Water Quality
Activated Carbon Filters: Chlorine, Taste and Membrane Protection
Activated carbon works by adsorption rather than straining. Its surface is riddled with pores giving an enormous internal surface area — a single gram can have hundreds of square metres — and dissolved organic molecules, chlorine and odour compounds adhere to that surface as water passes through.
In an Indian water purifier, carbon appears in two places and does two different jobs. Before the RO membrane, it removes chlorine, and this is genuinely critical: chlorine attacks the thin-film polyamide layer of an RO membrane and destroys it. After the membrane, a post-carbon polisher improves taste.
The pre-carbon role is the one worth understanding, because it is where neglect becomes expensive.
Why chlorine removal matters so much
Municipal water is chlorinated deliberately, and that is a good thing — it is what keeps supply microbiologically safe in transit. But RO membranes are made of thin-film composite polyamide, and chlorine oxidises that material. Continuous exposure progressively destroys the membrane's selective layer, and the damage is permanent: no cleaning restores it.
The failure pattern is characteristic and easy to miss. The membrane keeps producing water at a normal flow rate, so nothing seems wrong, but its salt rejection falls steadily. Output TDS creeps up over months while the purifier appears to work perfectly.
This is why an exhausted carbon pre-filter is not merely a filter that has stopped filtering — it is an active threat to the most expensive component in the system.
Important
GAC vs carbon block
Channelling is the main weakness of loose GAC: over time, water erodes preferential paths through the granules and passes through with much less contact than intended. A carbon block has no such paths, which is why blocks generally give more reliable chlorine reduction per unit volume.
| GAC (granular activated carbon) | Carbon block | |
|---|---|---|
| Form | Loose granules in a cartridge | Compressed, extruded solid block |
| Flow rate | Higher | Lower for the same size |
| Chlorine removal | Good | Better — longer contact path |
| Particulate filtration | Minimal | Also filters to ~0.5–5 micron |
| Channelling risk | Yes — water can find low-resistance paths | No |
| Cost | Lower | Higher |
| Typical use | Pre-filter, whole-house | Pre- and post-filter, drinking water polishing |
What carbon does and does not remove
| Contaminant | Carbon effectiveness |
|---|---|
| Free chlorine | Excellent — the primary function |
| Chloramine | Moderate; needs longer contact or catalytic carbon |
| Taste and odour compounds | Excellent |
| Many organic chemicals and pesticides | Good, depending on molecule and contact time |
| Trihalomethanes (chlorination by-products) | Good |
| Some heavy metals (e.g. lead, with specialised media) | Partial — not a dependable barrier |
| TDS / dissolved salts | None |
| Hardness | None |
| Bacteria and viruses | None — and wet carbon can support bacterial growth |
| Arsenic, fluoride, nitrate | None |
Replacement, and why 'it still tastes fine' is not the test
- Replace pre-carbon every 6–12 months, and sooner on heavily chlorinated supply.
- Replace post-carbon annually — it affects taste rather than protection.
- Do not wait for taste to deteriorate. Chlorine breakthrough happens before you notice a change, and the membrane damage starts then.
- Adsorption capacity is finite. Once exhausted, carbon can begin desorbing — releasing previously captured compounds back into the water.
- Wet carbon is a hospitable environment for bacteria, which is another reason not to run cartridges long past their life, especially in warm conditions.
- If the purifier has been unused for weeks, flush several litres through before drinking.
Where to buy
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