Air micro-nano bubbles combining with sodium hypochlorite to balance cleaning efficiency and aging mitigation of ultrafiltration membranes

During prolonged ultrafiltration operation in water treatment, severe membrane fouling was inevitable, necessitating frequent cleaning with high-concentration chemical solutions, which accelerated membrane aging. To balance the trade-off effects between chemical cleaning and membrane aging, air micro-nano bubbles (AMNBs) were introduced to improve cleaning efficiency while reducing the amount of cleaning agents. For single sodium hypochlorite (NaClO) solution, its optimized concentration was 0.1 % (volume percentage) for cleaning polyvinylidene fluoride ultrafiltration membranes fouled by polymeric organics (i.e., anionic polyacrylamide), which gave rise to serious erosion of membrane materials and decline in membrane performance. When combined with AMNBs, the required NaClO dosage was significantly reduced from 0.1 % to 0.03 % without compromising cleaning efficiency, thereby lessening membrane aging. The mechanisms behind the NaClO − AMNBs synchronization were proposed through concepts of micro functional zones, size effects, and aggregation stability. Micro functional zones were generated when hypochlorite anions (ClO⁻) were gathered onto AMNBs interfaces, as indicated by the intensified zeta potentials of ClO⁻+AMNB complexes. Alternatively, ClO⁻+AMNB particles with modest size scale were in favour of adhesion onto the foulant molecular chains and membrane interfaces, which facilitated the deconstruction of fouling layer. Additionally, aggregation stability of AMNBs was significantly enhanced by adding 0.03 % NaClO, which remarkably improved the cleaning efficiency. These findings suggest a dual benefit of enhancing cleaning effectiveness while reducing reagent usage, thus retardate membrane aging.