An alkaline filter is a cartridge of mineral media that water passes through slowly, dissolving a small quantity of calcium and magnesium and raising pH as a consequence. Fitted under a sink it is usually the last stage in a stack. What it delivers is a real and modest change in taste and chemistry, and what it is sold on is frequently a good deal more than that.
What the cartridge does
The media is consumed as it works, dissolving into the water rather than filtering it, so this is an additive stage rather than a removal one. The effect is largest on water that has very little dissolved content to begin with, which is why these cartridges are normally paired with reverse osmosis: fresh permeate is nearly mineral free and slightly acidic, and a small mineral addition lifts pH towards neutral and restores taste. On ordinary tap water that already carries minerals, the change is much smaller.
The two benefits that hold up
Taste is the honest one. Water stripped of minerals tastes flat, noticeably so in tea and coffee where mineral content affects extraction, and a small addition brings back the character people expect. The second is chemical stability: demineralized water is aggressive to metals it sits against, and adding mineral content reduces that. Both are real, both are modest, and both are reasons to fit the stage if you have reverse osmosis and find the water disappointing.
The claims that do not
The strong pitch is that drinking raised pH water changes body chemistry in beneficial ways. The stomach is strongly acidic and the body regulates blood pH within tight limits regardless of what is drunk, and the quantity of mineral a small cartridge adds is modest against what food supplies. Treat a pH figure printed on a box as a description of the water leaving the cartridge, not as a health specification, and be sceptical of anything promising more.
Fitting and replacing it
It goes last, after the membrane and after any polishing carbon, so nothing downstream removes what it has just added. Check tubing sizes before ordering a retrofit, and expect a slight reduction in flow to the faucet. Replace it on schedule rather than by symptom: the media is consumed, so an exhausted cartridge simply stops adding anything, with no pressure change and no clear taste cliff to warn you. Our under sink installation guide covers stage order in a full stack.
Questions people ask about under sink alkaline water filter
Does an alkaline filter remove contaminants?
No. It adds a small quantity of mineral. Removal is done by the stages ahead of it, typically a membrane or a carbon block.
Where in the stack does it go?
Last, after the membrane and any polishing carbon, so nothing downstream strips out the minerals it has just added.
Do the health claims about alkaline water hold up?
Treat them cautiously. The body regulates its own pH closely, and the mineral quantity a small cartridge adds is modest compared with diet.
How do I know when it needs replacing?
By schedule. The media is consumed rather than blocked, so it stops adding minerals without any pressure or obvious taste warning.