Opportunities and challenges of shortcut nitrogen removal in membrane-aerated biofilm reactors (MABRs)

Abstract

The membrane-aerated biofilm reactor (MABR) has attracted increasing attention from both academia and water practitioners due to its potential to enhance aeration efficiency and intensify treatment capacity. However, emerging evidence highlights a critical challenge in achieving shortcut nitrogen removal within MABRs because nitrite-oxidizing bacteria (NOB)—key enemies of shortcut nitrogen removal—reside in the innermost biofilm layer and are consequently well-protected. In response, extensive efforts over the past two decades have focused on developing strategies for stable NOB suppression to facilitate shortcut nitrogen removal in MABRs. This work offers a comprehensive review of the advancements made in this field, providing an in-depth analysis of the kinetic mechanisms that underpin the NOB suppression in MABRs. We propose that the current NOB suppression strategies in MABRs could be categorized into three groups according to their different kinetic mechanisms, while most were tested in laboratory with synthetic or high-strength wastewater and under controlled conditions. We present various strategies for the start-up, long-term maintenance and performance recovery after deterioration for shortcut nitrogen removal in MABRs. We also highlight MABRs as a potential platform for enriching and applying new nitrogen-cycling microorganisms. Finally, future research directions are suggested in both fundamental and engineering aspects, including exploring microbial ecology in biofilms and demonstrating NOB suppression strategies in real-world conditions. These efforts could further advance the understanding and application of MABRs in nitrogen removal processes.