A faucet attachment screws onto the aerator thread of a kitchen tap and diverts flow through a small carbon cartridge, usually with a lever that switches between filtered and unfiltered. It is the cheapest way to improve drinking water at a sink, needs no plumbing and no cupboard space, and is the right answer in rentals and in kitchens where nothing under the sink can be altered.
Will it fit your tap
This is the first question and it fails more purchases than performance does. The unit needs a removable aerator with a standard male or female thread, and adapters are usually supplied for both. It will not fit a pull out or pull down spray head, which is now the common kitchen tap, and it will not fit taps with a non standard or hidden aerator, or those where the aerator is a cache design needing a special key. Unscrew your aerator before ordering and see what thread is underneath. If nothing comes off, this is not your product.
What the cartridge treats
These are carbon devices. They address chlorine taste and odor, and depending on certification they may be rated for lead, cysts, some volatile organics and certain pesticides. What they do not do is remove dissolved minerals, so hardness, scale and spotting continue unchanged, and they are not a substitute for reverse osmosis if the concern is nitrate, arsenic, fluoride or total dissolved solids. Match the certification claims on the box to what your water actually contains rather than to the longest list of names printed on the packaging.
Living with one
The flow through the filtered path is slower than the tap's own, which is the trade for treating water in a small cartridge, and filling a large pot takes noticeably longer. Cartridge life is short compared with an under sink unit, because there is very little carbon in the housing, and hot water must never be run through the filtered path since it damages the media and can release what it has captured. The switching lever is a wear point on cheaper models. Our faucet filter comparison covers cartridge cost per year, which is where these stop being cheap.
When to spend more instead
If the household drinks and cooks a lot of filtered water, the cartridge cost of a faucet attachment overtakes an under sink system within a couple of years while delivering less capacity and slower flow. If the tap is a pull out spray, if the aerator will not come off, or if the concern needs reverse osmosis rather than carbon, then a countertop unit or an under sink installation is the honest recommendation and a faucet attachment simply will not reach it.
Questions people ask about water filter faucet attachment
Will it fit a pull out spray tap?
No. These need a plain removable aerator with a standard thread. Pull out and pull down spray heads cannot take a faucet attachment.
Does it soften the water?
No. It is a carbon device, so dissolved calcium and magnesium pass through and scale and spotting are unaffected.
Can I run hot water through it?
No. Hot water damages carbon media and can release what the cartridge has already captured. Switch to unfiltered for hot.
How long does the cartridge last?
Far less than an under sink unit, because the housing holds very little carbon. Heavy filtered water use makes an under sink system cheaper within a couple of years.