Critical minority fractions causing membrane fouling in reclaimed water: Fouling characteristics, mechanisms and control strategies

In regard to membrane-based technologies of wastewater reclamation, the reported key foulants were faced with dilemma that they could not be effectively separated and extracted from reclaimed water for thorough inves-tigation. In this study, the crucial foulants were proposed as critical minority fraction (FCM), representing the fraction with molecular weight (MW) > 100 kDa which could be easily separated by physical filtration using MW cut-off membrane of 100 kDa with fairly high recovery ratio. FCM with low dissolved organic carbon (DOC) concentration (-1 mg/L) accounted for less than 20% of the total DOC in reclaimed water, while contributed to more than 90% of the membrane fouling, and thus FCM could be considered as a perfect criminal causing membrane fouling. Furthermore, pivotal fouling mechanism was attributed to the significant attractive force between FCM and membranes, which led to severe fouling development due to the aggregation of FCM on membrane surface. Fluorescent chromophores of FCM were concentrated in regions of proteins and soluble mi-crobial products, with proteins and polysaccharides accounted for 45.2% and 25.1% of the total DOC, specif-ically. FCM was further fractionated into six fractions, among which hydrophobic acids and hydrophobic neutrals were the dominant components in terms of DOC content (-80%) as well as fouling contribution. Regarding to these pronounced properties of FCM, targeted fouling control strategies including ozonation and coagulation were applied and proved to achieve remarkable fouling control effect. High-performance size-exclusion chromatog-raphy results suggested that ozonation achieved distinct transformation of FCM into low MW fractions, while coagulation removed FCM directly, thus leading to effective fouling alleviation. Therefore, the investigation of the critical foulants was expected to help glean valuable insight into the fouling mechanism and develop targeted fouling control technologies in practical applications.

Automated summary

In this study, a crucial type of foulant, called the critical minority fraction (FCM), was identified as the main cause of membrane fouling in wastewater reclamation. FCM, with a molecular weight greater than 100 kDa, could be easily separated by physical filtration using a 100 kDa cut-off membrane. It was found that FCM, accounting for less than 20% of the total dissolved organic carbon (DOC) in reclaimed water, contributed to more than 90% of the membrane fouling. The interaction between FCM and membranes was identified as the pivotal fouling mechanism, leading to severe fouling development. Targeted fouling control strategies, such as ozonation and coagulation, were applied and proved to effectively alleviate membrane fouling.