Reverse Osmosis Systems

Why window cleaners purify their water

South African municipal water typically carries between 100 and 400 parts per million of dissolved minerals. Every one of them stays on the glass when the water evaporates, which is what dries as spotting and haze. Strip them out — below roughly 10 ppm — and the water evaporates leaving nothing at all. That is the entire principle behind pole-fed cleaning.

The equipment

75 GPD reverse osmosis system — R4,360, forces water through a membrane that rejects most dissolved solids
Reverse osmosis water trolley — R17,050, filtration and transport in one unit for site work
TDS meter — R290, reads parts per million directly so you know the water is actually pure

RO, resin, or both

Reverse osmosis handles volume cheaply and is where most cleaners start. Deionising resin polishes what the membrane leaves behind and is more expensive per litre, so it is usually used after RO rather than instead of it — see the mobile deionised water supply at R3,500. On hard feed water, run RO first and resin second.

Measure it, do not assume it

A membrane or a resin bed gives no warning when it is spent — the water simply stops being pure, and the first sign is usually a customer complaint. A TDS meter costs R290 and turns that into a number you check before you start. Above about 20 ppm you will see marks on dark glass in sunlight.

The purified water goes up a waterfed pole. Background: what water do window cleaners use. Full range: window cleaning equipment.