Pure Water and Reverse Osmosis Systems

Pure water cleans glass without detergent and dries without spotting. Ordinary mains water leaves marks because of the dissolved minerals in it; strip those out and the water dries clear, which is what makes water-fed pole cleaning work at all. Purity is measured in TDS — total dissolved solids — and the target is as close to zero as the supply allows.
How the stages work

Reverse osmosis forces water through a membrane that rejects most dissolved solids. It does the bulk of the work.
Deionisation polishes what the membrane leaves behind, taking the last minerals out via resin.
A TDS meter tells you when the resin is spent — the reading climbs before the glass starts spotting.
A mobile trolley carries the system to the job where there is no convenient tap.

South African mains water varies widely by region, so the TDS of your incoming supply decides how hard the system has to work and how often resin needs changing. Pure water feeds a water-fed pole; on its own it is not a cleaning method.