Micro-granular sludge driven by powder carrier in membrane bioreactor: highly-efficient N & P removal and membrane...

Membrane bioreactor (MBR) technology has emerged as a prominent solution in wastewater treatment due to its advantages, including high effluent quality, compact footprint, low residual sludge production, and ease of automation. However, persistent challenges-such as membrane fouling, high operational costs, and limited biological phosphorus removal efficiency under long sludge retention times (SRTs)-have hindered the sustainable application of MBR systems. This study proposes the integration of micro-granular sludge (MGS), induced by powder carriers, with MBR technology (MGS-MBR) to address these limitations for treating low-strength municipal wastewater. A pilot-scale evaluation demonstrated that powder carriers effectively promoted the formation of MGS with an average particle size of 50 μm in a conventional MBR system. This modification doubled the treatment capacity and achieved effluent total nitrogen (TN) and total phosphorus (TP) concentrations below 5 mg/L and 0.03 mg/L, respectively, even at a low C/N ratio (∼3.2) and short hydraulic retention time (HRT) of 6 hours. The transmembrane pressure (TMP) remained stable at ∼8 kPa over >20 days at a membrane flux of 20 L/(m²·h), indicating significantly prolonged cleaning intervals compared to traditional MBR systems. Fluorescence excitation-emission matrix (EEM) analysis revealed that protein- and fulvic-like substances in loosely bound extracellular polymeric substances (LB-EPS) were major contributors to reversible and irrecoverable membrane fouling. However, quorum sensing was found to be responsive to the increased microbial population density around powder carriers, leading to suppressed LB-EPS production by disrupting quorum sensing pathways. Additionally, microbial community analysis showed that typical denitrifying bacteria such as Pseudomonadota and Comamonadaceae were enriched under the influence of powder carriers coupled with hydrocyclone operation. The relative abundance of autotrophic denitrifiers, including Iamia, Hyphomicrobium, Rhodobacter, and Romboutsia, also increased, suggesting enhanced nitrogen removal capacity under low chemical oxygen demand (COD) conditions. Notably, Bacteroidota, known for hydrolyzing complex organics and supporting phosphorus-accumulating organisms (PAOs) through carbon provision, was enriched in the upflow sludge of the hydrocyclone. This likely contributed to the shortened nominal retention time of PAOs while maintaining efficient biological phosphorus removal. Collectively, these findings highlight a promising strategy to improve the sustainability of MBRs through enhanced fouling control and nutrient removal, while advancing mechanistic insights into microbial networks shaped by powder carriers.