The actual chemical used in treatment here is ozone, not oxygen. Ozone is not stable enough to be transported any great distance, so it has to be made on-site,

The actual chemical used in treatment here is ozone, not oxygen. Ozone is not stable enough to be transported any great distance, so it has to be made on-site, which involves zapping oxygen with electrical arcs. The oxygen feed must be a pure oxygen atmosphere, unless you want your ozone to be heavily contaminated with various nitrogen oxides. So you need pure oxygen in industrial quantities, which means shipping in liquid oxygen. Industrially, oxygen comes from filtering it out of the air--air is about 22% oxygen, after all. It's possible to do that on site, but the maintenance of the machine to filter it isn't really worth it compared to just ordering a new tanker of liquid oxygen every few days. Another issue with breaking up water molecules is you need a fairly pure source of water to do that. The water in a water treatment plant is anything but pure.