After the sediment filter, water passes through activated carbon. This stage is easy to forget, but it protects your RO membrane from chlorine and removes bad taste and smell. Here is what you need to know.
What does the carbon filter do?
- Removes chlorine from municipal water – chlorine damages the thin-film RO membrane.
- Reduces bad taste and odour, including the “medicine” smell of chlorinated water.
- Adsorbs some organic chemicals that give colour or smell.
It does not remove dissolved salts (TDS) – only the RO membrane does that.
CTO vs GAC: what is the difference?
| CTO (carbon block) | GAC (granular activated carbon) | |
|---|---|---|
| Form | Compressed carbon block | Loose carbon granules |
| Strength | Better chlorine and odour removal, finer filtration | Higher flow, lower pressure drop |
| Common use | Pre-carbon stage in domestic RO | Pre-carbon in some RO, commercial plants |
| Example | Blue Mount CTO 10 inch – ₹275 | Blue Mount BM 10 GAC – ₹275 |
Bigger systems use 20 inch CTO or 20 inch GAC cartridges. See all carbon filters.
Pre-carbon vs post-carbon
Pre-carbon (CTO/GAC) sits before the membrane to protect it. Post-carbon is a small inline filter after the tank that polishes taste just before the tap. Read post carbon filter guide.
How often to change the carbon filter
- Most homes: every 6–12 months.
- Heavily chlorinated or smelly supply: every 6 months.
- Change immediately if the RO water smells of chlorine or tastes odd.
Signs your carbon filter is exhausted
- Chlorine or bleach smell in the purified water.
- A musty taste even after the tank is drained and refilled.
- The membrane needs replacing much earlier than expected.
- It has been more than a year.
Changing filters yourself? Follow how to change RO filters at home.
Frequently asked questions
Can I use GAC instead of CTO?
Usually yes, if the size matches. CTO filters more finely; GAC gives slightly higher flow.
Does the carbon filter remove TDS?
No. Only the RO membrane lowers TDS.
Can a carbon filter be cleaned?
No – once the carbon is saturated it must be replaced.
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