Effect of the relative degree of foulant hydrophobicity on membrane fouling

Foulant hydrophobicity has been regarded as a key factor in membrane fouling. While hydrophobic/philic fractionation using DAX (or XAD) resin column has been extensively utilized in fouling studies, the critical retention factor (k'(cr), the ratio of water volume to column volume) has been long neglected which can alter the definition and relative degree of hydrophobicity of the obtained fractions and affect fouling results. To this end, this study compared fouling behavior of a membrane bioreactor (MBR) supernatant fractionated at varied k'(cr) and found that not the distinctively strong hydrophobic (k'(cr) > 50) or hydrophilic (k'(cr) < 5) fractions, but rather the nominally moderate hydrophobic/philic fractions had the greatest contribution to overall fouling. The joint influence of foulant hydrophobicity and coexisting factors (molecular weight, carboxylic group, UV/fluorescence properties and polysaccharide/protein/humic contents) was elucidated via multivariate statistical analysis. Therefore, special attention should be placed on the nominally moderate hydrophobic/ philic fractions for fouling control.