In-depth analysis of the steady operation of SAD (Simultaneous Anammox and Denitrification) process with added recirculation

The slow formation of simultaneous anammox and denitrification (SAD) granular sludge remains a major challenge for its practical application. This study developed a novel strategy for rapid cultivation of SAD granular sludge in lab-scale UASB reactors using synthetic wastewater and municipal sludge as inoculum. By introducing a recirculation system, we significantly enhanced sludge granulation, system stability, and nutrient removal performance. The recirculation-enabled reactor (R1) achieved over 90% removal of nitrogen and COD, promoted the secretion of extracellular polymeric substances (EPS), and increased the abundance of key anammox bacteria <i>(Candidatus Kuenenia</i>). Microbial and functional genes analyses revealed that recirculation up-regulated functional genes related to denitrification and granulation, improving both structural integrity and process efficiency. These findings demonstrate the effectiveness of recirculation in accelerating SAD sludge granulation and stability under controlled conditions. This work provides a feasible approach for optimizing SAD start-up and provide valuable insights into optimizing SAD granulation strategies, with important implications for energy-efficient nitrogen removal in wastewater treatment.