Effect of anionic fluidized ion exchange (FIX) pre-treatment on nanofiltration (NF) membrane fouling

Anionic Fluidized Ion Exchange (FIX) is used to improve the performance of downstream Nanofiltration (NF). The research is divided in three parts: (i) NOM removal by FIX, (ii) the effect of FIX treatment on NF fouling and (iii) FIX treatment in relation to biological stability. Pre-treated anaerobic groundwater was (i) fed directly to a 4-inch membrane element and (ii) fed to another 4-inch membrane element after anionic FIX treatment.The operational parameters of the membrane set-up were monitored during 42 days, followed by a membrane autopsy study in which accumulated biological, organic and inorganic fouling was determined. Parallel to this experiment, two small ion exchange (IEX) resin and glass beads filled columns were operated to study the effect of FIX on the biomass concentration of the feed water.FIX operated satisfactory and selectively removed humic substances (>90%) and hydrophobic organic carbon (HOC) (>80%) from the feed water. Furthermore, iron was substantially removed (71%) which was explained by complexation with humic substances.Removal of NOM by FIX did not reduce membrane fouling problems; the Membrane Transport Coefficient (MTC) decreased and the Normalized Pressure Drop (NPD) increased more rapidly for the NF membrane after FIX compared to the membrane without FIX pre-treatment. NOM removal by FIX did not reduce adsorption of organic matter onto the downstream membrane element, since predominantly humic substances were removed which did not adsorb to the membrane surface.FIX treatment resulted in higher biomass densities (400%) and slightly less iron deposition (20%) onto the membrane surface. Fouling of the membrane element after FIX treatment was dominated by biofouling and fouling of the reference membrane element experienced more colloidal iron fouling compared to the membrane element after FIX, both resulting in an increase in NPD.The microbiological water quality deteriorated after anionic FIX treatment, as was observed by an increase in ATP content. Growth of biomass onto the IEX resins was observed which was caused by both IEX materials and feed water components, such as NOM fractions.