Systems that were already using activated carbon or ion exchange for organics removal may have some treatment capacity, but otherwise the first systems specifically for treating drinking water are being designed and constructed now. There are contaminated sites that already have treatment or containment in place.
I don’t know the details about alum production (assuming that is what you are referring to), but there are many alternative coagulants available now. Sure the supply logistics would be incredibly challenging and many people would have to boil their water or use point-of-use filters, but this take is pretty doomer in my opinion. Most plants use alum because it’s cheap and easy, not because it’s their only option.