When to change your water filter (and what happens if you don’t)

Why filters don’t last forever

A water filter doesn’t make the unwanted stuff vanish — it catches it and holds onto it. A sediment cartridge traps sand, rust and grit in its layers. A carbon cartridge grabs the chlorine and chemical tastes and keeps them. Either way, the cartridge is a container, and containers fill up.

Think of a vacuum cleaner bag. When it’s new, it picks up everything. As it fills, the suction fades, and eventually it stops collecting altogether. A filter cartridge follows the same arc — it just happens invisibly, sealed inside a housing under your sink. That’s why a simple schedule beats waiting for something to look wrong.

The schedule, filter by filter

Sediment and carbon cartridges — the workhorses inside most undersink, countertop and whole-house systems — are due every three to six months. Closer to three if your water carries a lot of dirt or your household gets through plenty of water; closer to six on clean municipal supply with lighter use.

Membranes last much longer. The membrane is the very fine final barrier in a reverse osmosis or ultrafiltration system — the stage that stops what the other cartridges can’t — and it typically runs 12 to 24 months, depending on your water quality. It lasts that long partly because the cheaper cartridges in front of it take the punishment first. Which is a good reason not to skip the small changes: fresh pre-filters protect the expensive part.

UV lamps work on a yearly cycle. The lamp in a UV steriliser is rated for 8,000 hours — roughly a year of continuous use. Here’s the catch: an ageing lamp usually still glows, but glowing is not the same as sterilising. It weakens gradually, so change it by the calendar, not by eye.

Shower filter cartridges follow the same rhythm as the small cartridges: every three to six months, depending on how much water runs through them. A family showering daily should plan for the shorter end.

How a cartridge tells you it’s done

The clearest sign is flow. When your filtered tap slows from a steady stream to a dribble, the sediment cartridge has done its job and is now full — like that vacuum bag. It isn’t broken; it’s finished.

The second sign is taste. If the chlorine taste or smell you bought the filter to remove starts creeping back, the carbon has reached its capacity and water is passing through with less and less happening to it.

One honest caveat: carbon can quietly reach capacity without an obvious change in taste, and a UV lamp gives no signal at all. So treat these signs as early warnings, not as your system. The date on the calendar is the reliable one.

What actually happens past the due date

Nothing dramatic happens the week after a cartridge falls due — this is about slow drift, not a cliff edge. But the drift is real, and it runs in one direction.

Carbon works by adsorption — chlorine and chemical molecules stick to its surface, a bit like cars filling parking bays. Once every bay is taken, water flows through as normal but nothing more is being removed. Your tap still runs, the housing looks the same, and the filtering has quietly stopped. There is no warning light.

There’s a second, less obvious issue. A long-overdue cartridge sits damp and packed with the organic material it has collected over the months — a comfortable environment where bacteria can multiply. That’s not a reason to panic if you’re a few weeks late; it is the reason a cartridge should be swapped rather than left in place indefinitely.

And an overdue sediment cartridge slowly clogs, dropping your water pressure and making every stage after it work harder — including that membrane you’d rather replace every two years, not sooner.

Or let us do the remembering

Six months is exactly the sort of interval human memory is bad at, which is why SUPERPURE runs a free replacement-reminder programme. Register your filter — the QR code on every SUPERPURE box takes you straight there — and we’ll simply remind you when your cartridges fall due. No cost, no obligation, one less thing to hold in your head.

If you’d rather manage it yourself, that works too: write the install date on the housing with a marker and set a recurring reminder on your phone. Whichever route you choose, the filter you remember to change is the one that keeps doing its job.