alternative options. Staff estimated that, even accounting for electricity, the dryer option’s capital savings would shorten the payback period relative to digesters; one commissioner cited a rough 30‑year ROI comparison that did not favor digesters at large scale. Staff acknowledged the dryer consumes significant electricity (each unit rated at 600 amps, 480 volts in design materials) and said the city is investigating tariffs and supply-chain issues for imported equipment.

Timeline and next steps Dagonard said the project had reached 60% design and that staff planned to refine cost estimates and evaluate micro‑screen piloting and other pre‑drying options that might reduce dryer energy needs. He estimated about 18 months until construction could begin, subject to updated cost estimates and procurement.

Ending: Commissioners asked staff to continue refining capital and operating comparisons, to pilot pre‑drying or screening options where feasible, and to return with an updated cost estimate and schedule before moving to procurement.