Microbiological and process technological aspects of nanofiltration and reverse osmosis membrane biofouling

Abstract

High pressure membrane filtration processes such as nanofiltration (NF) and reverse osmosis (RO) are capable to produce high quality water, for industrial applications and human consumption, from virtually any feed water source. Biofouling, the unwanted biofilm formation causing negative effects on membrane performance, contributes significantly to the operational costs and downtime of membrane filtration installations. In this thesis, the microbiological aspects and process technological aspects of NF and RO membrane biofouling have been extensively investigated in full-scale industrial installations and in simplified laboratory systems. An operational definition of NF and RO biofouling is given and generally applied preventative and corrective measures to manage membrane biofouling are reviewed. Anoxic groundwater treating NF installations (including their microbial communities), have been studied and compared to oxic NF systems and RO systems. The anoxic groundwater treating NF installations were shown to be much more resistant towards biofouling development, when compared to their oxic feed water treating counterparts. Membrane cleaning, the first-choice remedy against biofouling development, was studied in full-scale installations and in a laboratory setup. It was shown that aged foulants could not be removed by chemical cleaning without compromising membrane integrity. It was concluded that the prevention of biofilm formation, rather than the control of biofilm formation (cleaning), should be the main focus of a successful anti-biofouling approach. Biofouling microorganisms from the Sphingomondaceae family, which have been isolated from the fouled membranes obtained from the full-scale studies, were then physiologically characterized. It was shown that the Sphingomonadaceae membrane isolates share many features that are uncommon for other members of the Sphingomonadaceae family. Those ‘uncommon’ features were then linked to the specific physiological traits that are required for successful membrane colonization. In a proof of principle biofouling experiment with one of the Sphingomonadaceae isolates, it could be confirmed that Sphingomonadaceae may not only play a crucial role in membrane colonization, but also in subsequent biofilm development and maturation. The results presented were then brought into perspective regarding full-scale membrane biofouling prevention and control. Finally, suggestions for future research are given.