Further understanding NaClO cleaning of bacteria-fouled ultrafiltration membrane: Variation of membrane structure and formation of halogenated by-products

NaClO has been extensively used for controlling membrane fouling in water and wastewater treatment processes , with bacteria being a common foulant in long-term ultrafiltration . However, the potential impact of NaClO cleaning on membrane structure and the formation of halogenated by-products remains unclear. This study comprehensively investigated the variation of membrane characteristics and formation of halogenated by-products during the chemical cleaning of Escherichia coli ( E. coli ) cell-fouled membrane using NaClO solution. The feed water containing E. coli cells resulted in severe membrane fouling by forming a dense cake layer, while NaClO cleaning achieved complete restoration of membrane flux under optimal conditions. The cleaning performance was compromised under high temperature and acidic conditions due to the lysis of E. coli cells and the limited oxidative capability of HClO. Excellent chemical resistance of polyethersulfone membrane against NaClO was confirmed under appropriate operational conditions. The biofouling and membrane materials played critical roles as precursors in the formation of halogenated chemicals during NaClO cleaning, and the generation pattern of by-products is highly dependent on the composition of membrane foulants. Increasing E. coli concentration, NaClO dosage, temperature and blending time led to the monotonical accumulation of almost all eleven representative by-products. As solution pH rose continuously, several by-products including trichloroacetic acid and chloral hydrate exhibited an initial increasing then decreasing trend, which could be attributed to the relative quantities between their formation and decomposition rates.