Abstract
Membrane-based filtration techniques have been widely applied in wastewater treatment. However, membrane fouling phenomenon, commonly resulting in a lower filtration efficiency and shortened lifespan of membranes is still an issue of concern. To explore fouling behaviors, the calcium-humic acid (Ca-HA) complexes colloids, which are non-rigid in contrast to rigid SiO2 aspheric colloids, were chosen as a foulant surrogate to investigate the size effect on the performance of nanofiltration (NF filtration). As-obtained results were fitted with Hermia model and the vertical distribution of foulants in nanofiltration membranes was analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our depth profile analysis indicated that the interaction between fouled membrane and foulants dominates the fouling development in nanofiltration membranes. In general, a thinner fouling was observed under neutral environment. Interestingly, a lower flux decline was found when the size of calcium-humic acid complexes reached around 800 nm in comparison with those with size around 300 nm. This was attributed to the shear-induced diffusion. Based upon our results, it is suggested that the shear-induced diffusion is the dominant effect controlling the fouling of large colloids (~ 800 nm) on nanofiltration membranes. Furthermore, it is proven that Hermia model is also capable of simulating the fouling behaviors caused by non-rigid colloids.