When to change every filter in your purifier

"How often should I change my RO filter" is really four different questions wearing one trench coat. A purifier has a sediment filter, a carbon filter, an RO membrane, and — if it has one — a UV lamp, and each wears out through a completely different mechanism on a completely different clock. The six-month sticker taped inside the cabinet is a manufacturer's average-case guess, not a measurement of your water.

The single biggest variable isn't the brand or the model — it's what's coming out of your tap before the purifier ever touches it. Sediment-heavy borewell water chews through a pre-filter in weeks; steady, low-turbidity municipal supply can stretch the same part past six months. Total dissolved solids, or TDS — the dissolved salts and minerals in water, measured in parts per million — comes up throughout what follows; the lower it is, the easier your filters have it. Use the ranges below as a starting point to calibrate against your own source water, not as a fixed calendar.

A realistic replacement schedule

These ranges reflect what water-treatment technicians and manufacturer service guides converge on for Indian conditions — treat the lower end as your assumption on rough source water and the upper end as what clean, low-TDS municipal supply can buy you.

• Sediment filter (pre-filter) — 3 to 6 months on treated municipal supply; as often as monthly to every 2-3 months on borewell or tanker water with heavy sediment, and faster still through monsoon when turbidity spikes.
• Carbon filter (pre- and post-carbon, sometimes called CTO) — 6 to 12 months. It's exhausted by chlorine and dissolved organics rather than grit, so it degrades on a steadier schedule than the sediment stage does.
• RO membrane — 2 to 3 years on decent, well-pre-filtered water; can fall to 12-18 months on consistently high-TDS borewell supply, or sooner if the sediment and carbon stages ahead of it were skipped. Its real end-of-life signal is output TDS creeping up, not a date.
• UV lamp — around 12 months, since most are rated near 8,000-9,000 hours of use. Germicidal output fades well before the lamp visibly stops lighting up, so a working-looking lamp isn't proof it's still doing the job.

Why your schedule won't match your neighbour's

Source water quality overwhelms every other variable here. Municipal supply is treated and runs comparatively low in sediment and TDS, so the upper end of every range above is realistic. Borewell and tanker water carries more suspended grit and generally higher TDS, both of which load the pre-filters and stress the membrane harder. Iron-heavy well water is a particularly rough case — dissolved iron oxidises and clogs the cartridge fast, cutting a sediment filter's working life from around six months down to as little as a couple of months. Pre-filter neglect compounds the problem: skip the sediment and carbon changes and you're no longer protecting the membrane, which is the expensive part, so it fouls early too.

Signs it needs changing now, not by the calendar

• A change in taste or smell — chemical, musty, metallic, or unusually salty water usually means the carbon stage has stopped adsorbing or the membrane is letting more through than it should.
• Noticeably slower flow or a longer fill time, which almost always means a clogged sediment or carbon cartridge restricting flow.
• Visible discoloration in the cartridge itself, if your housing lets you see it — sediment filters typically go from white to brown or grey as they load up.
• Output TDS drifting upward over weeks or months compared to what the unit read when new or last serviced — the clearest direct sign the membrane is wearing out.
• Water tasting oddly flat, which can mean a mineral/TDS-controller cartridge has exhausted early, on top of whatever the membrane is doing.

What the indicator light is actually telling you

The filter-change light on most units runs on a preset usage timer or litre counter calibrated at the factory to an assumed average water quality — it isn't a sensor reading your specific filter's actual condition. That means it can trigger early if you're on clean, low-usage municipal supply, or dangerously late if you're on rough borewell water that loads filters faster than the factory assumed. Treat it as a prompt to check, not as a verdict either way.

How to actually know it's working, not just running

A lit power light means the unit has electricity and the pump is running — nothing more. It doesn't confirm the cartridges still have capacity left or that the membrane isn't fouled.

A basic TDS meter gives you the one direct, cheap check that matters: test the output and compare it against what the unit read when it was new or last serviced. A healthy RO unit with a mineral/TDS controller typically outputs somewhere in the 50-150 ppm range — well under BIS's acceptable ceiling of 500 ppm, so don't chase that number as a target. What matters is the trend: a steady climb, or output creeping toward your untreated input reading, means the membrane is rejecting less than it used to.

A TDS meter has a real blind spot, though: it says nothing about bacteria or viruses. A UV lamp can be lit, look completely normal, and still be past useful germicidal strength — the only ways to catch that are replacing it on schedule and, if you're on a source with known microbial risk, an occasional lab or strip test rather than relying on the meter alone.

What it actually costs

• Sediment filter cartridge — roughly ₹150-350, the cheapest and most frequently replaced part.
• Carbon filter (pre- or post-carbon) — roughly ₹200-500 each; most units carry at least one of each.
• RO membrane — roughly ₹1,500-3,500, the single largest line item and the real cost of skipping upstream filter changes.
• UV lamp — roughly ₹500-1,200.

Add labour if a technician does the swap rather than you, and a realistic all-in annual cost for a typical domestic RO+UV(+UF) unit lands around ₹2,500-6,000 a year — toward the higher end on rough borewell water needing frequent pre-filter changes, lower on clean municipal supply with cartridges you swap yourself.

What BIS actually specifies

There's no statutory filter-change calendar. IS 10500:2012 sets the drinking-water quality bar your output has to clear — an acceptable TDS limit of 500 ppm among other chemical and microbial limits — regardless of how you get there. IS 16240:2023, the newer specification for point-of-use RO systems sold in India, governs how the machine has to perform when new and how it's tested, not how often you must replace parts. The actual interval is left to the manufacturer's manual and, more usefully, to what your own water and your own TDS readings tell you.