For emergency hydration needs there is a well known and very reliable water sterilizer called LifeStraw which my daughter said could be very useful to store for emergency preparedness. It retails in the UK for under £20. She said it has won an award last year and can be bought in 2-packs and 5-packs.

She also found this useful information and copied it to me: "LifeStraw removes minimum 99.9999% of waterborne bacteria, 99.9% of waterborne protozoan parasites, and filters to 0.2 microns; surpasses EU and EPA filter standards. It removes E. Coli, Giardia and Cryptosporidium oocysts, and many other waterborne contaminants "