SMBS dechlorination — why it matters and how much to dose
Short answer: Chlorine causes permanent, irreversible damage to thin film RO membranes. SMBS (sodium metabisulphite) removes it chemically. The dose ratio is roughly 3 parts SMBS to 1 part chlorine, with a margin — so for 1 ppm chlorine you dose around 3 to 5 ppm of SMBS.
If your feed comes from a municipal supply, chlorine is present, and without dechlorination your membranes are being damaged continuously.
Why chlorine destroys membranes
Thin film composite membranes have a polyamide active layer. Chlorine attacks the polyamide bonds directly, and the damage cannot be reversed by any cleaning.
The signature of chlorine damage is distinctive:
- Permeate flow increases — because the membrane now has holes
- Permeate TDS rises sharply — because it is no longer rejecting salts
How much chlorine is too much?
Membrane life is usually expressed in ppm-hours of chlorine exposure. What that means practically:
- A high chlorine level for a short time causes damage
- A low chlorine level continuously also causes damage, just more slowly
So the target before the membrane is zero, not "low".
How SMBS works
Sodium metabisulphite reacts with free chlorine and neutralises it. It is fast, inexpensive and the standard method in RO plants.
Working out the dose
Example: feed 1,000 LPH with 1 ppm free chlorine.
- SMBS needed: 1 ppm × 3 = 3 ppm, plus margin → say 4 ppm
- Grams per hour = (1,000 × 4) ÷ 1,000 = 4 grams per hour
Made up as a 5 percent solution, that is 80 ml per hour from the dosing pump. The method is the same as in our dosing calculation guide.
Where to inject
SMBS goes in upstream of the antiscalant injection point and well before the membrane, so there is time for the reaction to complete.
Order along the line:
- Raw water
- SMBS injection
- Cartridge filter
- Antiscalant injection
- High pressure pump
- Membranes
Verifying chlorine is at zero
Do not assume — test. Use a DPD test kit on the water just before the membrane. Free chlorine should read zero.
| Reading before membrane | Action |
|---|---|
| Zero | Correct |
| Any measurable chlorine | Increase SMBS dose, check the pump |
| Zero but membranes still failing | Check for chlorine spikes in the supply |
Watch for spikes. Municipal supplies sometimes raise chlorine levels sharply, and a dose set for the normal level will not cope. This is a common cause of unexplained membrane failure.
SMBS or activated carbon
| SMBS dosing | Activated carbon | |
|---|---|---|
| Method | Chemical reaction | Adsorption |
| Initial cost | Low (pump and tank) | Higher (vessel and media) |
| Running cost | Chemical, ongoing | Media replacement periodically |
| Reliability | Depends on the pump running | Passive, but capacity depletes |
| Bacterial risk | Low | Carbon beds can harbour bacteria |
Many plants use both — carbon for the bulk removal and SMBS as insurance against breakthrough. That combination is the most reliable.
Practical points
- SMBS solution should be made up fresh — it loses strength within 7 to 10 days
- Use a proper LLDPE dosing vessel
- Fit a level switch so dosing never stops unnoticed
- A standard PP dosing pump is fine — PVDF is not needed for SMBS
- Check the tank level weekly
Excess SMBS
Over-dosing is not dangerous to the membrane, but it does consume oxygen in the water, which in some circumstances encourages anaerobic bacterial growth. So dose with a sensible margin rather than a large excess.
For the product see our ZeroScale SMBS page.
Need SMBS or dechlorination advice?
Send your feed chlorine level and plant capacity, and we will confirm the dose and supply the chemical.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
Why must chlorine be removed before an RO membrane?
Chlorine attacks the polyamide active layer of thin film membranes and the damage is permanent. It cannot be reversed by any cleaning, so the membranes have to be replaced.
How much SMBS should be dosed?
Roughly three milligrams of SMBS per milligram of free chlorine, with a margin added. So for one ppm of chlorine, dose about three to five ppm of SMBS.
How do I know the chlorine is actually at zero?
Test the water just before the membrane with a DPD test kit. Free chlorine should read zero. Watch for spikes in municipal supply, because a dose set for normal levels will not cope with a sudden increase.
Where should SMBS be injected?
Upstream of the antiscalant injection point and well before the membrane, so the reaction has time and space to complete. Keep the two chemicals in separate tanks with separate pumps.
Should I use SMBS or activated carbon?
Many plants use both, with carbon for bulk removal and SMBS as insurance against breakthrough. Carbon is passive but its capacity depletes and beds can harbour bacteria, while SMBS depends on the dosing pump actually running.