Membrane aerated biofilm reactors for sustainable nitrogen management: Mechanisms, process integration, and engineering implications

doi:10.1016/j.biortech.2026.134354

Bioresource Technology

Volume 449, June 2026, 134354

https://doi.org/10.1016/j.biortech.2026.134354 Get rights and content

Highlights

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Identified counter-diffusion as the fundamental driver of nitrogen pathways in MABRs.
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Revealed oxygen flux as the key lever shaping biofilm stratification and stability.
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Reclassified MABR nitrogen removal by pathways rather than reactor configurations.
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Clarified trade-offs among SND, PN/A, and autotrophic denitrification in MABRs.
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Defined operational windows and N₂O risks as critical gaps for full-scale deployment.

Abstract

Membrane aerated biofilm reactors (MABRs) have emerged as a promising platform for sustainable nitrogen management in wastewater treatment, owing to their unique counter-diffusion biofilm architecture. This review critically examines how stratified redox microenvironments in MABRs govern nitrogen transformation pathways, enabling shortcut nitrogen removal and improved energy efficiency. We synthesize recent advances in gas diffusion and mass transfer modeling, microbial functional organization, and process integration strategies, with particular emphasis on simultaneous nitrification–denitrification (SND) and partial nitritation-anammox (PN/A) configurations. Beyond performance advantages, key sustainability challenges are discussed, including nitrite accumulation, nitrous oxide (N₂O) formation, operational stability, and scale-up limitations. By linking mechanistic insights with engineering implications, this review identifies critical knowledge gaps and control strategies for minimizing emissions and maximizing nitrogen removal efficiency. The analysis provides a framework for advancing MABR applications toward robust, low-energy, and low-emission nitrogen management in full-scale wastewater treatment systems.

Graphical abstract

Introduction

Nitrogen removal remains a persistent challenge in wastewater treatment due to its high energy demand, carbon dependency, and associated greenhouse gas emissions (Li and Zhang, 2017, Marque et al., 2025, Werkneh, 2022). Conventional biological nitrogen removal processes are characterized by intensive aeration and external carbon addition, which lead to substantial operational costs and environmental footprints (Deng et al., 2025, Li et al., 2023a, Li et al., 2023c). Intensifying regulatory pressures on energy efficiency and emissions have spurred growing interest in technologies that enable more sustainable nitrogen management through reduced oxygen input, enhanced process integration, and improved emission control.

Membrane aerated biofilm reactor (MABR) technology has garnered significant attention as an innovative and highly efficient platform for wastewater treatment (Liu et al., 2025). In MABRs, oxygen is delivered via gas-permeable membranes to biofilms growing on the membrane surface, which establishes a counter-diffusion configuration in which oxygen and substrates diffuse from opposite directions (Fig. 1) (Li et al., 2015, Lu et al., 2020). This distinctive architecture enables the formation of vertically stratified redox microenvironments within biofilms, fundamentally altering nitrogen transformation pathways compared with suspended-growth systems (Ahmar Siddiqui et al., 2022, Sun et al., 2023).

Over the past decade, substantial progress has been made in the development of MABR-based processes for nitrogen removal, ranging from conventional nitrification-denitrification to partial nitrification-Anammox (PN/A) and autotrophic denitrification driven by hydrogen or methane. Pilot- and full-scale applications have reported high oxygen transfer efficiency, reduced aeration demand, and improved footprint efficiency (Lv et al., 2021, Yuan et al., 2024). However, reported performance varies widely across studies, and long-term operational stability remains inconsistent. In particular, challenges such as nitrite accumulation, process controllability, and (nitrous oxide) N₂O emissions have increasingly been recognized as critical barriers to the sustainable deployment of MABRs.

Several reviews have summarized the development of MABR technology, focusing on membrane materials, module configurations, or overall process performance (He et al., 2021, Li et al., 2023a, Lu et al., 2020). While these contributions provide valuable overviews, many adopt a descriptive, study-by-study approach and place limited emphasis on the mechanistic links between counter-diffusion biofilm structure, nitrogen transformation pathways, and sustainability outcomes (Li et al., 2023b, Martin and Nerenberg, 2012, Syron and Casey, 2008). Moreover, key issues relevant to long-term sustainability --- such as nitrite looping, emission risks, and operational control windows --- are often discussed in isolation or treated as secondary considerations.

