Practical considerations for ageing of drinking water membranes

This dissertation advances our understanding of practical considerations for ageing of membranes used in drinking water treatment. Fouling and cleaning of membranes are well understood, but knowledge of long-term changes (i.e. ageing) have not been fully explored. To date, changes in physical and chemical characteristics, and changes in membrane performance, have been attributed to ageing. The three major sections in this dissertation were structured around membrane performance. Firstly, the existing literature was comprehensively reviewed. Important membrane performance factors were identified via this review: resistance, fouling rate, cleaning rate, and susceptibility to breach. How chemical and physical characteristics impact these performance factors was also systematically assessed. Almost all of the reviewed research involved bench-scale ageing of membranes, with scant attention to full-scale ageing, and even less on how bench-scale ageing can be useful in understanding full-scale ageing. This recognition led to the two further research projects. Secondly, we investigated changes in performance factors and chemical characteristics for PVDF membranes aged in 8 full-scale water treatment plants (14 treatment trains ranging from new to 10 years of age). Membranes were harvested by plant operators regularly and analyzed in a standardized laboratory setting. Membranes exhibited stable behaviour until about 5 years of operation; after this time, performance factors and chemical characteristics began to change significantly. Clean membrane resistance and fouling rate increased for aged membranes. The mechanical properties of aged membranes deteriorated, suggesting that their susceptibility to breach was heightened. These performance changes correlated with the removal of hydrophilic additives from the membranes. The final research project sought to link performance changes observed at bench-scale with performance changes observed in the full-scale study. Two bench-scale ageing techniques were used to probe changes in performance and characteristics: soaking membranes in NaOCl cleaning agent, and cycling them with foulant and cleaning agent. The changes in membrane chemistry were similar for bench-scale and full-scale ageing, but performance differed. By comparing the two bench-scale techniques with the full-scale ageing findings, it was established that irreversible foulants are critical components of full-scale ageing, and the involvement of these can be approximated to some extent by cycling membranes.Applied Science, Faculty ofCivil Engineering, Department ofGraduat