Sodium Bisulfite — Frequently Asked Questions

Because thin-film composite membranes are oxidised and permanently destroyed by free chlorine, in some cases within hours of exposure. Chlorine is nonetheless needed upstream to control biofouling in the feedwater, so it must be dosed and then scrubbed out immediately before the membranes. Sodium bisulfite reduces free chlorine to chloride almost instantly and is the standard means of doing it.

It reacts with dissolved oxygen to form sulfate, consuming the oxygen before it can reach the boiler steel. Dissolved oxygen causes pitting corrosion, which is localised and can perforate a tube long before general corrosion would be a concern. Mechanical deaeration removes most of it and a chemical scavenger deals with the remainder.

Acids, which liberate sulphur dioxide gas, and oxidising agents. Storage should be cool, sealed and well ventilated, and the solution form slowly oxidises to sulfate on standing so it has a limited shelf life. Strength should be checked before use on material that has been stored for an extended period.

Both. Solution grades are typically 38 to 40 percent and are convenient for direct dosing without make-up equipment, but most of the weight is water and shelf life is limited. Sodium metabisulfite is the solid form of the same chemistry, carrying more available sulphur dioxide per tonne with much better storage stability.