Impact of NaOCl ageing on reinforced PVDF hollow fiber membranes used in membrane bioreactor

Abstract

Reinforced hollow fiber membranes are commercially available but its information is limited on open literature. Its notable mechanical properties due to the reinforced layer makes it suitable for membrane bioreactor (MBR) application. On the other hand, polyvinylidene fluoride (PVDF) is an increasingly popular material choice for membrane used in MBR due to its good chemical resistance against chlorine present in sodium hypochlorite (NaOCl), which is commonly used as a cleaning agent. The impact of NaOCl on the PVDF membrane characteristics was reported in literature, but little information is available about the membrane performance of the aged PVDF membrane caused by NaOCl in MBR operation. In this study, the membrane performance of a reinforced PVDF hollow fiber membranes after NaOCl ageing were investigated thoroughly in MBR operations. A series of characterization on virgin and fouled membranes with/without NaOCl exposure were carried out. The accelerated NaOCl ageing experiments revealed that reinforced hollow fiber membranes have notable mechanical strength, which is not significantly affected by NaOCl ageing as compared to their non-reinforced counterpart. Additionally, there was no significant effect on the membrane behaviours in MBR after the membranes were exposed to a 5000 ppm NaOCl solution at pH 7 for 48 h, which is equivalent to 246,000 ppm.hr (~6.1 years) NaOCl exposure using a normal dose of NaOCl for regular membrane cleaning. However, after 524,000 ppm.hr (~13.1 years) of NaOCl exposure, the membrane failed to perform and a sharp transmembrane pressure (TMP) increase during MBR operation was observed. The severe fouling of aged membranes was found to be attributed to the changes in hydrophobicity, pore size and the foulants left on the membrane despite cleaning. Even though the membrane was fouled quickly after 524,000 ppm.hr (~13.1 years) of NaOCl exposure, the permeate quality was not affected much as indicated by >90% removal for dissolved organic carbon (DOC) and soluble chemical oxygen demand (sCOD), which suggests that the reinforced PVDF hollow fiber membranes used in current study exhibit considerable chemical resistance against chlorine and can be used in the MBR system for a long period of operation.