Reverse Osmosis RO

Reverse osmosis pushes ordinary tap water through a semi-permeable membrane under pressure. The membrane’s pores are small enough to block dissolved minerals, salts, and most other impurities while letting water molecules through, so what comes out the other side is water with almost nothing left dissolved in it.

What TDS means and why it is the number that matters

Total dissolved solids, or TDS, is a measurement of everything dissolved in water that is not water itself — calcium, magnesium, and other minerals, mostly. Tap water in this region typically reads well into the hundreds of parts per million. Water that has passed through an RO system usually reads in the single digits. That gap is the entire reason RO water behaves differently on glass than tap water does.

Why mineral-free water dries without spots

Ordinary water dries and leaves its dissolved minerals behind on the surface — that residue is what shows up as spotting once the water evaporates. Water with next to nothing dissolved in it has next to nothing to leave behind. It dries the way distilled water dries: clear, with no film and no ring. That property is what pure-water window cleaning is built on, and it is also why the same principle prevents hard water spotting from forming in the first place, rather than trying to remove it after the fact.

How RO differs from deionization

Deionization (DI) is the other common way to strip minerals from water, and the two get confused because they produce a similar result. RO is a physical filtration process — a membrane with pores small enough to block dissolved solids, driven by water pressure. DI is a chemical process, running water through resin beads that swap out mineral ions for hydrogen and hydroxide ions. DI can reach a slightly purer end result, but the resin has to be regenerated or replaced on a schedule and costs more to run at volume. For exterior window work, where the water is used continuously and in quantity, RO is the practical choice; DI shows up more often in smaller-scale or specialty applications where the last few parts per million matter.

Where this fits into the work

RO is the water-treatment step, not the cleaning method itself. The equipment that carries this water up to the glass, and the technique of cleaning with it instead of soap, are their own parts of the job — see pure-water window cleaning for the method and water-fed pole window cleaning for how it gets delivered on exterior work. It is worth asking a window-cleaning vendor whether they are actually running RO water or just calling a hose “pure water” — the TDS reading is the honest way to tell.