Particle size-dependent behavior of redox-active biochar to promote anaerobic ammonium oxidation (anammox)

Nitrate (NO3−) accumulation in effluent decreases the biological nitrogen performance of anammox process for wastewater treatment. Redox-active biochar has been demonstrated as an effective additive to improve microbial NO3− reduction via redox cycling of the surface functional groups. However, their redox reactivity toward anammox consortia remains unknown. In this study, we investigated the effects of corn stover-derived biochars with different particle sizes (10–30 μm (SP), 200–500 μm (MP) and 2–5 mm (LP)) on nitrogen removal performance of anammox reactors under continuous operation with varying nitrogen loading rates over the long term. The average nitrogen removal efficiencies of anammox reactors amended with SP-, MP- and LP-biochar were 97.9%, 97.5% and 87.9%, respectively, which were significantly higher than that of the control (80.5%). Effluent NO3− concentration was reduced by 40.7–45.2% with presence of biochar as compared to the control, which may be attributed to microbial interaction with Chloroflexi-affiliated heterotrophic denitrifying bacteria. The electrochemical analysis of the sludge extracellular polymeric substances (EPS) based on cyclic voltammetry and electrochemical impedance spectrum revealed that the extent of biochar to increase the electron-transport ability of EPS decreased with the biochar particle size (SP > MP > LP). Furthermore, SP-biochar outperformed LP-biochar for promoting the proliferation of anammox bacteria Candidatus Brocadia and increasing the abundance of anammox-associated functional genes (hzsA, hdh, nirS and napA) in the sludge. Together, these findings showed the stimulatory effect of redox-active biochar for anammox consortia and the particle size-dependence, which may potentially assist in operating anammox processes with high nitrogen removal performance.