Have you ever received a pop-up notification on your water purifier app saying, ‘The filter cartridge has reached the end of its lifespan; please replace it immediately’? As you look at the orange indicator light flashing on the machine, you can’t help but wonder: I’ve only been using it for four months—do I really need to replace it? Or is the retailer just trying to make a profit from selling replacement cartridges?
To be honest, 82 per cent of filter replacement reminders on the market are essentially just timers — they go off when a preset time is reached and have little to do with whether the filter is actually dirty. When deciding whether to replace a filter, it’s better to assess the water quality rather than rely on the time alone.
Different filters have vastly different lifespans
The various stages of filters in a water purifier perform different functions, so their lifespans naturally vary considerably.
The PP cotton filter acts as the first line of defence, specifically designed to trap large particles such as silt and rust. In new housing estates with normal water quality, a PP cotton filter will last 5 to 6 months without issue; however, in older estates or high-rise flats with secondary water supply systems, where pipes contain high levels of rust and silt, the PP cotton filter can become clogged in less than 3 months, causing the water flow to become weaker.
The activated carbon filter cartridge is responsible for adsorbing residual chlorine and odours. Ordinary granular activated carbon reaches saturation within 6 to 8 months, resulting in water that smells of bleach. Sintered coconut shell activated carbon has an adsorption capacity two to three times that of ordinary granular carbon and can typically last up to 12 months. If you simply follow the advice that ‘it needs replacing every 6 months’, you’ll be throwing money away on a perfectly good carbon filter.
The RO membrane is the heart of the water purifier, with a filtration precision of 0.0001 micrometres, capable of trapping heavy metals, bacteria and viruses. The lifespan of mainstream RO membranes is 2 to 3 years, whilst some products with anti-fouling coatings can last for over 3 years. However, do not be taken in by the ‘5-year replacement-free’ gimmick—behind such an extended lifespan, there is almost certainly a chemical scale inhibitor added; you must weigh up for yourself whether the water you drink is safe.
Ultrafiltration (UF) membranes require no electricity and produce no wastewater; they can trap bacteria and viruses but cannot filter out heavy metals or calcium and magnesium ions. They have a lifespan of one to two years; a noticeable reduction in water flow is a sign that they need replacing.
Don’t just stick to the manual; here are three ways to judge when to replace the filters
For pre-filters such as PP cotton and activated carbon, you can judge by sight and smell. Unscrew the filter housing and take a look: if the surface of the PP cotton is yellowed or blackened and has clearly hardened, it should be replaced even if it’s only been in use for two months; if the water from the activated carbon filter smells of chlorine or mould, it means it has reached adsorption saturation, and continued use will actually turn it into a breeding ground for bacteria.
When it comes to replacing the RO membrane, the TDS reading is the most reliable indicator. Buy a TDS meter for twenty or thirty yuan and measure the TDS of the purified water once in the morning and once in the evening for three consecutive days. The TDS of water from a newly installed RO system is generally within 10 ppm; if it consistently exceeds 30 ppm, you should be vigilant, and a stable reading above 50 ppm indicates a significant drop in desalination efficiency—replacement is essential.
There is also a traditional method: brew the same variety of green tea using both purified water and tap water, then leave them to stand for five minutes to observe the colour of the infusion. If the tea brewed with purified water appears cloudy and the tea leaves at the bottom look dark, this indicates that the activated carbon or RO membrane has developed a fault. Checking limescale when boiling water is another option—if the limescale at the bottom of the kettle after boiling purified water is as thick as that from tap water, the RO membrane is almost certainly no longer functioning properly.
A few of the most common sales pitches
‘PP cotton filters must be replaced every six months; failure to do so will cause secondary contamination’ – the most common ploy. Six months is merely a guideline; if your water quality is good and you use little water, the PP cotton filter may remain pristine even after eight months; if the water quality is poor, it may need replacing after just three months. Judging by water quality is far more reliable than sticking to a calendar.
