Nitrogen removal activity and functional microbial community structure in IFAS, activated sludge, and MBBR systems under different salinity conditions

A total of 30 bench-scale experiments were conducted to investigate the nitrogen removal performance of Integrated Fixed-Film Activated Sludge (IFAS), Activated Sludge (AS), and Moving Bed Biofilm Reactor (MBBR) systems under different salinity conditions. Under non-saline conditions, the IFAS system exhibited the highest nitrogen removal efficiency, achieving an ammonium nitrogen (NH₄ + -N) removal rate of 89.2 % within the first 420 min of nitrification, with an average ammonia oxidation rate of 6.2 mg/(L·h). As salinity increased, the inhibitory effect on nitrogen removal intensified across all three systems. At low salinity, notable nitrite nitrogen (NO₂ − -N) accumulation was observed, with accumulation rates of 50.7 %, 47.9 %, and 20.1 % in the IFAS, AS, and MBBR systems, respectively, at a salinity of 8 g/L. nder high salinity (32 g/L), nitrogen removal was severely inhibited, with NH₄ + -N removal rates dropping to 19.8 % (IFAS), 5.2 % (AS), and 1.4 % (MBBR). During the gradual increase in salinity, the IFAS system consistently exhibited the highest NH₄ + -N removal efficiency and demonstrated the best pollutant removal performance under short-term salinity shock. 16S rRNA gene sequencing showed that Pseudomonadota, Chloroflexota, and Bacteroidota were dominant at the phylum level, with notable genus-level differences, especially in Dechloromonas and Nitrospira abundances. Both AS and MBBR systems were sourced from the same section of a wastewater treatment plant and configured according to their respective characteristics. AS and MBBR each enriched different dominant functional microorganisms , while IFAS combined both, consistently achieving the highest nitrogen removal performance. This advantage may also explain its superior resistance to short-term salinity shock. • Under different salinity conditions, the nitrogen removal performance of the three systems is ranked as: IFAS > AS > MBBR. • Under low salinity conditions, significant NO 2 − -N accumulation is observed in the IFAS, AS, and MBBR systems. • At the phylum level, the top three microorganisms are mainly Pseudomonadota, Chloroflexota, and Bacteroidota. • At the genus level, there is a significant difference in microbial composition between the biofilm and activated sludge.