An activated carbon filter treats water by adsorption: the media is processed to create an enormous internal surface area, and organic molecules, chlorine byproducts and the compounds behind bad tastes and smells stick to that surface as water passes through. Chlorine itself is handled slightly differently, by a chemical reaction at the carbon surface rather than by sticking. Both mechanisms depend on the water spending enough time in contact with the media.
Adsorption, and why contact time is everything
Carbon works because a small volume of media holds a colossal surface area of pores, and the target molecules are held there as water flows past. The controlling variable is contact time, sometimes written as empty bed contact time, which is simply how long the water spends in the media. Push water through faster than the bed is designed for and the same carbon removes noticeably less, which is why an undersized cartridge on a high-flow line disappoints even when it is brand new. It is also why whole-house carbon is sold as a deep tank of media rather than a small cartridge: the depth is what buys the time.
Granular carbon, carbon block and catalytic carbon
Granular activated carbon is loose media in a tank or cartridge. It gives low pressure drop and high flow, but water can find preferential paths between the grains, a problem called channeling, which reduces effective contact. A carbon block is powdered carbon bound into a solid cylinder, so water must pass through the structure itself. That gives better contact and, importantly, a mechanical filtration rating as well, so a block traps particles while it adsorbs. The trade-off is a higher pressure drop and faster clogging on dirty water. Catalytic carbon is surface-modified to promote chemical reduction, which is the usual answer for chloramine and for hydrogen sulfide odor.
What carbon does not remove
Carbon is not a universal filter, and treating it as one is a common and expensive mistake. It does not soften water, because hardness minerals are not adsorbed and pass straight through. It does not remove nitrate, fluoride or most dissolved salts. It has limited effect on many dissolved metals, and it is not a disinfectant: bacteria are not removed by carbon, and an exhausted bed sitting warm and wet can actually support biological growth, which is one reason cartridges should not be run indefinitely. If the water report shows hardness, nitrate or bacteria, the answer is a softener, a reverse osmosis membrane or a disinfection stage, with carbon alongside rather than instead.
Knowing when it is spent
Carbon gives almost no warning. It does not clog dramatically when its adsorption capacity is used up, and by the time chlorine taste has come back through the filter, the media has been passing other things for a while. That is why replacement is scheduled from the manufacturer's rated capacity and your water use rather than from taste. Whole-house tanks backwash themselves to keep the bed loose and avoid packing, and are rebedded after years rather than cleaned. Vendors publish their carbon systems in capacity and flow steps, which is the fair way to compare, and our whole house water filter guide covers where the carbon stage sits in the wider treatment train.
Questions people ask about activated carbon filter water
Does an activated carbon filter remove chlorine?
Yes, that is its most reliable job, along with the tastes and odors that come with it. Chloramine is harder and usually needs catalytic carbon with a longer contact time.
Will activated carbon soften hard water?
No. Hardness is dissolved calcium and magnesium, which carbon does not adsorb. Softening requires ion exchange, and scale control requires a conditioner, not a carbon filter.
Does carbon remove bacteria from water?
No. Carbon is not a disinfectant, and an exhausted bed can support biological growth. Use ultraviolet treatment, chlorination or an appropriately rated membrane where bacteria are a concern.
How do I know when to change a carbon filter?
Use the manufacturer's rated capacity and your household water use, not taste. Carbon stops adsorbing quietly, so waiting for the chlorine flavor to return means running past the point of effectiveness.