Granular dimension in Anammox: Linking particle morphogenesis to nitrogen removal efficiency

Anaerobic ammonia oxidation (Anammox) granular sludge has a series of advantages, with its nitrogen removal performance varying according to particle size. This review systematically delineates the mechanism by which anammox granular sludge (AnGS) particle size regulates the nitrogen removal process through two primary pathways: mass transfer mechanisms and microbial community restructuring. It analyzes the influence of particle size on its physicochemical characteristics, anammox activity, extracellular polymeric substances (EPS), and microbial community structure, and accordingly proposes targeted strategies for particle size regulation. With regards to EPS content, a direct correlation is established with particle size but drops markedly beyond 3 mm. The proportion of polysaccharides (PS) increases in large-sized granules (>2 mm). Microbial community analysis confirms that increased particle size leads to reduced community diversity, yet within the optimal particle size range (0.5-2.0 mm), an optimal balance between functional microbial activity and system stability can be achieved. By adjusting reactor parameters, or by crushing and re-granulation oversized granular sludge combined with supplementing exogenous substances such as iron-based materials, polyvalent cations, and exogenous signaling molecules, the ideal particle size distribution can be precisely maintained. Future research should clarify mass transfer-microbe couplings and optimize exogenous dosing to support targeted sludge control.