Polyamide RO membranes are sensitive to free chlorine. Sodium metabisulfite (SMBS) is commonly used as a reducing agent upstream of RO to remove residual oxidant after chlorinated pretreatment.
Start with the chlorine load
The theoretical reaction requirement can be estimated stoichiometrically, but practical design normally includes an appropriate margin and a validated residual-control strategy. The correct setpoint should be based on feed chlorine, contact/mixing conditions and the membrane supplier’s requirements.
Convert an approved SMBS dose to product usage
If the selected active dose is 3 mg/L at 100 m³/hr:
Active SMBS = 100 × 3 ÷ 1000 = 0.30 kg/hr.
If dosing a prepared 10% w/w solution, solution requirement = 0.30 ÷ 0.10 = 3.0 kg/hr. Divide by the solution density to obtain L/hr.
Monitoring
ORP can be a useful operational indicator, but the correct target depends on the system. Verify that free chlorine is absent at the membrane inlet using a suitable analytical method and maintain the dosing equipment.
Solution preparation
SMBS solutions oxidize over time. Prepare solution strength and tank turnover according to site procedure. Avoid excessively long storage of dilute solution.
Safety
Keep SMBS away from acids. Acidification can release sulfur dioxide gas. Use appropriate ventilation, PPE and chemical segregation.