Spatial distribution and nitrogen metabolism behaviors of anammox biofilms in bioelectrochemical system regulated by continuous/intermittent weak electrical stimulation

A bioelectrochemical anammox system (BEAnS) was constructed and operated at low temperature to investigate the effect of weak electrical stimulation (continuous or intermittent) on metabolism behaviors, electrochemical activity, nitrogen removal, and spatial distribution of anammox biofilms. Results showed that the continuously stimulated biocathode exhibited the highest electron transfer system activity (34.2 ± 6.9 μg mg−1 h−1), the fastest electron transfer coefficient (0.0412 s−1), the largest current density (0.25 A cm−2), and the smallest polarization resistance (∼4 Ω). This led to an enhanced nitrogen removal of 60.6 ± 5.6% and energy recovery of 289.1 mW m−3, compared with that of the control. Confocal laser scanning microscopy and fluorescence in situ hybridization analysis confirmed that continuous weak electrical stimulation could accelerate the secretion of extracellular polymer substances, which facilitated the aggregation/adhesion of single cell and subsequent development of compact and stratified co-existing biofilms on cathode. Microbial analysis further demonstrated the proliferation of electrotrophic denitrifying microorganisms (i.e., Thauera, Hydrogenophaga, and Ignavibacterium), nitrifier (Candidatus_Nitrotoga), and denitrifier (Denitratisoma), except for anammox bacteria (i.e., Candidatus_Brocadia, and Candidatus_Jettenia), implying the critical role of weak electrical stimulation in diversifying N-degrading microbial community and creating a more robust nitrogen metabolic pathway. These findings can provide a novel insight into autotrophic electroactive anammox biofilm enrichment and their activity enhancement by weak electrical stimulation.