Purified drinking water at the kitchen tap.

RO pushes water through a semi-permeable membrane that rejects most dissolved contaminants. Residential systems typically use 4–6 stages: sediment and carbon pre-filters protect the membrane, the RO membrane does the heavy reduction, and a post-filter (sometimes with remineralization) polishes taste before a small storage tank and dedicated faucet.

Because the membrane is fine, feed water should be reasonably clear and not extremely hard — which is why many Canadian homes pre-treat with whole-home carbon and/or a softener.

RO shines at the kitchen for drinking, cooking, coffee and ice. It is not a whole-house softener and does not replace UV disinfection on a contaminated well — though it can be the drinking-water stage in a larger well train.

Traditional residential RO sends some reject water to drain. Newer high-efficiency membranes and permeate pumps improve the product-to-waste ratio. Your installer should explain recovery rates, tank size and under-sink space before you buy.