I’ve got a question for your readership, the answer to which may save a great number of lives.
The metropolitan Phoenix area is one of the half-dozen most populous in the United States. Between 3 and 4 million people live there. The river which Phoenix was built on (the site of a previous civilization whose population vanished around 500-600 years ago) is now dammed upstream, and usually bone dry. The population is mostly dependent on grid-up well-pumped or canal/dam-diverted water for its entire supply.
Some 3.5 million people are going to be in a world of hurt if the grid goes down.
One bright spot: given the high temperatures, swimming pools are very common, even in working-class neighborhoods. A hike to the top of any of the several urban park mountains in the city reveals dozens or hundreds of backyard swimming pools scattered everywhere. These manmade cisterns will presumably hold several thousand gallons of water even after a SHTF  event.
If the grid goes down, these are going to be the best available source of water for millions of people.
What is the best low-tech, grid-down way of de-chlorinating, or otherwise rendering drinkable, swimming-pool water?
Thanks! – K.F. in Phoenix