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Encapsulating microorganisms to enhance biological nitrogen removal in wastewater: recent advancements and future opportunities

Encapsulating microorganisms is promising to enhance biological nitrogen removal (BNR) in wastewater, with benefits of increased efficiency, reduced inhibition, and improved stability. Encapsulation technology has advanced, with recent findings in new encapsulation materials, pure and enrichment culture studies with novel nitrogen-converting microorganisms, and improved mathematical models and molecular tools to enable more predictive applications of encapsulation. Nevertheless, interactions between encapsulated microorganisms and between microorganisms and their surrounding matrices remain unclear. This review aims to summarize recent insights regarding our understanding and application of encapsulation for BNR. The review addresses the need to reevaluate the stability, permeability, and sustainability of encapsulation materials under realistic wastewater treatment conditions. In addition, comparing the kinetic and stoichiometric parameters of key microorganisms in BNR processes suggests that recently discovered groups of microorganisms, such as ammonia oxidizing Archaea, comammox, heterotrophic nitrifiers, and anammox bacteria could be a favorable choice for encapsulation. With respect to future opportunities, microorganism–encapsulant interactions in BNR should be further studied and understood using a combination of microscopic and molecular biology tools with predictions from mathematical models, further enabling the predictive application of encapsulation for BNR. The mechanistic understanding gained from studying encapsulated systems can also be extended to other treatment processes involving microbial immobilization.