What does a water filter actually remove? A plain-English guide

“Removes impurities” is one of the most common phrases on water filter packaging — and one of the least useful. Different filters use different technologies, and each one is good at dealing with some substances and does little for others. Understanding the basics helps you choose a filter that solves your actual problem, instead of paying for claims that do not apply to your home.

Activated carbon: taste, smell and chlorine

Activated carbon is in almost every home water filter, from jugs to under-sink systems. It is carbon treated to have an enormous internal surface area, which traps certain substances as water passes through.

Activated carbon is especially good at reducing chlorine, along with many of the compounds responsible for unpleasant tastes and smells. That is why it is the go-to choice if your main complaint is that tap water tastes “of the swimming pool”. The Drinking Water Inspectorate mentions activated carbon filters as one option for reducing chlorine taste, provided you follow the manufacturer's instructions carefully.

What carbon does not do well on its own is remove dissolved minerals. It will not, by itself, stop limescale.

Ion exchange resin: limescale and some metals

Ion exchange resin consists of tiny beads that swap certain dissolved substances in the water for others. In filter jugs, the resin mainly targets calcium and magnesium — the minerals that make water hard and form limescale in kettles. Some resins also reduce certain metals such as copper and lead.

Most filter jug cartridges in the UK combine activated carbon and ion exchange resin, which is why they improve taste and reduce limescale at the same time. The resin has a limited capacity, though: in a very hard water area it is used up faster, which is one reason to change cartridges on time.

Membrane filtration: from bacteria to chemicals

Membrane filters work by physically blocking particles larger than their pores. The US Centers for Disease Control and Prevention describes several levels:

• Microfiltration (pores of about 0.1 micron) removes parasites and is partly effective against bacteria.

• Ultrafiltration (about 0.01 micron) removes parasites and bacteria and is partly effective against viruses.

• Nanofiltration (about 0.001 micron) also deals with viruses and some chemicals.

• Reverse osmosis (about 0.0001 micron) removes germs and a range of chemicals, including lead and copper.

For homes on a mains supply in the UK, germs are not normally the concern — public water is treated and monitored before it reaches you. Membrane systems are more relevant if you have a specific chemical concern, or if your home is on a private supply such as a well or borehole.

What else is on the list

• Distillation boils water and collects the steam, removing germs and many chemicals, but it uses a lot of energy.

• Ultraviolet (UV) treatment disinfects water against germs and is usually combined with pre-filtration. It does not remove chemicals.

• Water softeners remove only hardness minerals such as calcium and magnesium. They protect pipes and appliances but are not drinking water filters.

How to read the claims on the box

The most reliable way to know what a filter does is independent certification. In the US and internationally, filters are commonly certified against NSF/ANSI standards. Two of the most relevant are:

• NSF/ANSI 42 — covers aesthetic effects such as chlorine taste and odour, and particle reduction.

• NSF/ANSI 53 — covers health-related contaminants. The US Environmental Protection Agency points people looking for lead reduction to filters certified by an accredited third party against both standards 53 and 42.

Certification applies to specific substances, not to the filter in general. A product certified to reduce chlorine is not automatically certified to reduce lead. When you compare filters, look for the exact substance you care about on the certificate or product sheet, not just a general claim.

What a filter will not do

• A filter cannot fix problems caused by your plumbing if the pipes themselves are the source. Old lead pipes, for example, are best replaced — see our guide on lead pipes in older homes.

• A poorly maintained filter can make water worse rather than better, as deposits and bacteria build up in an old cartridge.

• Most jug and tap filters are not designed to make untreated water safe to drink. If your water comes from a private supply, get it tested and get specialist advice.

The short version

Carbon for taste and chlorine, ion exchange for limescale, membranes such as reverse osmosis for a wider range of contaminants. Decide what you want to reduce, then choose a filter certified for that. For an overview of filter types and how to compare them, read Jug, tap or under-sink: which water filter suits your home?