Reverse osmosis

This is the part sales pages are vaguest about, so it is worth being blunt. No single stage treats everything, and buying the wrong one for your water is the most common way people end up with expensive equipment that changes nothing.

Osmosis is water moving naturally through a membrane from a dilute solution toward a concentrated one. Reverse osmosis uses household water pressure to push it the other way — forcing water through a semi-permeable membrane while leaving dissolved contaminants behind on the pressure side.

The membrane's pores are small enough to exclude individual dissolved ions, which is what makes RO effective against things nothing else in the train touches. It is also why it is slow: pushing water through openings that fine takes time, so RO systems produce a few gallons an hour and store the output in a small tank ready for use.

A residential unit is several stages in one: a sediment pre-filter, a carbon pre-filter to protect the membrane from chlorine, the membrane itself, and a carbon post-filter to polish taste on the way to the tap.

People reasonably ask why they cannot simply treat the whole house this way, given RO removes more than anything else. Three reasons.

Production rate. A whole-house RO system would need enormous membrane area and a large storage tank to meet a household's peak demand, because the process is inherently slow. The cost and space involved are on a different scale entirely.

Waste. RO produces a concentrate stream that goes to drain — typically two to four gallons for every gallon of treated water on a residential unit. That is acceptable for drinking and cooking water. Applied to every shower, toilet flush and washing machine cycle it becomes a serious drain on a well.

And there is simply no benefit. Water this thoroughly demineralised is not needed for bathing or laundry, and being aggressive toward metal, it is arguably worse for the plumbing. Treating the two or three gallons a day you actually drink is the sensible engineering answer.

This is where RO stops being a taste upgrade and becomes the actual answer to a health problem.

Nitrate and arsenic are the two health-based contaminants that turn up regularly in private wells, and neither is touched by the equipment most households already have. A softener does not remove them. A carbon filter does not remove them. Boiling makes nitrate worse, by evaporating water and concentrating what remains.

Both are also undetectable without a test — no taste, no smell, no colour. So the sequence is: test, and if nitrate is above 10 mg/L or arsenic above 0.010 mg/L, fit RO at the drinking tap or use an alternative supply while you work out a longer-term answer.

One caveat on arsenic: it exists in two forms, and RO removes arsenic V far more completely than arsenic III. Where arsenic is the reason for the system, the water should be tested for speciation, and pre-oxidation may be needed to convert one to the other.

Check your nitrate and arsenic results→

It does remove calcium, magnesium and trace minerals along with everything else — that is unavoidable given how it works, and it is the most common objection raised against it.

In dietary terms the contribution is small. Drinking water supplies a minor fraction of mineral intake for most people compared with food. Remineralisation cartridges are available and do add minerals back, and some people prefer the taste, but framing them as a health necessity overstates the case considerably.

The genuine trade-off is between removing everything including things you would rather keep, and leaving in place a contaminant like nitrate or arsenic. Where there is a health-based contaminant to remove, that is not a close call.