Critical fractions in reclaimed water responsible for membrane fouling: Isolation, fouling characteristics, quantitative and qualitative variations in practical application.

Considering the different fouling characteristics between model foulants and organic components in real reclaimed water, it is of great importance to identify the critical foulants responsible for membrane fouling. This study identified and isolated the fraction with molecular weight (MW) >100 kDa as the critical foulant in secondary effluent by MW cut-off membrane of 100 kDa with high efficiency. This fraction accounted for 92.2 % membrane fouling of raw water, including 28.7 %, 29.7 % and 33.8 % fouling contribution by subfractions with MW between 100 and 300, 300-500 and >500 kDa. Specifically, the critical fraction with MW > 100 kDa were mainly distributed in two parts: <0.22 μm and >0.45 μm, corresponding to 41.9 % and 56.9 % fouling contribution of this fraction. Furthermore, both total organic carbon (TOC) and fouling potential of fraction with MW > 100 kDa were monitored, presenting about threefold increase from September to January in next year. Membrane fouling contribution of this critical fraction in raw secondary effluent were mainly distributed in 85- 95 % throughout the 5 months, demonstrating its predominant fouling propensity. Moreover, the TOC concentration of fraction with MW > 100 kDa presented distinct positive correlation with the fouling potential of raw secondary effluent (R2  = 0.947), which was promising to be a surrogate for predicting membrane fouling in practical application.