Granular Stratification Enables Stable and Efficient Nitrogen Removal in Anammox Sludge Bed Reactors

The granule-based anammox process is a promising biotechnology for nitrogen removal; however, it is challenging to achieve a high rate and efficiency simultaneously. In this study, we addressed this challenge by coupling two granule types with complementary kinetics in an anammox sludge bed reactor. Bright red granules (GR) exhibited higher maximum specific anammox activity (SAA) but lower substrate affinity; they predominantly occupied the reactor's lower section, where substrate concentrations are high, forming a high-capacity layer. Yellow-brown granules (GY) showed higher substrate affinity but lower maximum SAA; they mainly populated the upper low-substrate zone, forming an efficient substrate-scavenging layer. This self-organized granule stratification enabled both a high nitrogen removal rate (14.5 kg-N/(m³·d)) and high efficiency (90.2%). Physicochemical property and microbial community analyses of granules showed that the stratification of "affinity" was mainly governed by the granular structure and mass transfer rather than intrinsic microbial traits. By actively withdrawing excessive sludge from the fast-growing lower zone while preserving biomass in the upper zone, such stratification can be stably maintained. Collectively, these findings advance our understanding of granule heterogeneity in anammox sludge bed bioreactors and offer a practical strategy for performance optimization.