A water-insoluble polymeric oxidizing medium is contemplated that has a plurality of polymerized N-pyridinium vinylbenzyl triiodide or tribromide moieties whose pyridinium rings bear two substituents, R.sup.1 and R.sup.2, that are independently a hydrido or a C.sub.1-C.sub.4 alkyl group, and correspond in structure to the formula A process for preparing aseptic water and an apparatus useful for carrying out that process, both of which utilize a contemplated polymeric oxidizing