Long-term stability of two-stage aerobic granular sludge system for combined organic matter and nitrogen removal from aerobically digested centrate wastewater

This study evaluates a two-stage moving bed biofilm reactor (MBBR)-aerobic granular sludge (AGS) system developed to treat high COD and high ammonia biosolid autothermal thermophilic aerobic digestion (ATAD) centrate. The MBBR efficiently handled high organic loading rates (OLRs) exceeding 20 kg COD/m³ /d and reduced the carbon to nitrogen (C/N) ratios from 6 to 3. Treated effluent was then processed in the AGS, where nitrogen removal was primarily accomplished via the nitritation/denitritation pathway, with removal efficiencies reaching 98.9 % for NH⁺₄-N and 91.7 % for total inorganic nitrogen (TIN). After 200 days of operation, the AGS system had an optimized hydraulic retention time (HRT) of 10 h and maintained a sludge volume index (SVI) between 40 and 80 mL/g, achieving a nitrogen treatment capacity of 1.8 kg N/m³ /d. Ammonia oxidizing bacteria (AOB) demonstrated a substantial nitrogen conversion efficiency, evidenced by nitrogen conversion rates of 0.57 ± 0.02 g N/g VSS/d and denitrification rates of 1.27 ± 0.01 g N/g VSS/d. Microbial analysis revealed the enrichment of Nitrosomonas (from 0.74 % to 6.34 %) and key denitrifying genera including Thauera, Pseudomonas, and Paracoccus, supporting the system’s high and sustained nitrogen removal performance. These findings underscore the potential of the two-stage MBBR-AGS process in treating high COD high ammonia wastewater, suggesting its viability for large-scale industrial applications.