Sodium Metabisulfite (SMBS)
Na2S2O5 · 2–5 mg/L · 97–98%
High-purity (97–98%) dechlorination agent that protects polyamide RO membranes from oxidative degradation — membranes begin to fail after roughly 1,000–2,000 ppm-hours of chlorine exposure. Dosed upstream of the RO cartridge filter at ~1–1.5 ppm per 1 ppm free chlorine; also used as a biostatic preservative for stored elements.
Specifications
| Chemical Formula | Na2S2O5 |
|---|---|
| Primary Function | Dechlorination — protects polyamide RO membranes from oxidation |
| Physical Form | White powder (97–98% purity) |
| Application Point | Upstream of RO cartridge filter |
| Typical Dosage | 2–5 mg/L (~1–1.5 ppm per 1 ppm free chlorine) |
| Packaging | 25 kg bags |
Where it's used
- RO Feed Dechlorination
- Membrane Preservation
- RO / NF Pre-treatment
- Biostatic Shock Treatment
- Chlorinated Feed Water
Related solutions
What it is and when to use it
Sodium metabisulphite (SMBS, Na₂S₂O₅) is a 97–98% pure dechlorination agent that removes free chlorine from RO feed water. Use it to protect polyamide RO membranes from oxidative damage, and as a preservative solution for storing membrane elements during shutdowns.
Polyamide RO membranes are damaged by chlorine. NWPL RO manuals set a free-chlorine limit of 0.1 mg/L, and membranes begin to fail after roughly 1,000–2,000 ppm-hours of exposure. SMBS reacts with free chlorine and neutralises it, so it is dosed upstream of the RO cartridge filter when feed water is chlorinated for disinfection or biofouling control.
The guide dose is about 1–1.5 ppm SMBS per 1 ppm of free chlorine, with a typical dosage of 2–5 mg/L. Residual chlorine or ORP should be measured after the dose point. It is supplied as white powder in 25 kg bags. Prepared solutions slowly lose strength when exposed to air, so make up fresh solution regularly.
Use it for
- Dechlorinating municipal or chlorinated feed before RO or NF
- Neutralising upstream hypochlorite dosed for biofouling control
- Preserving membrane elements during shutdown or storage
- Protecting chlorine-sensitive equipment downstream of disinfection
How to choose
- Measure the maximum free chlorine in the feed and dose at about 1–1.5 ppm SMBS per 1 ppm chlorine
- Install the dose point upstream of the cartridge filter to allow mixing and reaction time
- Monitor residual chlorine or ORP after dosing, with an alarm or interlock
- Prepare fresh solution regularly and keep containers closed
- Avoid large overdoses, which waste chemical and can encourage biological growth
Frequently asked questions
How much SMBS is needed to remove chlorine?
As a guide, dose about 1–1.5 ppm of SMBS for every 1 ppm of free chlorine in the feed, with a typical range of 2–5 mg/L. Base the design on the highest expected chlorine level, then confirm complete removal by measuring residual chlorine or ORP after the dose point.
Why must chlorine be removed before RO membranes?
Chlorine oxidises the polyamide layer that performs the salt rejection, causing permanent damage and falling rejection. NWPL RO manuals set a free-chlorine limit of 0.1 mg/L, and membranes begin to fail after roughly 1,000–2,000 ppm-hours of exposure. SMBS neutralises chlorine before it reaches the membranes.
Can SMBS be used to preserve stored RO membranes?
Yes. SMBS solution is widely used as a biostatic preservative for RO elements during shutdowns or storage, limiting biological growth while the plant is offline. Follow the membrane manufacturer's preservation procedure for concentration and replacement intervals, and flush the preservative out thoroughly before restarting.
Technically reviewed by Prashant Dwivedi, Co-Founder & Director ·