Carbon water filtration works by adsorption: dissolved molecules stick to the enormous internal surface area of activated carbon as water passes through it. It is the workhorse of household water treatment because it handles the problems people actually notice, chlorine taste and odor and a wide range of organic compounds, and because it protects other equipment downstream.
Adsorption, not straining
Activated carbon is made from coconut shell, coal or wood, processed so that each grain is riddled with pores giving a vast internal surface. Contaminants with an affinity for that surface attach to it and stay there while the water carries on. Because the mechanism is chemical attraction rather than physical blocking, two things follow. Carbon has a finite capacity and eventually saturates, at which point it stops working while still flowing perfectly. And it is selective: it is very good at chlorine, taste, odor and organics, and does essentially nothing for hardness, nitrate or dissolved minerals, which have no affinity for the surface.
Granular, block and catalytic
Granular activated carbon is loose media, used in tank systems and in some cartridges. Water moves through the bed easily, so pressure drop is low and flow is high, but water can find preferential channels through the grains and get less contact than the bed size suggests. A carbon block is powdered carbon bound into a solid extruded cylinder, giving far more consistent contact and a mechanical filtration rating as a bonus, at the cost of higher pressure drop and a shorter life on dirty water. Catalytic carbon is a modified surface that handles chloramine, which ordinary carbon struggles with, and is the correct specification where a utility uses chloramine rather than free chlorine.
Contact time and correct sizing
Adsorption needs time, so the amount of carbon and the speed of the water through it decide how well a filter performs. A whole house tank is sized against peak household flow so the water spends long enough in the bed, and a cartridge is rated for a flow beyond which its performance falls away even though water still comes out of the tap. This is the reason a small under sink cartridge cannot be scaled up to a whole house by fitting it to a bigger pipe. If you are planning a full treatment train, our well water filtration system guide covers where carbon sits relative to sediment, iron and softening stages.
Exhaustion, backwashing and bacteria
Carbon gives no warning when it is spent, which is why replacement intervals are set by volume or time rather than by taste. Tank based systems backwash periodically to lift and reclassify the bed, clearing trapped sediment and preventing packing, and their media is changed on a multi year cycle. Cartridges are replaced whole. Two practical cautions: a carbon bed that sits wet and unused can support bacterial growth, so a filter in a vacation home should be sanitized or replaced on return; and carbon removes the chlorine that was protecting the plumbing downstream, which is why carbon and UV are often specified together on treated supplies.
What carbon should be paired with
Carbon is nearly always one stage in a sequence. Sediment filtration goes ahead of it so grit does not blind the surface and shorten its life. On municipal supplies carbon protects softener resin and reverse osmosis membranes from chlorine, which is a large part of why it is fitted at all. Where hardness, iron, nitrate or arsenic are the problem, carbon is not the answer and the correct media or process goes in alongside it. Judged as a single purchase carbon looks limited; judged as the stage that fixes taste and protects everything else, it is the most useful filter in the house.
Questions people ask about carbon water filtration
Does carbon water filtration remove hardness?
No. Calcium and magnesium have no affinity for the carbon surface, so hardness passes straight through. Softening requires ion exchange with a resin bed, which is a different appliance entirely.
What is catalytic carbon for?
Chloramine. Utilities that use chloramine rather than free chlorine defeat ordinary activated carbon, so catalytic carbon with its modified surface is specified where chloramine is in the supply.
How do I know when a carbon filter is exhausted?
You generally cannot tell by looking or tasting, since many adsorbed compounds are tasteless. Replace on the rated volume or time interval rather than waiting for a change at the tap.
Is a carbon block better than granular carbon?
It gives more consistent contact and adds mechanical filtration, but at higher pressure drop and shorter life on dirty water. Granular beds suit high flow whole house duty where backwashing is available.