I may sound like a broken ...

I may sound like a broken record but ozone is a powerful oxidant and disinfectant.  Used in drinking water for over a hundred years around the world.  It is a molecular form of oxygen.  If the water to be treated is pre-filtered (it may not need to be) then ozone will kill all microbial pathogens and leave no obvious residues.  You can make it in situ; either by electrolysing the water (simple power source eg batteries or PV are enough to provide power), or using a conventional air-fed ozone generator and dissolution device.  I can give you more advice if you need it.