Repeated-cycle operational response and residual fouling characteristics under NaOCl and vanillin-assisted NaOCl backwashing in membrane bioreactors

Membrane biofouling remains a major operational constraint in membrane bioreactors (MBRs), increasing transmembrane pressure (TMP) and chemical-cleaning demand. This study compared four backwashing conditions—deionized (DI) water, vanillin-only, sodium hypochlorite (NaOCl)-only, and vanillin-assisted NaOCl backwashing – in lab-scale MBRs over an initial baseline filtration period and four post-backwash filtration cycles, using one independent reactor per condition. Although vanillin has been reported to interfere with quorum sensing, no quorum-sensing-related endpoints were measured in this study. Cumulative operation times were 5.7, 6.9, 14.8, and 13.0 d for DI-water, vanillin-only, NaOCl-only, and vanillin-assisted NaOCl backwashing, respectively. NaOCl alone provided the longest cumulative operation and higher operational-time recovery, defined as post-backwash filtration time relative to baseline filtration time, in Cycles I–II. Vanillin-assisted NaOCl backwashing showed numerically higher recovery in Cycles III–IV (Cycle IV: 31.0% versus 17.0%). Endpoint carbohydrate, protein, and biofilm biomass were lowest under NaOCl-only backwashing. Under vanillin-assisted NaOCl backwashing, total residual carbohydrate and protein were 51.6% and 87.1% of the DI-water values, respectively. Carbohydrate was predominantly soluble, while the soluble-carbohydrate concentration remained below the DI-water value. ATR-FTIR spectra showed attenuated protein-associated bands under both NaOCl-containing conditions. NaOCl alone provided the strongest overall cleaning response within the four-cycle test window. Without independent reactor replication, the later-cycle crossover remains a descriptive observation and does not demonstrate a vanillin-specific effect. Replicated, longer-term evaluation with direct measurements of residual chlorine, parent vanillin and its transformation products, and fouling-layer properties is required. NaOCl-only backwashing provided the longest cumulative operation over four cycles.A numerical crossover between the two NaOCl protocols appeared after Cycle II.NaOCl-only backwashing produced the lowest endpoint carbohydrate, protein, and biofilm biomass.Vanillin-assisted NaOCl produced an intermediate residual-carbohydrate level dominated by the soluble fraction. NaOCl-only backwashing provided the longest cumulative operation over four cycles. A numerical crossover between the two NaOCl protocols appeared after Cycle II. NaOCl-only backwashing produced the lowest endpoint carbohydrate, protein, and biofilm biomass. Vanillin-assisted NaOCl produced an intermediate residual-carbohydrate level dominated by the soluble fraction.