Fouling mechanisms in nanofiltration membranes for the treatment of high DOC and varying hardness water

This study investigates the behavior of flux decline and change in fouling mechanisms in nanofiltration membranes, for the treatment of high dissolved organic carbon (DOC) water in the presence of divalent cations (Ca2+) at different concentrations. A model synthetic water composed of sodium alginate concentration of 17 mg/L as DOC and calcium chloride of three concentrations: low 50, medium 200, and high 350 mg/L as CaCO3, was used for the fouling study. The synthetic water quality was based on the DOC and hardness concentrations reported in three potable water sources in the Province of Manitoba and Ontario, Canada. A thin-film composite (TFC) NF90 membrane (DOW Filmtec., USA) was used for the fouling studies. Resistance-in-series model and other fouling models were used to describe the permeate flux decline and changing fouling mechanisms in NF90 membranes. The fouling study demonstrated that low calcium concentration formed an irreversible fouling layer with a relatively low surface roughness of 15.38 nm. This was attributed to high adhesive forces between the foulant (primarily alginate molecules) and the rough membrane surface. At high calcium concentration, a dense, compact and reversible fouling layer was formed, due to increased cohesive forces between calcium and alginate molecules, resulting in maximum Ca-Alginate complexation and aggregation. The aggregates further linked together to form a cross-linked foulant structure with a surface, roughness of 38.17 nm. However, for medium calcium concentration a complex (nonuniform porosity) matrix of foulants was formed with a very high surface roughness of 67.97 nm, due to the insufficient calcium available to fulfill the binding sites on alginate molecules. The fouling layer so formed resulted in offering highest flux decline and total hydraulic resistance. Results of fouling experiments identified that the reversibility of alginate fouling increased with increasing calcium concentration. However, the flux decline and the total hydraulic resistance were dependent on both alginate and calcium concentrations for maximum complexation and aggregation. In addition to the aforementioned effects of Ca2+, results also show that the dominance of gel layer formation fouling mechanism increases with increase in calcium concentrations.