Surface shear stress and retention of emerging contaminants during ultrafiltration for drinking water treatment

This study investigated the impact of surface shear stress, to represent air sparging employed for fouling control, on the retention of organic micropollutants during ultrafiltration for drinking water treatment. The retention of 16 different pharmaceutically-active and endocrine disrupting compounds was examined during ultrafiltration of three natural surface waters (two lake, one river) under four different surface shear stress regimes: no shear stress, low peak shear stress (representative of continuous coarse bubble sparging), sustained peak shear stress (representative of intermittent coarse bubble sparging), and high peak shear stress (representative of large pulse bubble sparging). Results indicate that surface shear stress does impact the retention of emerging contaminants; however, it is dependent on water matrix and compound properties. The greatest retention of micropollutants was observed in waters with a higher concentrations of organic matter, and for conditions where no surface shear stress was applied (average 32% retention), and under conditions representative of large pulse bubble sparging (average 34% retention). The observed retention under conditions of no shear stress was likely due to a heavy fouling layer that altered the membrane selectivity and was able to entrap organic micropollutants of larger molecular weight. Under conditions that mimicked air sparging, increasing the shear stress (quantified as the root mean square applied shear) resulted in increased retention of organic micropollutants, particularly those that are neutral and hydrophobic in nature. This may be related to solute–solute complexes, which are kept in solution when shear stress is applied, or related to modification of the fouling layer by the shear stress induced onto the membrane surface. The results suggest that there may be value added with respect to removal of organic micropollutants, such as pharmaceuticals, when employing air sparging as a fouling control strategy during ultrafiltration.