Preservation of reverse osmosis membranes with non oxidizing biocides – comparison with SMBS

Sodium metabisulphite (SMBS) is the current standard preservation chemical used in RO plants during shut down. It is a cheap and efficient preservative, but its tendency to oxidize easily has several drawbacks. The use of a non-oxidizing biocide instead could solve some of the issues currently seen with the SMBS, but little has been reported about membrane compatibility and preservation efficiency in the long-term mode. Long-term membrane preservation trials have been executed with three different non-oxidizing biocides: DBNPA (2,2-dibromo-3-nitrilopropionamide), CMIT/MIT (5-chloro-2-methyl-4-isothiazolin-3-one (CMIT) and 2-methyl-4-isothiazolin-3-one (MIT), OIT (2-octyl-2H-isothiazol-3-one) as well as SMBS as the reference chemical. The suitability of these chemicals in this application was confirmed using both new Brackish Water Reverse Osmosis (BWRO) and used membranes with various membrane chemistries (Nanofiltration (NF), BWRO, Sea Water Reverse Osmosis (SWRO)). The preservation trial with new membranes confirmed the long-term stability of the product when stored in the biocide solution while the trial with used elements is closer to realistic plant conditions and validated the efficiency of the biocide against biofouling in the long-term. These results show that the biocides can be equivalent preservatives to SMBS and that the application is economically feasible. The used active concentrations for biocides are storage time and temperature dependent and this should be taken into account when first applying them in the field.