A novel alginate-assisted immobilization strategy for enhanced biofilm development and nitrogen removal in membrane aerated biofilm reactors (MABRs)

The prolonged startup period caused by slow and unstable biofilm formation remains a major barrier to large-scale application of membrane-aerated biofilm reactors (MABRs). This study developed a novel sodium alginate–assisted immobilization strategy to accelerate biofilm establishment and enhance nitrogen removal. A homogeneous mixture of sodium alginate and activated sludge was spray-coated onto plasma-treated PVDF hollow-fiber membranes, followed by CaCl₂ mist crosslinking to form a thin gel layer immobilizing active biomass. The coated MABR achieved 85 ± 5 % surface coverage within seven days—over twice that of the control—and complete ammonium oxidation (>90 %) in 10 days, reducing startup time by about 50 %. Digital microscopy and SEM revealed rapid transition from the synthetic alginate matrix to a self-sustained EPS network, while 16S rRNA sequencing confirmed enriched Nitrosomonas and Nitrospira with enhanced microbial diversity. The oxygen transfer rate initially decreased by 18 % after coating but recovered to 95 % of the original after partial scaffold dissolution, showing reversible mass-transfer adaptability. Mechanistically, the biodegradable Ca 2+ –alginate scaffold provided an initial attachment platform evolving into a stable stratified biofilm enabling simultaneous nitrification and denitrification. This study establishes a mechanistic and practical basis for biopolymer-assisted biofilm engineering toward rapid-startup, high-efficiency MABR systems.