Running a mill filled with tons of earth for a day at 450 RPM seems to be an enormous amount of energy expenditure relative to the PFAS you’d be able to destroy at the concentrations in the US.
For their context, in which it was firefighting material in a small training zone, it seems like a great fit, but for widespread deployment this seems like a stretch.
You can't object "but then you have to build X" because you have to build stupid ball mills too. They aren't free.
But yeah this whole PFAS cleanup project is going to be a long haul.
Nobody sat there with the first combustion engine and said "well this is too much effort, nobody will ever make another one of these" with the same sort of thought process.
Usually used during a short period of time before the diesel generator can be fully operational.
The energy of the wheel is going into repeatedly lifting the material. That energy is lost: it's not transferring that energy back into the rotational energy of the wheel every time it falls, but into breaking up the material against the bottom of the wheel, more or less perpendicularly to the direction of the wheel's travel at that point.