Membrane ageing in drinking water treatment : assessing the impact of natural organic matter foulants and hypochlorite interactions

The research investigates the impact of interactions between natural organic matter (NOM), present on membranes as foulants, and hypochlorite, used for chemical cleaning, on the ageing of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes. Ageing, defined as the progressive deterioration of physical and chemical characteristics and hydraulic performance of membranes, represents a critical challenge to the sustainable operation of UF membrane facilities. The role of NOM foulant-hypochlorite interactions was poorly understood. Using laboratory accelerated soak and cyclic ageing approaches, and common model NOM components (bovine serum albumin (BSA), sodium alginate (SA), and humic acid (HA)), the research demonstrates that radical oxidants are generated from NOM foulant – hypochlorite interactions under conditions relevant to chemical cleaning, and that these radical oxidants contribute to accelerating membrane ageing. BSA, a model protein type NOM, was identified as the primary contributor to radical oxidant generation when interacting with hypochlorite; other model NOM considered did not interact with hypochlorite. The impact of ageing was quantified based on changes in physical and chemical characteristics (characterized using differential Fourier Transform Infrared (FTIR) spectroscopy, contact angle and scanning electron microscopy (SEM)), and hydraulic performance (characterized using clean water resistance). The research demonstrated that full-scale facilities with feed waters containing higher protein type NOM concentrations are likely to generate more radical oxidants during chemical cleaning, and that these radical oxidants are likely to increase the rate at which changes in physical and chemical characteristics and hydraulic performance occur during membrane ageing. Results for laboratory accelerated cyclic ageing were similar to those for ageing at full-scale when considered with respect to the loading of protein type NOM from feed waters. Addition of radical scavengers to the chemical cleaning solution effectively mitigated the detrimental effects of radical oxidants on membrane ageing. Permeation of hypochlorite through membranes during chemical cleaning, rather than simply soaking membranes in hypochlorite as typical practice at full-scale, improved hydraulic performance by removing accumulated foulants within membrane pores. The research outcomes advance current understanding of NOM in membrane ageing and offer practical strategies to extend membrane lifespan, consequently improving sustainability of membrane filtration systems in drinking water treatment.