Know which “filter” you mean
An RO system can contain sediment, carbon, membrane, post-carbon and remineralisation stages. They age for different reasons. A manufacturer advertising a “one-year filter” may package several stages into one cartridge; another system may have separate six-month prefilters and a multi-year membrane.
Time versus capacity
Some cartridges have both a time limit and a litre/gallon capacity. Heavy use can reach capacity first. Very light use does not necessarily make a wet cartridge safe indefinitely; hygiene and material ageing still matter.
Why carbon timing matters to the membrane
Where the membrane is chlorine-sensitive, exhausted carbon can expose it to oxidant and shorten membrane life. Waiting for TDS rejection to collapse is therefore not a sensible way to decide when to replace every prefilter.
When to investigate early
Unexpected flow loss, taste/odour, pressure change, a sustained decline in rejection, filter-life warnings or known poor feed-water events deserve attention. Diagnose rather than blindly replacing the most expensive part.
After long non-use
Follow the manufacturer's restart/storage rules. Standing water and wet cartridges create different hygiene considerations from ordinary daily operation.
Keep dates on the machine
A simple service label or phone calendar entry prevents the most common maintenance failure: forgetting when the cartridge went in.
For the complete routine, see RO maintenance and troubleshooting.
A cartridge can look clean while its adsorption capacity is exhausted, and a discoloured sediment filter is not automatically unsafe. Replacement decisions should follow the maker's schedule, capacity and performance indicators rather than colour alone.
Do not buy filters purely by appearance
If a carbon prefilter is rated for twelve months or a stated capacity, a two-person household and a large family may reach the limits differently. Likewise, a membrane fed with hard, high-TDS water at marginal pressure may not age like the same membrane on softer, well-pretreated water. Calendar reminders are useful, but they should sit alongside the manufacturer's capacity and operating limits.
A practical example
Sources and further reading
For factual claims that matter to health, regulation or product performance, we prefer primary and official sources. Useful references for this guide include:
- Drinking Water Inspectorate — Physical removal of contaminants
- NSF — NSF/ANSI 58 reverse osmosis systems
Related guides
Last reviewed: 6 October 2026. This guide is written for a UK audience. Always follow current supplier, regulator, manufacturer and healthcare advice where it applies.