That is a GREAT question. I don't know the answer.

I recently worked on a project for a large / well known national youth oriented gym type organization that involved a swimming pool. The facility originally was designed for chlorination, but due to the deterioration the chlorine was causing (structure, doors, etc) they switched to a UV system. The cost was staggering, but again we're talking about filtering an Olympic sized pool's water (that has kids swimming and peeing in it).

Chlorination will remain the only cost effective, low tech purification option. A gallon of UNSCENTED bleach is about two dollars. It only takes 8 drops of bleach per gallon of water so the cost per gallon is literally nothing. However, bleach won't kill all the nasty stuff in water, and it has a shelf life. Bleach starts losing its efficacy quickly so rotate your storage and learn about its diminishing potency so you can increase the bleach-to-water ratio in a protracted emergency.

http://water.epa.gov/drink/emerprep/emergencydisinfection.cfm