Establishing and maintaining stable anammox pathway for nitrogen removal in a pure floc sludge A2/O reactor treating low COD/N municipal wastewater

Most municipal wastewater treatment plants worldwide rely on the traditional activated sludge process for nutrient removal and recovery. However, current research and applications of mainstream anammox primarily utilize biofilms or granular sludge as media, which inevitably increases capital costs and operational complexity. This study, for the first time, demonstrates the long-term stability of partial denitrification/anammox (PD/A) in a pure floc sludge system treating real municipal wastewater (COD/N: 2.3-3.3) employing the widely-applied anaerobic-anoxic-oxic (A(2)/O) process. It reveals the crucial role of extended sludge retention time (> 150 days) and low COD/N (2.7-3.0) in achieving efficient nitrogen removal. Anammox bacteria rapidly accumulated in the floc sludge via natural shedding from the PD/A biofilms and continued to proliferate after the system shifted to a pure floc sludge configuration, ultimately reaching 2.16 x 10(9) copies/g SS. The pure floc system sustained stable PD/A for 280 days, with anammox contributing 31.7-45.3 % of nitrogen removal. The system achieved a nitrogen removal efficiency of 79.2 +/- 3.8 %, with an effluent concentration of 9.8 +/- 1.7 mg-N/L. This study highlights the potential of pure floc sludge systems for mainstream anammox implementation, presenting a simpler, more cost-effective alternative to biofilms and granular sludge systems.