http://rparticle.web-p.cisti.nrc.ca/rparticle/AbstractTemplateServlet?calyLang=eng&journal=cjm&volume=45&year=1999&issue=8&msno=w99-063

In the USA many $BILLIONS have recently been spent in attempting to reduce the 'chlorine resistants' from potable water .... and thats in the 'developed' world.
Sorry, there are too many chlorine resistant species (cysts/bacteria/viruses, etc.) to take a chance in the more remote places - you really need to prevent them from entering the tank.