Like many, we rely on a large cisterna (35,000L) which is filled by rainwater and maybe one 15000L delivery by lorry per year for €90. The pool uses most of it. Keeping the water good is easy, just keep the light and bugs out. We use a simple 1 micron cartridge filter before using the water in the house for washing. We used a reverse osmosis filter for drinking until reading a WHO report that warned against long term use due to mineral depletion. Might try a UV filter instead but it's bottled water for drinking at present.