The filters in an under-sink reverse osmosis system each have a distinct job, and they fail in different ways and on different schedules. Knowing which cartridge does what tells you which one to blame when the taste changes, which one protects the expensive part, and which one you can stretch when money is tight.
Sediment prefilter: the sacrificial stage
The first cartridge is a sediment filter, usually spun or pleated polypropylene, and its job is to stop grit, rust flakes and silt before they reach anything else. It is the cheapest cartridge in the system and it takes the most punishment, and on a well or on an older municipal main it can load up fast. The symptom of an overdue sediment filter is reduced production: the storage tank takes longer to fill and the dedicated faucet gives a weaker stream. Change it on its own schedule rather than waiting to do the whole set at once.
Carbon prefilter: the one that saves the membrane
The second stage is carbon, and it is the most important cartridge in the system even though it is not the most expensive. Chlorine and chloramine attack the thin film that a reverse osmosis membrane is made from, and a membrane that has taken a chlorine hit loses rejection permanently. The carbon prefilter absorbs that oxidant so the membrane never sees it. This is why a lapsed carbon change turns into a membrane bill later, and why the carbon interval is set conservatively. It also removes the taste and odor compounds the membrane would otherwise pass.
Membrane and postfilter
The membrane does the actual separation, rejecting dissolved solids to the drain line and sending treated water to the storage tank. It lasts years rather than months when the prefilters are maintained, and it is replaced when rejection falls, which you detect with a total dissolved solids meter comparing feed and product rather than by taste. After the tank sits a polishing carbon postfilter that removes any flatness or tank taste picked up in storage; it is changed annually in most systems. Some systems add a remineralization cartridge after that, which is a taste choice rather than a treatment one.
Setting intervals and changing them properly
Manufacturer intervals assume average feed water. Yours may be dirtier or cleaner, so treat the printed figure as a starting point and adjust from what you observe: falling production means sediment, returning chlorine taste means carbon, rising product readings mean the membrane. When you change cartridges, shut the feed valve and the tank valve, relieve pressure at the faucet, and sanitize the housings if the system has stood idle. Flush the first tank of water to drain afterward. Our best under-sink water filter reverse osmosis guide compares the systems on cartridge cost as well as performance.
Questions people ask about reverse osmosis filter under sink
Which reverse osmosis filter should I change most often?
The sediment prefilter, because it takes the particulate load. The carbon prefilter is the most important to keep on schedule, since it protects the membrane from chlorine.
How do I know when the membrane needs replacing?
Compare total dissolved solids in the feed water and the product water with a meter. When rejection falls off despite fresh prefilters, the membrane is done.
Can I change just one cartridge instead of the whole set?
Yes, and it is usually cheaper. Sediment and carbon stages run on different schedules, and the membrane lasts far longer than either.
Why does the water taste odd after a filter change?
New carbon sheds fines and the system traps air. Flush a full tank to drain and let the system refill before judging the taste.