Accordingly, this review provides a critical synthesis of recent advances in MABR-based nitrogen removal with a specific focus on sustainable nitrogen management. Rather than cataloging individual studies, we emphasize mechanistic insights into gas diffusion and mass transfer, redox stratification, and microbial functional organization, and examining how these factors shape nitrogen transformation pathways. Process integration strategies, including simultaneous nitrification-denitrification (SND), PN/A, and autotrophic denitrification, are evaluated from both performance and sustainability perspectives. Finally, engineering challenges related to scale-up, operational stability, and N₂O mitigation are discussed to identify key knowledge gaps and control priorities for future research and full-scale implementation.

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Section snippets

Membrane modules and oxygen delivery characteristics

The performance of MABRs is fundamentally governed by how oxygen is delivered to the biofilm through membrane modules. Unlike conventional aeration methods, the gas supply mechanism of MABRs involves gas diffusion from the membrane compartment into the biofilm, while liquid-phase substrates simultaneously diffuse inward from the main liquid phase (He and Daigger, 2023, Wei et al., 2012). This characteristic is particularly critical for nitrogen transformation processes that rely on controlled

Gas diffusion and mass transfer modeling in MABRs

Counter-diffusion gas transport is the defining mass transfer mechanism governing biofilm behavior MABRs. Unlike conventional biofilms that rely on oxygen supplied from the bulk liquid, MABRs deliver oxygen directly through gas-permeable membranes, while nitrogenous substrates (e.g., ammonium, nitrate) diffuse inward from the liquid phase. This counter-current diffusion of reactants establishes steep, opposing concentration gradients that fundamentally reshape the mass-transfer landscape and

Function guilds driving nitrogen transformations

Nitrogen transformation in MABRs is governed not merely by the overall microbial community composition, but by the spatial organization and metabolic interactions of distinct functional guilds. Table 2 lists the bacterial communities involved in nitrogen metabolism within MABR systems. These microbial groups form a stratified structure within the biofilm: AOB, such as Nitrosomonas, thrive in the oxygen-rich zone near the membrane surface, where high dissolved oxygen supports ammonia oxidation.

Conventional nitrogen removal in MABRs: simultaneous nitrification–denitrification and shortcut pathways

SND is the most established nitrogen removal mode in MABRs, enabled by the counter-diffusion architecture that spatially separates aerobic and anoxic reactions within a single biofilm. Oxygen supplied through the membrane sustains ammonia oxidation near the membrane surface, while denitrification proceeds in outer anoxic layers adjacent to the bulk liquid. This intrinsic stratification allows integrated nitrogen conversion without multiple reactors or external carbon supplementation,

Full-scale and pilot-scale performance of MABR

MABRs have progressed from laboratory-scale studies to pilot and commercial installations for municipal and industrial wastewater. The bubble-free oxygen delivery mechanism typically achieves oxygen transfer efficiencies of 60-90%, substantially higher than conventional aeration (10-30%), enabling 20-50% reductions in aeration energy demand while maintaining high nitrogen removal (Downing and Nerenberg, 2008, Martin and Nerenberg, 2012)

In pilot-scale applications, several studies have

Research Needs and Future Direction

MABRs are emerging as a promising platform for energy-efficient nitrogen removal, yet several fundamental and engineering challenges still constrain their full-scale implementation. A central knowledge gap lies in the quantitative coupling between oxygen flux, biofilm architecture, and nitrogen transformation pathways. Although microbial functions are often inferred from metagenomic profiles, the mechanistic links between oxygen penetration depth, redox stratification, and the relative

Conclusions

Membrane aerated biofilm reactors represent a transformative platform for sustainable nitrogen removal by decoupling oxygen supply from bulk liquid hydraulics and establishing counter-diffusion biofilms with stratified redox environments. This configuration enables multiple nitrogen transformation pathways, including shortcut nitrification, anammox, and autotrophic denitrification, to coexist within a single reactor while significantly improving oxygen utilization efficiency and reducing

CRediT authorship contribution statement

Yuxin Qin: Writing – review & editing, Writing – original draft, Software, Data curation, Conceptualization. Jun Zhang: Methodology, Data curation. Peiyao Yuan: Software, Data curation. Yongjiao Ding: Validation, Formal analysis. Rongbo Guo: Resources, Funding acquisition. Chengxian Wang: Software, Data curation, Conceptualization. Bingrui Ma: Supervision, Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The project was supported by National Key Research and Development Program (Grant No. 2024YFC3907200, 2024YFE0117300), the Natural Science Foundation of Shandong Province (Grant No. ZR2024QB307, 2023TZXD020), Key Technology Research, Development and Industrialization Demonstration Project of Qingdao (No. 25-1-1-gjgg-78-nsh).