Three-dimensional structured carriers enhance the performance of hybrid anammox reactor: Start-up kinetics and granule–biofilm interactions

Hybrid anammox reactors integrating granular sludge and biofilm carriers are effective in maximizing biomass retention and enhancing nitrogen removal. However, the specific influence of carrier structure on reactor performance and related co-existing mechanisms from different biomass remains poorly understood. This study employed spatially designed hybrid anammox reactors with planar and three-dimensional (3D) structured carriers to investigate start-up kinetics and granule–biofilm interaction mechanisms. The nitrogen loading rate increased 11–17 times within 90 d in the hybrid anammox reactors. A 28 % shorter doubling time of nitrogen removal rate was observed in the reactor with 3D structured carriers than in the reactor with planar structured carriers. The 3D structured carrier promoted rapid biofilm colonization and formation via interception and attachment, with a biomass retention 3.1 times higher than that of the planar structured carrier, significantly enhancing the contribution of the biofilm to nitrogen removal. The granules and biofilms in the planar structured carrier exhibited similar microbial compositions and functions, suggesting a parallel nitrogen removal mode in the hybrid anammox reactor. The reactor incorporating 3D structured carriers exhibited greater differentiation between granules and biofilms, enhancing functional complementarity and microbial interaction. Granules reinforced anammox metabolism, while biofilms supplemented denitrification and nitrogen assimilation pathways. These findings provide novel insights into reactor configuration design and suggest that hybrid anammox reactors equipped with 3D structured carriers are a promising approach for anammox process applications. • Spatially designed granule–biofilm hybrid enabled fast start-up of anammox reactor. • 3D structure carrier-hybrid reactor had 28 % shorter doubling time than planar type. • 3D structure carrier reached high biomass retention and microbial diversity. • Synergistic and parallel nitrogen removal modes depended on carrier structure. • Granule–biofilm interaction enhanced nitrogen removal in hybrid anammox reactor.