The same medium, two packages
Activated carbon removes taste, odour and a broad range of organic compounds by adsorption: contaminants stick to an enormous internal surface area (a single gram of activated carbon carries the internal surface of a tennis court or more). Free chlorine is handled differently: the carbon surface catalytically destroys it, reducing it to chloride. Both cartridge styles exploit this; the difference is the format.
Granular activated carbon (GAC) cartridges are a container of loose carbon granules the water flows through. Carbon block (CTO: chlorine, taste, odour) cartridges grind the carbon fine, mix it with a binder, and extrude it into a solid porous block the water is forced through.
What the format changes
| GAC | Carbon block | |
|---|---|---|
| Contact with carbon | Water finds paths between granules | Every drop forced through the block |
| Channelling | Yes — flow wears preferential paths | None |
| Carbon particle size | Coarse granules | Fine powder = far more accessible surface |
| Chlorine/taste/odour performance | Good when new, degrades as channels form | Better, and holds its rating for its rated volume |
| Sediment removal | Effectively none | Doubles as a 1 – 10 µm nominal sediment filter |
| Pressure drop | Very low | Meaningful — check the flow rating |
| Carbon fines in water | Releases fines at start-up (flush required) | Minimal |
| Cost | Lower | Higher |
The performance difference has one root cause: channelling. In a GAC bed, water gradually carves preferred paths and an increasing fraction slips past most of the carbon. A block cannot channel. That is why an equal-sized block outperforms GAC on chlorine, and why the demanding certified claims such as cyst and lead reduction are block territory: they lean on the fine, uniform pore structure a block also gives you. That is not the same as saying GAC cannot be certified. Certified granular-carbon cartridges are common for chlorine, taste and odour. Certification attaches to a tested product against a named standard, never to a construction type.
Choosing between them
- High flow, low restriction, "improve the taste": GAC. Whole-house duty, tank polishing, anywhere the pressure budget is tight and the goal is general improvement rather than a guaranteed number.
- Point-of-use drinking water where the claim must be provable: usually a carbon block, but specify the certified product rather than the material. Blocks carry the widest range of certified claims (chlorine, taste, odour, and on some of them lead, cysts and VOCs), while certified GAC cartridges cover the chlorine, taste and odour end. Ask for five things: the certifying body, the standard, the contaminant claimed, the rated flow, and the capacity in litres. A claim missing any of those is marketing, not certification.
- Protecting an RO membrane: either works for free chlorine when correctly sized, but remember the block's pressure drop on small pumped systems, and remember any carbon stage is consumable. A spent carbon cartridge protects nothing.
- Chloraminated municipal supply: standard carbon of either format struggles; specify catalytic carbon and accept a larger contact volume.
Sizing honestly
Adsorption needs contact time. Every carbon cartridge has a rated flow, and running double the rated flow does not halve performance. It can collapse it, because contact time falls below what adsorption needs. If the duty flow exceeds the cartridge rating, fit cartridges in parallel or move to a GAC media vessel, which is the same choice at the next scale up.
And always put a sediment cartridge ahead of carbon: dirt that blinds the carbon's surface is dirt occupying capacity you paid for. A 5 µm spun-bonded cartridge in front extends a block's life, by an amount the silt load sets rather than a fixed multiple.
Rule of thumb: GAC for flow, block for the widest range of certified claims. Where a number must be met at the tap, work from the certificate for that exact cartridge rather than from its construction; where the water just needs to taste better without losing pressure, GAC earns its keep.