What a water filter can't do
Every filter technology has a line it does not cross, and knowing where those lines fall is the fastest way to buy the right thing once instead of the wrong thing twice. Here is the honest list.
4 min read
No single filter does everything
This is the idea underneath all the others. Filtration is a set of specialised tools, not a ladder from worse to better, which is why systems are built in stages, with each stage doing one job the others cannot.
Most disappointment with water filters traces back to one mismatch: a filter doing exactly what it was designed to do, for a problem it was never designed to solve.
Sediment filters do not improve taste
A sediment cartridge is a mechanical strainer. It catches sand, rust and grit by size, and that is the whole mechanism. It has no effect on chlorine taste or odour.
If your water is clear but tastes of a swimming pool, a finer sediment cartridge will not help. That is carbon's job.
Carbon does not reduce dissolved solids
Activated carbon is excellent at chlorine, tastes, odours and organic compounds. It works by holding those molecules on its surface.
What it cannot do is remove dissolved salts and minerals — the bulk of what a TDS reading measures. (Carbon does hold certain dissolved organic chemicals; that is precisely its job. Salts are a different matter and pass straight through.) If your water tastes salty or brackish, which is common on some borehole supplies, no carbon cartridge will change it. That genuinely is a job for a reverse osmosis membrane, and it is worth knowing before you spend on the wrong stage.
UV does not filter anything out
A UV steriliser deals with microorganisms only. It removes no sediment, no chemicals, no tastes and no odours — nothing physically leaves the water.
It is designed as the final stage after your filters, because it needs clear water to work through, and it needs water to pass within its rated flow — too fast, and the dose is incomplete. It only disinfects while it has mains power, so water drawn during an outage passes through untreated. And 'disinfect' is the honest word rather than 'sterilise': UV inactivates microorganisms to a very high degree under the right conditions; it does not promise a total kill of everything under all conditions.
A filter cannot treat water it never sees
A fridge filter improves one dispenser. An under-counter system improves one tap. A shower filter treats the shower it is fitted to. None of them do anything for the rest of the house.
Only a whole-house system, fitted where water enters your home, treats every tap. Even then, the standard 3-stage systems handle sediment, chlorine, VOCs and odours, not bacteria or viruses.
And no filter works forever
Every cartridge is a container that fills up. Replace each at its own model's stated time or litre limit, whichever comes first. As a rule of thumb, sediment and carbon cartridges are due every three to six months; membranes typically run 12 to 24 months; a UV lamp is rated for 8,000 hours, about 11 months of continuous running.
The awkward part is that carbon can reach capacity without any obvious change in taste, and a UV lamp gives no signal at all. Your tap still runs, the housing looks the same, and the filtering has quietly stopped. Change them by the calendar, not by eye.
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