DI (deionising) resin polishes water to 0 ppm by swapping dissolved mineral ions for hydrogen and hydroxide ions, which combine into pure water. Resin is a consumable: the higher your tap water’s TDS, the faster it exhausts. Feeding resin with RO-filtered water instead of tap water extends its life several times over.
What DI resin is
Mixed-bed DI resin is a blend of cation and anion exchange beads. Cation beads capture calcium, magnesium and sodium. Anion beads capture chlorides, sulphates and carbonates. The output is water with effectively nothing dissolved in it — 000 on a TDS meter.
How long resin lasts
Resin life is arithmetic, not luck: it exhausts in proportion to the ions it removes. Cut the input TDS in half and the same resin purifies roughly twice the volume.
| Feed water | Typical input TDS | Relative resin life |
|---|---|---|
| Municipal tap, inland SA | 150–300 ppm | shortest |
| Municipal tap, coastal/Cape | 50–150 ppm | 2–3× longer |
| RO-filtered water | 5–20 ppm | 10× longer and more |
Watch the meter, not the calendar. When output creeps above 10 ppm, the resin is spent — spots will start appearing on glass even though the water looks identical.
The running-cost decision
DI-only is cheap to buy and expensive to run in high-TDS areas; RO+DI costs more upfront and almost nothing per litre after. The crossover comes fast for anyone cleaning weekly. The decision logic sits in RO vs DI for window cleaning; the chemistry of why even 20 ppm leaves marks is in why pure water dries spot-free.
Getting more from every litre of resin
Three habits stretch resin life. Pre-filter with a reverse osmosis stage so the resin only polishes. Close the flow when the brush is off the glass — resin exhausts by volume, and half the water some users run never touches a window. Store resin sealed; air exposure degrades the beads.
Equipment that manages resin for you
The 6 m waterfed pole kit and 9 m waterfed pole kit include the filtration stage. High-volume users cut resin cost with the reverse osmosis system (R3,900) in front of the resin, or go fully mobile with the RO water trolley (R15,210). Verify every setup with a TDS meter (R260).