In-depth insight into improvement of simultaneous nitrification and denitrification/biofouling control by increasing sludge concentration in membrane reactor: performance, microbial assembly and metagenomic analysis

[Display omitted] •High sludge concentration simultaneously improved TN removal/biofouling control.•High sludge concentration drove microbial assembly for excellent MBR performance.•Microbial composition between bulk sludge and bio-cake displayed differently.•Up-regulated nitrogen metabolism and NADH/ATP production facilitated SND.•Active amino acid degradation offered molecular explanation for fouling reduction. Currently, severe membrane fouling and inefficient nitrogen removal were two main issues that hindered the sustainable operation and further application of membrane bioreactor (MBR). This study aimed to simultaneously alleviate membrane fouling and improve nitrogen removal by applying high sludge concentration in MBR. Results showed that high sludge concentration (12000 mg/L) enhanced total nitrogen removal efficiency (78 %) and reduced transmembrane pressure development rate. Microbial community analysis revealed that high sludge concentration enriched functional bacteria associated with nitrogen removal, increased filamentous bacteria fraction in bio-cake and inhibited Thiothrix overgrowth in bulk sludge. From molecular level, the key genes involved in nitrogen metabolism, electron donor/adenosine triphosphate production and amino acid degradation were up-regulated under high sludge concentration. Overall, high sludge concentration improved microbial assembly and functional gene abundance, which not only enhanced nitrogen removal but also alleviated membrane fouling. This study provided an effective strategy for sustainable operation of MBR.