Wausau utility debates PFAS target, costly GAC change-outs as carbon shows breakthrough

Wausau staff asked the Waterworks Commission on Aug. 5 to consider an internal trigger for replacing granular activated carbon (GAC) used to remove PFAS from drinking water, after monthly sampling showed early breakthrough of PFOA and PFOS in the plant's lead vessels.

Staff said the city’s treatment system — 12 tall GAC vessels operating in lead‑lag pairs — has been online a little over nine months and that monitoring shows PFOA and PFOS as the primary compounds of concern. City consultants presented graphs tracking concentrations through sample ports in each vessel and predicted some vessels could reach regulatory thresholds in months, not years.

The commission’s recommendation matters because three separate standards influence the utility’s response: the U.S. Environmental Protection Agency’s recommended maximum contaminant level (MCL) of 4 parts per trillion (ppt) for PFOA and PFOS; the Wisconsin Department of Natural Resources’ (DNR) enforcement standard of 70 ppt; and the Wisconsin Department of Health Services’ (DHS) 20‑ppt advisory level. Staff said DNR currently requires public notice if levels exceed 20 ppt, but that the federal 4‑ppt MCL is expected to be adopted into state code in future rulemaking.

Consultant Susan (Donahue) explained the plant’s lead‑lag operation and sampling strategy, and said monitoring shows the lead vessels are “using up the carbon” and that a widening mass‑transfer zone indicates progressive loading through the beds. She presented order‑of‑magnitude replacement costs: replacing carbon in all 12 vessels was shown on the planning slides as roughly $1.3 million to $1.8 million depending on assumptions; reactivated carbon (reprocessing used carbon) might reduce that cost by an estimated 20–30 percent. Staff also identified a one‑time, congressionally directed federal grant of about $1.7 million that can cover the city’s first change‑out and part of a second.

City staff recommended the commission set an operational trigger at the EPA's eventual MCL level of 4 ppt to schedule replacement rather than proactively replacing carbon at lower advisory levels. Staff said doing so would both align internal practice with the likely future federal requirement and produce data about how many bed volumes the current system can treat before breakthrough to 4 ppt. Staff cautioned that replacing carbon earlier to stay under 20 ppt now could require more frequent change‑outs and may increase long‑term costs, including the risk that both lead and lag beds become loaded and must be replaced together.

Commissioners asked about alternatives, testing time and vendor options. Susan said the tall vessels were selected to allow flexibility in media choice; besides GAC, the vessels can accept ion‑exchange media or newer sorbents such as Fluorozorb, which staff said may be less expensive but is not yet widely used in Wisconsin and likely would require pilot testing. Staff said switching brands of GAC or bidding fills now could provide better price information; a pilot for alternative media would likely take many months.

Staff also warned of downstream effects: wastewater staff noted the municipal wastewater treatment plant is due a new discharge permit and will conduct PFAS testing on effluent during the first two years of the permit term; higher PFAS in drinking‑water treatment backwash or other sources could affect wastewater results and trigger minimization planning.

The commission did not set a final policy at the meeting. Staff said they will return next month with additional data and refined cost estimates and recommended scheduling the first change‑out once distribution‑system samples reach 4 ppt in the areas tied to the most quickly loading vessels.