Regarding low volume "spare emergency water" that needs no maintenance or purification:



Consider the hot water heater- a sealed unit that allows new water in as "old" water is removed- and, of course it is heated. Remove the heating component, leaving a sealed unit (read: tank). I have a large poly tank with an input pressure regulator connected to the water supply. Output is household water. As water is removed, new water enters (just like a hot water tank)

This provides 500 gallons of constantly "refreshed" water that requires no treatment and should city water cease (or the well go dry), provides a 500 gallon reserve. Input pressure provides output pressure (sealed tank).

Quick disconnects allow me to sever input and output ...and did I mention the tank is mobile?

-carl