Mashable covered a countertop water filtration system. Water filters are one of those products bought largely on marketing, and the categories genuinely do different things, so it is worth knowing which problem each one solves.
Activated carbon
This is what is inside almost every jug filter and most fridge and tap filters.
Carbon is processed to be extraordinarily porous, giving an enormous internal surface area. Contaminants stick to that surface, a process called adsorption, and it works well for the things that make water taste unpleasant: chlorine, chloramine to a lesser degree, and many organic compounds.
What carbon does not remove is dissolved minerals, nitrates, fluoride or most salts. Those pass straight through, which is why a jug filter does not soften hard water despite many people believing it does.
Carbon block filters, where the carbon is compressed into a solid, perform considerably better than loose granular carbon because the water cannot channel around the media. Lead reduction claims generally depend on a carbon block with a specific treatment, not on carbon alone.
Reverse osmosis
RO pushes water under pressure through a membrane with pores small enough to reject dissolved ions. It removes the widest range of contaminants of any household method, including the dissolved solids carbon leaves behind.
The trade-offs are real. It is slow, so systems use a storage tank. It wastes water, historically several litres rejected per litre produced, though newer systems are better. It strips beneficial minerals along with everything else, which is why many units add a remineralisation stage. And it needs adequate mains pressure.
RO is the right answer for a specific, identified contaminant problem. It is overkill for water that simply tastes of chlorine.
Ion exchange
Resin beads swap one ion for another. Water softeners exchange calcium and magnesium for sodium, which is why softened water feels different and why it adds a small amount of sodium.
This is the technology that actually addresses limescale. Many jug filters include a small amount of ion exchange resin alongside carbon, which is why they reduce scale in a kettle modestly, but the capacity is far too small to soften a household supply.
UV, and what it is not for
Ultraviolet light disrupts the DNA of bacteria and viruses so they cannot reproduce. It is effective disinfection and adds nothing to the taste.
It removes no chemicals whatsoever, and it requires clear water to work, since particles shade organisms from the light. UV is a companion to filtration, never a replacement, and in a treated mains supply it is usually solving a problem you do not have.
Start with what is in your water
This is the step nearly everyone skips, and it determines everything else.
In the UK, the EU and the US, your water supplier publishes a quality report for your area, free and usually searchable by postcode. It lists hardness, chlorine treatment, and results for regulated contaminants.
Read it before buying anything. If the answer is “hard water and a chlorine taste”, a carbon jug and a kettle descaler solve it for very little money. If there is a specific contaminant of concern, that dictates a specific technology, and general-purpose filters may do nothing for it.
Older properties are the main exception worth acting on independently, because lead service pipes are a property-level issue that a district-level report will not reveal.
The maintenance point that matters
Change cartridges on schedule. A saturated carbon filter stops adsorbing, and a wet filter left in a warm jug is a good environment for bacterial growth.
An overdue filter is worse than no filter, which is the opposite of how most people treat them.
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