The Study Of Membrane Fouling During Ultrafiltration And Forward Osmosis By Organic Matters

This study investigated the organic macromolecules fouling on the pressure-driven ultrafiltration (UF) membrane and osmotically-driven forward osmosis (FO) membrane. Protein and humic acid, two types of ubiquitous identified membrane organic foulants, were chosen as the model foulants. It was found that the hydrodynamic conditions (initial flux and cross-flow velocity) and feedwater composition (foulant concentration, pH, ionic strength, and divalent ions concentration) played a significant role on the organic fouling of these two types of membranes.During the bovine serum albumin (BSA) ultrafiltration, drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux can not be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point (IEP) of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the IEP. Increasing the ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8, effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient behavior was also affected by the BSA retention of the membrane.In the FO process, the water and salt flux performance was strongly influenced by the internal concentration polarization (ICP) and also dependent on the membrane orientation. Increasing the foulant concentration, ionic strength and divalent ions concentration in the feed solution as well as salt concentration in the draw solution and lowering the pH promoted the humic acid fouling on the porous support layer of the FO membrane, whereas the cross-flow velocity had little effect on the flux decline with the FO porous support layer towards the feed solution. In addition, humic acid fouling on the FO membrane was less severe with the active layer facing the feed solution than that with the active layer facing the draw solution.UF and FO membrane fouling by the organic macromolecules was affected by the coupled chemical and physical aspects. Despite of different types of organic foulants and membranes, electrostatic repulsion between the foulant-foulant and foulant-membrane was one of the most significant factors affecting fouling. Foulant-membrane interaction dominated the initial flux decline, while the long-term flux behavior was governed by the foulant-foulant interaction.