@flu: I just purchased and installed one of the reverse osmosis systems you are referring to. It was about $190 from costco and installs under the sink. I did a test and to make 3 gallons of water, it sent 13 gallons down the sink drain. Luckily it’s pretty easy to collect this water and use it for gardening, etc. I debated returning the unit, but thought it would be cool to have ultra pure water and figured out that it didn’t waste much water compared to other things in the house. I think we only use probably a max of a few cups a day, so it’s probably way less water wasted than say showering for a minute longer or adjusting the fill level of the toilet. Brita / Pur won’t waste any water, but those are charcoal only systems. If you are one of those concerned about fluoride and other things, then reverse osmosis is the only practical solution besides buying bottled. I think the RO system would still be much cheaper than bottled even with the waste water it generates. Hope this helps.