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GREENSBORO — The city water department is embarking on a long, costly effort to counteract pollution from firefighting foams that apparently have accumulated over the years in part of the city’s watershed.

The first act played out last week, when officials at the Mitchell Water Plant christened a new treatment device that the city is renting for $9,000 a month and that costs about $1,000 per day to run.

They plan to buy one of these ‘PAC Feed’ systems to replace the rental unit at a cost of $80,000 to $120,000 within six months.

The game plan calls for only running the machinery when weekly tests show that the concentration of firefighting chemical PFOS and a sister compound reach the federal health advisory level of 70 parts per trillion (ppt) in the plant’s Lake Brandt water supply.

But testing for PFOS, the related chemical PFOA and other perfluorinated compounds costs another $1,500 per week.

And with test results coming in at that pace, it means that once cranked up, the PAC Feed system likely will run for days at a time using roughly $1,000 per day in filtering materials before a new round of chemical testing gives plant operators the all-clear to shut it down. Or not.

‘This figure is very close to four times the current Mitchell daily cost of all other chemicals combined,’ city Water Supply Manager Dell Harney said of PAC Feed’s operating cost in a recent report.

The newly leased PAC Feed system’s goal will be to keep the combined total of PFOS and PFOA at least 10 percent below the nationwide health advisory, meaning that the maximum concentration would be 63 ppt, Harney said.

Looking ahead, the off-and-on PAC technology could define the Mitchell plant’s operating routine — and its financial drain on the city — for the next four or five years.

That’s the rough time frame for planning, designing and installing a permanent replacement system that city water administrators see as the ultimate PFOS solution — at an estimated cost up to $30 million.

They plan to hire an engineering firm to design that permanent replacement system in the next budget year, said Mike Borchers, assistant director of the Water Resources Department.

‘Construction could start as early as two years out,’ Borchers said of the permanent replacement. ‘Due to the complexity of the project, it will likely take two years, so that places us in fiscal year 2022-23 for having the full-scale system in place and operational.’

The replacement system would use a different, carbon-based technology to screen out PFOS and other pollutants. That replacement technology is equally effective, but less costly to operate day-to-day than what the city unveiled Wednesday at the Mitchell plant.

The leased PAC equipment that the city debuted last week relies on powdered activated carbon to chemically attract PFOS and many other pollutants that form tight bonds. PAC is the acronym commonly used to describe the technology.

The leased machinery was added to a concrete slab beside the Mitchell plant in recent weeks, and city officials gained speedy approval from state regulators to include it as part of their treatment process.

The system works by crushing small bits of carbon into a powder, which is fed into a pressurized stream of water to form a thick fluid that is injected into untreated water during the early stages of purification.

The carbon in that injected fluid binds with PFOS and many other types of contaminants in the ‘raw’ water, forming a sludge that is removed for incineration at the city’s Osborne Wastewater Treatment Plant.

The city’s longer-range plan would use a similar technology known as ‘granular activated carbon,’ or more commonly by its GAC acronym. This technology adds a step to the treatment process by running future drinking water across filters of charcoal-colored carbon that do not have to be replenished as frequently as the bulk bags of PAC Feed carbon.”