Activated carbon is the most widely used media in domestic water treatment and the most widely misunderstood, because adsorption is selective in a way straining is not. Carbon holds onto molecules that stick to its enormous internal surface area, and everything that does not stick passes through, which produces a fairly clean division between what it handles and what it ignores.
What it removes well
Free chlorine, quickly and reliably, which is the most common reason it is fitted. The taste and odour compounds behind most complaints about tap water, including the earthy and musty notes from natural organic material. Many organic chemicals, including a range of solvents and pesticides, depending on the grade and the contact time. And, in a fine carbon block, particles down to the block's rated size, which is how blocks can carry certifications for cysts and for particulate lead that loose media cannot.
What it handles only in the right form
Chloramine needs catalytic carbon and substantially more contact time, because it is deliberately stable; standard carbon reduces it slowly and incompletely, which is the most common disappointment in this category. Lead is addressed only by blocks specifically certified for it, since a general taste filter is not tested for it and should not be relied on. And volatile organic compounds vary widely, so a certification naming the specific compound is worth more than a general claim.
What it does not touch
Hardness, so it will not stop limescale, and a household fitting carbon for scale will see no change at all. Dissolved iron and manganese in any useful quantity. Nitrate, fluoride, dissolved salts and total dissolved solids, which come out roughly as they went in. And it does not disinfect, although a block filters mechanically. Those jobs belong to ion exchange, oxidizing filters, membranes and disinfection stages, and carbon usually works alongside one of them rather than instead of it.
Why performance drops without warning
Carbon has a capacity rather than a visible end point. Spent media looks identical to fresh, gives no pressure warning and often no taste warning until well past the point where it stopped working, and a warm wet cartridge held past capacity is somewhere bacteria will grow. Contact time also decides everything: the same cartridge performs better at a slow tap than on a whole house line at peak flow. Change on throughput, and put sediment ahead of it. Our replacement filter guide covers matching cartridges.
Questions people ask about what do carbon filters remove from water
Do carbon filters remove hardness?
No. Calcium and magnesium pass straight through, so a carbon filter changes nothing about limescale.
Do they remove lead?
Only blocks specifically certified for lead reduction. A general taste and odour filter has not been tested for it.
Why does my carbon filter not deal with chloramine?
Chloramine needs catalytic carbon and much longer contact time. Standard carbon reduces it slowly and incompletely.
How do I know when carbon is spent?
By throughput, not appearance. Exhausted media looks identical to fresh and gives no pressure or taste warning.