Activated carbon and SMBS dosing are the two ways of neutralising free chlorine ahead of a membrane. When to prefer which.
Because membranes have a low tolerance to chlorine, free chlorine has to be removed before it reaches the membrane. There are two common ways of doing that in the field: an activated carbon filter and SMBS (sodium metabisulfite) dosing.
Activated carbon retains chlorine on its surface; SMBS reduces it chemically, that is, neutralises it. The two can be alternatives to each other, and on critical systems they are also used together.
When the bed is correctly sized and the contact time is sufficient, it removes the great majority of free chlorine. It requires no chemical dosing and is simple to operate.
On the other hand the bed’s capacity fills over time and it can start letting chlorine through without warning. Measuring free chlorine at the carbon outlet is therefore safer than trusting the filter.
SMBS dosing is a more controlled method of chlorine removal: the dose is set against a measurement and the result can be verified. It requires chemical stock and maintenance of the dosing pump.
Setting the dose correctly matters; too little lets chlorine through, while too much means wasted chemical and, in some cases, other problems in the system.
The decision depends on the chlorine level in the raw water, the flow rate and operating preference. If chemical-free operation is the goal, activated carbon comes first; if you want protection that can be measured and verified, SMBS does.
On critical processes, arranging both together and placing a chlorine measurement at the carbon outlet is the safest way to protect the membrane.
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