Enhanced nitrogen removal in microgranular activated carbon–integrated membrane bioreactors

Abstract

Achieving simultaneous high-efficiency nitrogen removal and membrane fouling control remains a critical challenge in Membrane Bioreactor (MBR) technology. In this study, we investigated the integration of microgranular activated carbon (μGAC) into a flat-sheet MBR to enhance system performance. By strategically optimizing the sludge retention time (SRT) to 20 d, the system maintained a high concentration of functional biomass, which, when coupled with the fluidized state of μGAC, established a highly efficient simultaneous nitrification and denitrification (SND) environment. The biofilm on μGAC exhibited a relatively high total abundance of nitrogen-removing and organic-degrading bacteria, and the SND rate in the μGAC–MBR was 85.53%, representing a 12.00% increase over the control MBR (C–MBR). The long-term running μGAC–MBR system achieved an average total nitrogen (TN) removal efficiency of 82.74% and near-complete ammonia oxidation of 99.73%. The addition of μGAC modified sludge properties by reducing the protein-to-polysaccharide (PN/PS) ratio and increasing sludge hydrophilicity, resulting in a 14.5% reduction in membrane flux decline. Metagenomic analysis revealed that the μGAC–MBR selectively enriched K-strategist genera, specifically Nitrosospira and Nitrospira, which increased in abundance by 11.22% within the membrane cake layer. Furthermore, the significantly upregulated expression of key functional genes involved in carbon metabolism (yvaK and exaA) and nitrogen cycling (amoC, nirB, and norC) demonstrated a strengthened metabolic capacity for ammonium conversion. These results suggest that μGAC acts as a selective catalyst for a robust, K-strategist–dominated microbial community, thus providing enhanced denitrification and superior fouling resistance.

Graphical abstract

Keywords

Membrane bioreactor (MBR) / Microgranular activated carbon / Sludge retention time / K-strategist bacteria / Nitrogen metabolism

Highlight

	● μGAC in MBRs significantly improved TN removal and reduced fouling.
	● μGAC addition enriched nitrifying K-strategist bacteria within MBR systems.
	● μGAC addition promoted key nitrogen-cycling genes for enhanced denitrification.

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