Utilities manager outlines tertiary treatment upgrade to reduce phosphorus and meet DEQ permit limits

Craig Vorpahl, utilities manager for the City of Post Falls, gave a staff update Dec. 17 on tertiary treatment being added to the city's water reclamation facility to reduce phosphorus and comply with permit limits.

Vorpahl said the existing secondary biological treatment removes about 96% of incoming total phosphorus but that the facility still discharges roughly 7 pounds per day. The city's permit requires a final effluent of 3.19 pounds per day, so staff conducted pilot studies and selected a combination of tertiary clarification and membrane filtration to achieve the lower limit.

Staff described the tertiary clarification step as a chemically driven process that aggregates phosphorus for removal and the membrane filtration as fine filtration through fibers with a nominal pore size of about 0.3 microns. Vorpahl said the new facilities are in startup and currently treat up to 5.2 million gallons per day in design (the facility is presently seeing about 3.1 million gallons per day). He said optimization work is underway to set chemical doses and the split between clarification and membrane filtration to meet the effluent limit without unnecessary cost.

Vorpahl also noted that tertiary treatment will enable land application to recycled water sites in the future, but that land application is governed by a separate DEQ permit that focuses more on nitrogen limits. Councilmembers asked clarifying questions about permit differences and operational impacts; no council action was required at the meeting.