Reverse osmosis explained

RO removes by size and charge rather than by adsorption, so its coverage is broad: lead, nitrate and nitrite, arsenic, fluoride, sodium, sulfate, copper, chromium, many pharmaceutical residues, and calcium and magnesium — RO permeate is effectively soft. NSF/ANSI 58 is the standard residential RO systems are certified against, and a certified unit lists exactly which contaminant reductions were verified.

It is weaker on dissolved gases and small uncharged molecules. Chlorine passes and also damages thin-film composite membranes, which is why every practical system puts a carbon prefilter ahead of the membrane and usually a carbon polishing filter after it. Hydrogen sulfide and radon are gases and are not membrane targets.

• Sediment prefilter — protects the downstream stages from particulate.
• Carbon prefilter — removes chlorine and chloramine, which would otherwise oxidise the membrane. This is the stage that determines membrane lifespan.
• RO membrane — the actual separation. Replaced every two to five years depending on feed water and use.
• Storage tank — because production is slow, the tank buffers a few gallons so the tap delivers at a usable rate.
• Post-carbon filter — removes any taste picked up in the tank.

Every RO system sends some water to drain. Older or poorly configured units run at ratios as unfavourable as 4:1 — four gallons to drain per gallon produced. Modern permeate-pump or high-efficiency configurations commonly reach 1:1 or better. On a household drinking-water volume the absolute numbers are small, but the ratio is a fair question to put to any installer, and it is also the figure most likely to be glossed over in a sales conversation.

Hard feed water makes the ratio worse and shortens membrane life, because scale forms on the membrane surface. That is the one place where hardness and RO interact: on a very hard supply, softening upstream of the RO unit is a membrane-protection measure, not a redundancy.

RO permeate is low in minerals and slightly acidic, and it tastes flat to many people. Remineralisation cartridges add back small amounts of calcium and magnesium, which restores mouthfeel and raises pH.

On the nutrition argument, be careful with what is claimed. The WHO has reviewed drinking water as a mineral source and notes that water contributes a minor share of dietary calcium and magnesium for people eating an ordinary diet; it also discusses the possibility that very low-mineral water is a less favourable source. That is a genuinely open area rather than a settled one. The honest summary: remineralisation is primarily a taste decision, and anyone told it is a health necessity is being sold past the evidence.

RO earns its place when there is a specific dissolved contaminant to remove — a nitrate detection, lead risk from service lines, arsenic in a private well — or when you want one tap producing consistently low-solids water for drinking and cooking. It is not the answer to scale on the shower door, and it is not a substitute for fixing a taste problem that carbon alone would solve at a fraction of the cost.

For coverage trade-offs against a whole-house configuration, see whole-house RO vs under-sink RO, and for the softening comparison, filtration vs softening.