What is pure water window cleaning, and why do window cleaners use reverse osmosis water?
Team OPPY4 min read
Pure water window cleaning uses tap water that has been pushed under pressure through a fine membrane, which holds back limescale and other dissolved minerals. Window cleaners use it because pure water dissolves dirt and dries without spots or streaks. Nobody has to wipe the glass afterwards, even at height or when a drone does the rinsing.
Anyone who has washed windows with tap water knows the problem: the glass looks clean until it dries, and then the spots appear. Pure water solves exactly that. Below you can read what it is, how it is made and why window cleaners use it, especially for glass at height.
What is pure water, exactly?
In window cleaning, pure water is water from which almost all dissolved substances have been removed, often by reverse osmosis. What remains contains so few minerals that it leaves no visible residue when it dries.
Tap water naturally contains calcium and magnesium, and the more it contains, the harder the water (De Watergroep). That is no problem for drinking, but it is on glass: when a droplet evaporates, the minerals stay behind as a white ring. Pure water no longer carries those minerals, so nothing is left on the glass.
How is reverse osmosis water made?
A pump pushes tap water under pressure through a semi-permeable membrane. Water molecules pass through, while most dissolved salts and minerals stay behind.
In natural osmosis, water flows from a weaker solution towards a stronger one. In reverse osmosis, a pressure higher than the osmotic pressure is applied, so water crosses the membrane the other way and leaves the dissolved substances behind. According to membrane manufacturer DuPont, a reverse osmosis membrane rejects 95 to 99 per cent or more of dissolved salts, depending on the membrane type, the feed water, the temperature and the system design (DuPont).
A system for window cleaning often combines several stages. The Unger HydroPower RO, for example, pairs a carbon and sediment pre-filter with a reverse osmosis membrane and a resin filter (a DI polisher) that removes the last minerals. A built-in meter shows the purity in ppm, parts per million (Unger USA). Unger writes that the membrane removes on average 85 to 95 per cent of impurities and the resin the last 5 to 15 per cent, and that water only counts as 100 per cent pure at 0 ppm of total dissolved solids (Unger Europe).
Pure water window cleaning vs traditional: what is the difference?
Traditional window cleaning removes the dirty water with a squeegee before it dries; with pure water, the glass is rinsed and left to dry on its own. That makes it possible to clean from a distance, with a water-fed pole or a drone.
With the bucket, detergent and squeegee method, the window cleaner has to reach every pane by hand. With pure water, that last step disappears: Unger points out that pure water lets you cut out steps such as wiping and polishing (Unger Europe). Pure water also dissolves dirt on its own, precisely because it does not yet carry anything dissolved. That is why it suits a brush on a long pole from the ground, and a drone rinsing glass that nobody can reach by hand. For the latter, see how drone window cleaning works.
Is reverse osmosis water the same as deionised or demineralised water?
For cleaning, yes. Demineralised water is the umbrella term for water with its minerals removed, and reverse osmosis is one way of making it.
The difference lies in the technique. An ion exchanger swaps the minerals in the water for other ions using a resin, which is why it is called deionised water. Distillation boils the water and collects the steam. Reverse osmosis filters it through a membrane. In window cleaning, membrane and resin are often combined: the membrane does most of the work, the resin finishes it.
Four kinds of water for cleaning glass
| Tap water | Reverse osmosis water | Deionised water (resin) | Distilled water | |
|---|---|---|---|---|
| How it is made | No treatment for cleaning | Pressure through a membrane, often followed by a resin filter | Ion exchange in a resin filter | Boiling and collecting the steam |
| Minerals | Calcium and magnesium, depending on the region | Almost none | Almost none | Almost none |
| After drying on glass | Risk of limescale spots and streaks | No visible residue | No visible residue | No visible residue |
| Practical for window cleaning | Only with a squeegee or wiping | Yes, made on site | Yes, but the resin saturates quickly in hard water | Not practical in large volumes |
Is pure water expensive?
The water itself is ordinary tap water; the cost lies in the equipment, the filters and the water that runs off as concentrate. The harder the water, the faster the filters wear.
A reverse osmosis system splits the supply into two streams: pure water and a concentrate that carries away the rejected minerals (Unger Europe). That concentrate goes to the drain. On top of that, the pre-filter, membrane and resin need replacing on time. In hard water it is mainly the resin that saturates faster, which is why the membrane is placed in front of it: it removes most of the minerals and makes the resin last longer. For you as a client, that cost is simply part of the price per clean.
How we do it at OPPY
We clean glass and facades with reverse osmosis water, at low pressure or, where needed, up to 200 bar, using a drone flown from the ground. Because the water contains no limescale, the glass dries without streaks or spots and nobody has to wipe it. On facades, we use a product suited to the surface and the type of soiling where needed, and rinse with reverse osmosis water.
For glass at height, see Window cleaning.
Sources
- Reverse osmosis (RO) technology, DuPont Water Solutions
- Streak-free cleaning with pure water, Unger Europe
- HydroPower RO System (product sheet RO20S), Unger USA
- Waterhardheid (water hardness, in Dutch), De Watergroep
Frequently asked questions
Is reverse osmosis water the same as deionised water?
For cleaning glass, yes. Demineralised water is the umbrella term for water with its minerals removed, whether by reverse osmosis, ion exchange (deionisation) or distillation. Reverse osmosis is one way to make it, and in window cleaning it is often followed by a deionising resin.
Do you need detergent for pure water window cleaning?
For ordinary dirt on glass, usually not: pure water dissolves the dirt and dries without residue, so no wiping is needed. For greasy dirt, bird droppings or old deposits, a contractor may add a suitable product.
Is pure water window cleaning expensive?
The cost lies mainly in the equipment and its upkeep: the pre-filter, the membrane, the polishing resin and the water that runs off as concentrate. The harder the local tap water, the faster the filters wear. For a client, that cost is part of the price per clean.
Can solar panels be cleaned with pure water?
Yes, for the same reason as glass: the water dries without limescale spots on the glass of the panels. Always follow the panel manufacturer's cleaning instructions.
About the author
Team OPPY
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