The same medium, two packages
Activated carbon removes chlorine, 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 surface area of roughly a rugby field). 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 — which 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.