Hybrid MBR-carrier system enables efficient nitrogen removal from semiconductor wastewater through engineered microbial niche differentiation: Process optimization, biofilm-enhanced kinetics, and multi-omics insights

Nitrogen removal from semiconductor wastewater is constrained by low carbon to nitrogen ratios and the requirement for specialized microorganisms capable of degrading organoamines such as tetramethylammonium hydroxide (TMAH), N-methyl-2-pyrrolidone (NMP), and monoethanolamine (MEA). To address these challenges, a hybrid treatment system was developed by integrating a multi-stage anoxic-oxic-anoxic-oxic (AOAO) process with a long sludge retention time membrane bioreactor (MBR) and polyurethane fillers, thereby intentionally creating three distinct ecological niches within a single system, namely suspended sludge, filler-attached biofilm, and MBR-retained biomass. With external carbon addition, the system achieved 73.4 percent total nitrogen (TN) removal, corresponding to an effluent TN of 8 to 11 mg L-1, while comparable performance was obtained without external carbon through a two-point influent feeding strategy (A1:A2 = 8:1), resulting in 71.9 percent TN removal and effluent TN of 7 to 13 mg L-1. Following filler incorporation and optimization of a back-loaded hydraulic retention time distribution (A1:O1:A2:O2 = 2:3:2:6 h) combined with 150 percent internal recycle, stable TN removal of approximately 73 percent and effluent TN of 7 to 9 mg L-1 were achieved under carbon-limited conditions. Batch kinetic analyses showed that the filler biofilm exhibited a 2.1-fold higher specific denitrification rate than suspended sludge (7.3 versus 3.4 mg N g-1 MLSS h-1) and an 18 percent improvement in carbon utilization efficiency (2.16 versus 2.34 mg TOC mg-1 N). Normalized stochasticity ratio analysis revealed that the stable anoxic microenvironment within the fillers shifted microbial community assembly from predominantly stochastic processes in suspended sludge (NST 75.7 percent) to more deterministic processes in the filler biofilm (NST 37.3 percent), selectively enriching Methanomethylovorans, which was 5.1-fold more abundant than in the MBR biomass and is associated with anaerobic TMAH demethylation, together with Hyphomicrobium responsible for methylotrophic denitrification. Untargeted metabolomic profiling further identified depletion of succinic acid and consumption of tryptophan in the filler biofilm, and strong correlations between metabolites and functional genes (tryptophan-norB, r = 0.90; succinate-dmmA, r = 0.83) linked organoamine catabolism with respiratory denitrification, while co-occurrence network analysis showed that the filler biofilm exhibited the highest connectivity (216 edges, density 0.53), indicating tightly coupled anaerobic demethylation and denitrification modules; collectively, these results demonstrate that engineered niche differentiation, achieved by deliberately constructing distinct microbial habitats with complementary metabolic functions, represents an effective design paradigm for overcoming carbon limitation in organoamine-rich industrial wastewater treatment.