Short answer
Reverse osmosis pushes tap water through a very fine membrane that holds back most of the dissolved minerals. A deionisation resin then removes what’s left. The result is pure water with almost nothing dissolved in it, so it dries on glass without leaving spots. I make it in my van before every clean.
Why take the minerals out at all?
Tap water carries dissolved minerals, mainly calcium and magnesium. When a drop of tap water dries on glass, the water goes and those minerals stay behind as a small white mark. In hard-water areas there’s more of them. Essex & Suffolk Water’s report for its Rayleigh supply zone, for example, describes the water as hard. Take the minerals out first, and there’s nothing left to dry onto the window. Does hard water leave spots on windows? explains it in more detail.
Step 1: filtering out the bits
Before the water reaches the membrane, it passes through pre-filters. These catch sediment and treat the water so the membrane lasts longer. Think of them as the coarse sieve before the fine one.
Step 2: reverse osmosis
Osmosis is what happens naturally when water moves through a membrane from a weaker solution to a stronger one. Reverse osmosis does the opposite: pressure pushes the water through the membrane the other way, so clean water comes through and most of the dissolved minerals are held back and flushed away. The membrane does the heavy lifting.
Step 3: deionisation
Reverse osmosis takes out most of the minerals, but not quite all. A deionisation vessel, filled with resin beads, removes the rest. The resin swaps the remaining charged particles for ones that combine back into water. When the resin is used up, it’s replaced.
Step 4: checking it
Pure water is checked with a TDS meter, which stands for total dissolved solids. It reads how much is dissolved in the water. Trade sources say that if the reading creeps up, the system needs attention, because water that isn’t pure enough can leave marks. That’s one of the things I keep an eye on.
What happens to the pure water?
It’s carried in a tank in the van, pumped up a hose and through a water-fed pole to a soft brush. I scrub the glass, rinse, and leave it to dry. No soap goes on your windows.
Is it safe?
It’s tap water with the dissolved minerals taken out, so it’s safe on your glass, frames, plants and paving. It isn’t meant for drinking, and I don’t offer it for that.
Does it clean better than soapy water?
For outside glass, it can, because there’s no soap left behind to attract dirt and nothing left to dry into spots. Inside, the traditional washer and squeegee is still the right method, because indoors there’s nowhere for rinse water to go. Is pure water window cleaning better? covers where it wins and where it doesn’t.
Why make it in the van?
Making it on board means I arrive with the water I need and don’t need your tap or electricity. It also means I’m in control of the quality, rather than relying on someone else’s system.
Common questions
Does pure water damage frames or seals?
Not on normal uPVC and glazed windows. Take care with cracked paint and oiled or varnished timber, which is why I ask about frames on the quote form.
Why do my windows look wet afterwards?
Because I leave the pure water to dry rather than wiping it. Why windows look wet explains it.
Can I make pure water at home?
Small units exist. For your own windows, the simplest tip is not to let tap water dry on the glass.
Is a water softener the same thing?
No. A softener swaps hardness minerals for others, so the water still has dissolved solids in it and can still leave marks. Reverse osmosis and deionisation take them out.
