Chlorine is added to public water to prevent bacterial growth, but it also creates taste, odour, and filtration problems at home. Many people want to know if reverse osmosis can remove chlorine and make water cleaner and safer to use.
The answer is simple. An RO system does remove chlorine, but it is not the membrane that does the job. The carbon prefilters handle chlorine removal before water reaches the membrane. This filtration step protects the system and keeps the membrane working as intended.
This guide explains how chlorine is removed, why prefilters matter, and how RO systems treat both chlorine and chloramine.
How an RO System Works
A reverse osmosis system pushes water through a semi-permeable membrane that blocks dissolved solids, minerals, and many other contaminants. The membrane performs well only when chlorine is removed first. Even small amounts of chlorine can break down the membrane surface and lower its output.
RO systems prevent this by placing the carbon prefilters before the membrane.
Why Chlorine Cannot Reach the RO Membrane
The TFC membrane is sensitive to oxidants. Chlorine attacks the membrane material and causes permanent damage. This leads to poor flow, weak filtration, and costly repairs.
The membrane does not remove chlorine. It must be protected by filters designed for chemical removal.
What Removes Chlorine in an RO System?
Chlorine removal happens in the prefiltration stage. RO systems typically use two main filters before the membrane.
Sediment Prefilter
This filter removes sand, rust, dirt, and visible particles. Its only job is to protect the carbon filter and prevent clogging.
Carbon Prefilter
This is the filter that removes chlorine. Activated carbon captures chlorine through adsorption. As water passes through, chlorine sticks to the carbon surface and is removed before it can reach the membrane.
The carbon prefilter also improves taste and odor, which makes the final water cleaner and more pleasant to use.
How Carbon Filters Remove Chlorine
Activated carbon has a high surface area. When water flows through it, chlorine reacts with the carbon surface and becomes trapped. This is why carbon is used in almost every RO system.
Two primary carbon designs are used.
Granular Activated Carbon (GAC)
GAC contains loose carbon granules. It allows higher flow but can create small channels where water moves faster, lowering contact time.
Carbon Block
Carbon block filters are compressed into a solid structure. This increases contact time between water and carbon, which improves chlorine removal and reduces channelling.
Carbon block filters are often used in RO systems because they offer strong chlorine capture at consistent flow rates.
Free Chlorine vs Combined Chlorine
Public water can contain two forms of chlorine.
Free Chlorine
This is the active form that gives water its taste and smell. It reacts quickly and is easier to remove with standard carbon filters.
Combined Chlorine
This is chlorine that has already reacted with organic material. It is harder to remove and may require longer contact time or denser carbon media.
Chlorine vs Chloramine Removal
Municipal systems may use chloramine instead of chlorine. Chloramine is a mix of chlorine and ammonia, and it stays stable in water for a longer time. Standard carbon filters struggle with chloramine removal because the reaction is slower.
Catalytic Carbon for Chloramine
Catalytic carbon is treated at high temperatures to make it more reactive. It breaks the chlorine ammonia bond and removes chloramine more effectively than standard carbon.
RO systems that receive chloraminated water often include a catalytic carbon prefilter to protect the membrane and ensure proper chemical removal.
Why Contact Time Matters
Chlorine removal depends on how long the water stays in contact with the carbon media. Faster flow reduces contact time and weakens performance. RO systems balance flow, filter size, and carbon density to ensure proper chemical removal.
This contact time is one of the main reasons carbon block filters perform better for chlorine and chloramine.
Additional Stages in a Complete RO System
Many RO units include an extra carbon stage after the membrane.
Post Carbon Filter
This filter improves taste and removes any trace chemicals that may have passed through earlier stages. It does not handle chlorine removal but helps polish the final water quality.
Signs of Membrane Damage from Chlorine Exposure
If the carbon filter fails to remove chlorine, the membrane will show early damage. Common signs include:
- a sudden increase in TDS levels
- weak output pressure
- reduced water production
- faster fouling
Does an RO System Make Water Chlorine Free?
Yes. A properly designed RO system removes chlorine before the membrane. If the carbon filter is fresh and sized correctly, the water reaching the membrane will be free of chlorine.
If chlorine appears in RO water, it usually means the carbon filter is exhausted and needs to be replaced.
When to Replace Carbon Prefilters
Carbon filters should be replaced on time to maintain chlorine removal. Signs of exhaustion include:
- noticeable chlorine smell
- reduced RO output
- faster membrane fouling
- poor tasting water
Most systems require carbon filter changes every six to twelve months, depending on water usage and incoming chlorine levels.
Conclusion
Reverse osmosis removes chlorine, but the membrane does not handle this step. The carbon prefilters capture chlorine before it reaches the membrane, protect the system, and improve final water quality. With the right carbon media, RO systems can remove both chlorine and chloramine and deliver clean, great tasting water.