A reverse osmosis system holds several cartridges that look similar and are not, and they run on different clocks. Two are changed on a calendar, one on measurement and one is routinely forgotten. Getting them straight is what keeps the system performing and prevents the expensive component being damaged by a cheap one that was left in too long.
The two prefilters
A sediment cartridge first, taking the mechanical load, which usually announces itself by slowing production. Then a carbon cartridge, whose job is to remove chlorine before it reaches the membrane, because chlorine degrades thin film composite material permanently. That one shows nothing at all when it is spent: no pressure change, no taste warning, just chlorine passing through to the membrane. Both are replaced annually in a typical household, sooner on dirty feed water, and this is the cheap job that protects the expensive one.
The membrane
Physically different, housed in its own pressure vessel rather than a filter sump, and described by a size code and a production rating. It is replaced on evidence rather than on a calendar: compare a dissolved solids reading of the feed with one from the membrane output taken before the storage tank, and replace when the readings have converged. Note that a higher rated membrane of the same size will fit but changes the system's balance, since the flow restrictor is matched to production.
The polishing and remineralization stages
A small carbon filter after the tank gives the water its finished taste and goes on the same annual schedule as the prefilters. A remineralization cartridge, where fitted, is the one that gets forgotten: its media is consumed rather than blocked, so it produces no pressure change and no clear taste cliff, and the water simply goes flat again over months. Put it on the annual schedule rather than waiting for a symptom that arrives too gradually to notice.
Buying and fitting the set
Buy the annual set as a kit for your specific system rather than piece by piece, and add spare housing o-rings and food grade silicone grease, since the o-ring is what turns a filter change into a leak. Note which cartridges are directional and fit them the right way round. Sanitize while the housings are open, with the membrane and carbon removed first. Then flush as the manual states before drinking. Our reverse osmosis guide covers matching cartridges to the common systems.
Questions people ask about reverse osmosis filter cartridges
How many cartridges does an RO system have?
Typically a sediment prefilter, a carbon prefilter, the membrane, a polishing carbon and sometimes a remineralization cartridge.
Which run on a calendar and which on measurement?
Prefilters, polishing and remineralization on a calendar; the membrane on a dissolved solids comparison before the tank.
Which one is most often forgotten?
The remineralization cartridge. Its media is consumed silently, so the water goes flat again with no other warning.
Can I fit a higher rated membrane?
Physically yes, if the size code matches, but the flow restrictor is matched to production and usually has to change with it.