- →Carbon works through adsorption — chemicals bind to the surface. When all the binding sites are full, the filter is exhausted — even if water flows normally.
- →Carbon block outperforms GAC. It forces water through the full matrix with no channelling, giving consistent contact time and better contaminant removal.
- !For Sydney, Brisbane, Adelaide and GWW Melbourne — standard GAC has limited effectiveness against chloramine. You need catalytic carbon with NSF 42 chloramine claim.
- ✗Carbon cannot remove fluoride, hardness, nitrates, PFAS or lead (without NSF 53). These require reverse osmosis.
- !An expired carbon filter can harbour bacteria. Replace on schedule — do not wait until you notice a taste change.
Adsorption — not absorption
Activated carbon removes chemicals through adsorption — contaminants bind to the surface of the carbon, not into it. The distinction matters because the carbon has a finite number of binding sites. When they are all occupied, the filter is exhausted — and water flows through normally with no indication that filtration has stopped.
A single gram of activated carbon has approximately 500 square metres of internal surface area — roughly the floor area of a large house. This enormous surface area is what makes carbon so effective at capturing taste and odour compounds, chlorine, chloramine, and volatile organic compounds.
GAC vs Carbon Block
Granular Activated Carbon (GAC) uses loose carbon granules. Water can find the path of least resistance between granules — called channelling — reducing contact time with the carbon surface. GAC is used in pitcher filters, some whole-home pre-filters, and inline fridge filters. Cheaper to make, less consistent performance.
Carbon Block compresses carbon into a solid matrix with a defined structure. Water is forced through the full cross-section, ensuring consistent contact time. More effective for chloramine, lead (when NSF 53 certified), and volatile compounds. The standard for quality under-sink and whole-home systems.
Contact time is everything in carbon filtration. Shower filters fail against chloramine not because the carbon is poor quality — it is because water passes through in under one second at shower temperatures and pressures. The physics simply do not allow meaningful adsorption at that speed.
The chloramine problem
Standard activated carbon removes free chlorine effectively. Chloramine is different. Monochloramine requires either much longer contact time or a different type of carbon.
Catalytic carbon has a modified surface chemistry that specifically facilitates the breakdown of chloramine. If you are in Sydney, Brisbane, Adelaide, or Greater Western Water areas of Melbourne — all chloramine cities — standard GAC or basic carbon block has limited effectiveness. You need catalytic carbon with an NSF 42 chloramine claim. Verify at nsf.org.
What activated carbon cannot do
Carbon is frequently marketed in ways that imply broad-spectrum filtration. The reality is specific and limited:
- Fluoride — essentially no effect. Only RO removes fluoride meaningfully.
- Hardness — dissolved calcium and magnesium pass straight through carbon.
- Nitrates — not addressed. RO removes 87–90%.
- Lead — standard carbon does not remove lead. NSF 53 certified carbon block with a specific lead claim does.
- Bacteria — GAC and most carbon blocks do not remove bacteria. Expired carbon in warm conditions can actually support bacterial growth.
- PFAS — partial reduction at long contact times only. NSF 58 RO achieves 90–96%.
An expired carbon cartridge provides the same flow rate as a new one but zero filtration benefit — and may harbour bacteria. Replace on schedule regardless of whether you notice a taste change.
For chloramine cities (Sydney, Brisbane, Adelaide, GWW Melbourne): catalytic carbon with NSF 42 chloramine claim only. Standard GAC pitcher filters provide limited benefit in these cities.
For fluoride, nitrates, lead, PFAS, or hardness: carbon alone is insufficient regardless of quality or certification. Those require RO or dedicated treatment stages.