Key foulants and their interactive effect in organic fouling of nanofiltration membranes

Abstract

Membrane fouling especially organic fouling of nanofiltration (NF) membranes is notorious but yet remains not fully understood. This study was conducted to identify the key organic substances responsible for organic fouling and quantify the individual and interactive effects of these key foulants on organic fouling, primarily by using statistical methods. A total of ten different water, covering lake water and water collected from water and wastewater treatment plants, were filtered to obtain the fouling potential and determined for organic and inorganic components. Pearson correlation showed that fouling potential could correlate with TOC with a moderate significance. Partial correlation revealed that, in addition to polysaccharides and protein-like substances, fulvic acid-like substances could act as key foulants to organic fouling of NF membranes, though protein-like and fulvic-like substances were mutually highly correlated. Generally, most polysaccharides were macromolecules (>10 kDa) while protein-like and fulvic acid-like substances were more of low molecular weight. A regression equation was established between fouling potential and the three types of key foulants; the calculated value by this equation matched well with the experimental result especially when the fouling potential was relatively high. Variance partitioning analysis showed that individual contributions to organic fouling by polysaccharides, protein-like and fulvic acid-like substances were all lower than 30%, while the interactive effect among these three components was as high as 66% ± 7%. This suggested that organic fouling was a complex process with the three key types of foulants involved simultaneously. Spearman correlation showed that higher contents of polysaccharides and protein-like substances in feed water generally led to a faster accumulation of these two types of foulants (determined from FTIR results) onto membrane, respectively, but not necessarily resulted in a heavier organic fouling.