Sodium metabisulfite (SMBS) is not a disinfectant. SMBS dissolved in water converts to sodium bisulfite (SBS). It is effective as a preservative because it is an oxygen scavenger and therefore prevents growth of aerobic microorganisms. However, to scavenge dissolved oxygen while dosing online, a tremendously high dosage would be required, making it extremely uneconomical. Furthermore, only aerobic bacteria require dissolved oxygen, so anaerobic bacteria, some of which can also produce biofilm, would thrive. Should SBS be dosed at a lower dosage that doesn’t fully scavenge all dissolved oxygen (maintaining an aerobic environment), it would act as an electron donor to Sulfur Oxidizing Bacteria (SOB). The reduction of dissolved oxygen would also create a stressful environment for the SOB, causing them to produce a dense biofilm that is especially difficult to penetrate during CIP. Time and again, high SBS dosages have been associated with increased biological growth, especially where the water contains high Assimilable Organic Carbon (AOC) levels. Chlorination followed by SBS dechlorination has been show to increase AOC levels through oxidation of TOC to more assimilable organics.
Fungus requires nitrogen for growth, making polyamide membranes especially susceptible. Fungi and heterotrophic bacteria obtain the carbon needed for growth from organic compounds. Some fungi and bacteria are also chemotrophs, meaning that they can obtain their energy from inorganic chemical compounds such as SBS which act as electron donors or receptors (oxidizing bacteria vs reducing bacteria depending on the environment). It is therefore essential to clean the membranes well and remove all organic foulants from the membrane surface prior to long term storage in bisulfite solution. Failing to do so could result in severe membrane deterioration during the storage period.
See also:
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