Reverse Osmosis

Reverse osmosis pushes water through a semipermeable membrane, a sheet whose openings are fine enough that water passes and most of what is dissolved in it does not. Left alone, water drifts across such a membrane towards the saltier side; pressure applied to the dirty side reverses that drift, which is where the name comes from. What comes through is left roughly 90 percent free of mineral and biological contaminants - most inorganic chemicals, microorganisms and many organic compounds. One unit is normally fitted to a single tap and treats only the water used for drinking and cooking, which is what point-of-use means.

It needs pressure, and it needs protecting. A feed above about 4 bar (60 psi) is what drives water across the membrane, so a tank fed by gravity from a metre or two of height will not run one and a pump has to stand in front of it. Two stages sit ahead of it in a fixed order: a mechanical filter to keep sediment off the membrane, then activated carbon for organic compounds. Both consumables are changed on a clock rather than in response to symptoms - carbon roughly every 6 to 12 months, the membrane cartridge every 12 to 24 - because clogging and lost pressure show up as reduced output long before the membrane itself has failed.

The real cost is water. Three parts or more are discarded with the contaminants for every part delivered, so a household on a limited or rainwater-fed supply spends several units of stored water for each unit it drinks, continuously rather than now and then. On a small supply that one ratio usually decides whether it belongs at all. Its failure is also invisible: a torn membrane still delivers water that looks and tastes treated, so testing the output on a schedule is the only way to know it is working. A product certification helps but does not settle it, because it states which contaminants the device was shown to reduce rather than promising the treated water meets any particular drinking-water limit.