A reverse osmosis system has stages with different lives, and treating them as one service item is how households either waste money or ruin a membrane. The prefilters are changed often and on a calendar. The membrane is changed rarely and on evidence. The polishing filter is changed with the prefilters. Doing it in the right order, cleanly, takes under an hour.
Which stage has which life
The sediment prefilter takes the mechanical load and is usually the first to block, which shows as slow production. The carbon prefilter protects the membrane from chlorine and has a capacity rather than a visible end point, so it goes on the calendar whatever it looks like. The membrane itself lasts years when those two are maintained, and is judged by output quality rather than by time. The final polishing carbon sits after the tank and gives the water its finished taste. Changing prefilters on schedule is what buys membrane life, so the cheap job protects the expensive one.
The sequence, step by step
Close the feed valve and the tank valve, then open the RO faucet to release pressure and leave it open. Have a bowl ready, because each housing holds water. Work from the first stage forward, taking one housing off at a time so nothing is mixed up. Wipe each housing out, clean the o-ring groove, lubricate the seal with food grade silicone and reseat it properly, since a twisted o-ring is the usual cause of a leak found the next morning. Fit the new cartridge the right way round, hand tighten, and only then move to the next stage.
Sanitize while it is open, then flush properly
The inside of an open housing is the one moment you can sanitize the system, and it is worth taking. Follow the manufacturer's method, which usually means a measured dose of unscented household bleach or a proprietary sanitizer introduced into the first housing with the membrane and any carbon cartridges left out, circulated, then thoroughly flushed. Never leave the membrane in place while sanitizing with chlorine, because chlorine destroys it. Afterwards, refit everything, open the feed slowly, check for leaks by hand and again an hour later, then fill and drain the tank the number of times the manual states.
Knowing when the membrane is actually finished
Do not replace a membrane on a calendar. Measure. Take a total dissolved solids reading of the feed water and another of the treated water taken before the tank, and compare. A membrane doing its job leaves a wide gap between the two, and a spent one lets them converge. Slow production alone is not evidence, since a blocked prefilter, a waterlogged tank and low line pressure all cause it. Our RO filter guide covers matching replacement cartridges to a system and keeping the intervals from slipping.
Questions people ask about changing ro filters
How often should RO prefilters be changed?
On the manufacturer's schedule, typically annually for a household, and sooner where the feed water is dirty. They protect the membrane, so slipping the interval is expensive.
Can I sanitize the system with the membrane in place?
No. Chlorine based sanitizer destroys the membrane permanently. Remove the membrane and any carbon cartridges before sanitizing, then refit and flush.
Why is the water still slow after new prefilters?
Check the storage tank air charge and the line pressure. A tank that has lost its charge or a partly closed feed valve mimics a blocked filter closely.
Do I need to flush after every filter change?
Yes. New carbon releases fines and new cartridges carry manufacturing residues, so fill and drain the tank as the manual states before drinking any of it.