Hydraulic backwashing decision method based on fouling index trend analysis in full-scale ultrafiltration systems

In full-scale ultrafiltration (UF) systems, timely hydraulic backwashing is essential for maintaining filtration efficiency and membrane service life. Conventional fixed-interval hydraulic backwashing strategies cannot account for cycle-to-cycle variations in membrane fouling, which may lead to premature or delayed intervention. To address this problem, this study proposes a trend fouling index (TFI)-based hydraulic backwashing decision method for full-scale UF operation. A resistance-in-series-derived fouling index model was first established, and each production cycle was divided into fast and stable water production stages. The trend feature of TFI during the stable stage was then extracted to formulate a slope-based hydraulic backwashing criterion. The proposed method was validated in a full-scale water treatment plant and compared with the conventional fixed-interval strategy. The results show that the proposed strategy could adaptively adjust the filtration duration according to the fouling condition of different production cycles. The fouling trend analysis and transmembrane pressure distribution further confirmed that the additional fouling caused by the extended filtration period remained within the range of physically removable contamination. In addition, the analysis of different initial pressure definitions for TFI calculation provides a reference for long-term operational assessment and maintenance of full-scale UF systems.