“Failure to replace filter cartridges can cause cancer” — pure scare-mongering. Whilst expired filter cartridges do lead to a decline in water quality and potentially excessive bacterial levels, this is a world away from causing cancer. Vendors say this simply to make you anxious; once you’re anxious, you’ll obediently hand over your money.
“Replace all filters annually” — pure marketing spin. Actual test data shows that only the PP cotton and activated carbon filters have clear performance decline thresholds; the RO membrane can last for over 30 months provided the TDS value remains stable. Bundling all filters into a single replacement cycle isn’t about convenience; it’s about selling you a few extra filters.
‘RO membrane requires no replacement for 5 years’ – RO membranes with exceptionally long lifespans essentially rely on scale inhibitors to prolong their service life. Although phosphate-based scale inhibitors comply with current national standards for single-use water safety, these standards do not account for the cumulative risks associated with long-term consumption. For households with elderly people or children, it is safer to choose models that require no scale inhibitor additives.
Practical filter replacement: replacing as needed is more scientific than replacing by a set schedule
Where you live determines how long your filters will last
Water quality in China varies drastically between the north and south; the lifespan of the same filter cartridge can differ by as much as a factor of two depending on the region. In North and North-West China, water is relatively hard, with tap water TDS levels frequently exceeding 300 ppm, placing a heavy burden on both the PP cotton and the RO membrane. Field tests in an older residential estate in Beijing showed that, without a pre-filter installed, the PP cotton filter became completely clogged in less than three months. In these regions, it is recommended to install a pre-filter; a 40-micron filter can trap over 95 per cent of rust and silt, extending the lifespan of the PP cotton filter by more than double.
In southern regions, the water is generally softer, with TDS levels typically below 100 ppm; the PP cotton filter can last six to eight months without issue. However, in some southern cities where the water source contains higher levels of organic matter, the activated carbon becomes saturated more quickly; if the treated water develops an unpleasant odour, it must be replaced without waiting for the six-month mark.
In residential estates with secondary water supply systems in high-rise buildings—regardless of whether they are in the north or south—it is relatively common for water tanks not to be cleaned thoroughly, resulting in water containing significantly more impurities than that supplied directly by the municipal water network. In such cases, the PP cotton filter must be replaced within three months, whilst the activated carbon should be replaced every six months as a mandatory measure.
Water quality testing you can do at home
The most practical tool is a TDS meter, which costs around twenty to thirty yuan each; simply calibrate it with purified water before use. If the TDS value at the purified water outlet of a reverse osmosis (RO) system exceeds 30 ppm, this warrants attention; if it exceeds 50 ppm, the RO membrane must be replaced. Ultrafiltration systems do not alter the TDS value, so this indicator is not useful for ultrafiltration. Testing for residual chlorine is also straightforward. Residual chlorine test kits are available online for a few yuan a bottle; add two drops to the treated water – if the solution turns pale yellow or orange-red, this indicates that the activated carbon has become saturated and is no longer retaining residual chlorine.
The simplest method is to smell and taste the water. Turn on the tap and let the water run for 30 seconds, then take a sniff; if there is a smell of bleach, the activated carbon has failed. If the water tastes bitter or has a sweet aftertaste followed by astringency, this is also a sign that the filter cartridge needs replacing. Boil a kettle of purified water; if there is noticeably more limescale at the bottom of the kettle than before, it is highly likely that there is a problem with the RO membrane or the pre-filter cartridge.
Regular checks are more important than replacing filters on a fixed schedule. Take the PP cotton filter apart for a quick inspection every three months; replace the activated carbon filter if the water has an unusual odour; and monitor the TDS reading for the RO membrane—if you stick to these three points, you won’t be led by the nose by manufacturers’ filter replacement reminders.
There’s no one-size-fits-all answer for filter replacement; it can range from three months to three years.
It ultimately depends on your household’s water quality, water consumption and the quality of the filter cartridges.
Rather than being held hostage by manufacturers’ timers, why not spend twenty or thirty yuan on a TDS pen,
and let the data guide your decisions?
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