Propagation of side-stream partial nitritation–anammox disturbance to mainstream wastewater greenhouse gas emissions

Side-stream partial nitritation-anammox (PN/A) process is energy efficient for nitrogen removal but highly sensitive to disturbances such as high organic loadings from anaerobic digestion (AD) liquor. How such disturbance affects downstream treatment performance and greenhouse gas (GHG) emissions in full-scale plants remains insufficiently understood. Here, a full-scale Anoxic-Aerobic-AD-PN/A wastewater treatment plant was compared during satisfied and disturbed PN/A operation. Under disturbed operation, the mainstream apparent liquid-phase TN removal rate decreased from 472.4 to 298.2 kg-N/d and estimated N 2 O and CH 4 emissions increased by 69.39 kg-N 2 O/day and 10.99 kg-CH 4 /day, respectively. Estimated mainstream GHG emissions rose from 61784.2 to 82768.9 CO 2 -eq.kg/day. A broader comparison including PN/A N 2 O estimates and biogas-electricity offset also suggested increased climate burden (58997.2 to 77742.1 CO 2 -eq kg/day). Metagenomic analysis showed marked reductions in the relative abundance of Ca. Brocadia and Nitrosomonas in PN/A, together with a strong depletion of anammox genes, hdh / hzo . Elevated Nitrosomonas -associated cytochrome P460 and higher abundances of nirK/norBC (assigned to Nitrosomonas and Nitrospira ) in the Anoxic and Aerobic tanks were consistent with increased nitrifier-associated N 2 O-production potential. These findings indicate that disturbance of side-stream PN/A can propagate to the mainstream and increase estimated mainstream GHG emissions, highlighting the need to balance AD intensification with stable PN/A operation.