Ammonia inhibition in anaerobic digestion and weakly electrical stimulation strategies: Exploring mitigation mechanisms and economic benefits-A review

Anaerobic digestion (AD) is widely used in waste treatment to produce clean energy such as biogas. However, the excessive ammonia produced during the AD process has a significant inhibitory effect on the microbial system. At present, strategies for alleviating ammonia inhibition have been studied. Among them, microbial electrolysis cell (MEC) emerged as a promising and effective approach in recent years. However, there is still a lack of systematic summary and comprehensive analysis of the mitigation mechanism. Here, ammonia inhibition mechanisms, mitigation mechanisms of MEC and the regulation of microorganisms were comprehensively reviewed. The results show that high ammonia MEC-AD system enhanced the expression of key proteins in hydrogenotrophic methanogens, and methane production shifted from acetoclastic pathway to the hydrogenotrophic pathway. NH₄⁺ is primarily oxidized at the anode through electrode-mediated pathways and microbial cooperation, enhancing anammox and denitrification gene abundance, promoting NH₄⁺ and NO₂^– conversion to nitrogen, thereby alleviating ammonia inhibition. This article provides a comparative assessment of MEC-AD and other technologies from the perspectives of technology, environmental sustainability, and economic feasibility. Supported by life cycle assessment, the MEC-AD system exhibits notable advantages in environmental and economic sustainability. However, mechanisms regulating substance conversion and enzyme activity are still unclear in MEC-AD system. In particular, the relationship between active functional microorganisms and their gene expression profiles. Overall, this paper presents the first systematic review of microbial metabolic regulation in high ammonia MEC-AD systems, providing a valuable framework for elucidating the mechanisms of MEC-AD technology alleviates ammonia inhibition. Abbreviations: AD, anaerobic digestion; FAN, free ammonia; VFA, volatile fatty acids; IET, interspecies electron transfer; EPS, extracellular polymeric substance; LCA, life cycle assessment; LCC, life cycle cost; AK, acetate kinase; AmtB, ammonium transfer protein; EAM, electroactive microorganisms; GHGs, greenhouse gas; MF, magnetic field-assistance; AS, air stripping; TAN, total ammonia nitrogen; CH₄, methane; EET, extracellular electron transfer; DIET, direct interspecies electron transfer; C/N, carbon/nitrogen; MEC, microbial electrolysis cell; HM, hydrogenotrophic methanogenesis; H₄MPT, tetrahydromethotrexate; ETS, electron transport system; AS, air stripping; SCOD, soluble chemical oxygen demand; CCC-MF, current-carrying-coil-based magnetic field.