Full-scale process intensification via partial denitrification-anammox and densification without dedicated physical selectors under strict nitrogen limits

This study presents an over 1200-day full-scale demonstration integrating partial denitrification–anammox (PdNA) and biological selection-driven sludge densification to achieve process intensification under stringent nitrogen limits. The work was conducted at a 181,000 m 3 /d water resource recovery facility employing a continuous-flow step-feed activated sludge process for secondary treatment followed by tertiary moving bed biofilm reactors (MBBRs). PdNA was successfully initiated in the tertiary MBBR without anammox seeding, representing the first reported unseeded mainstream PdNA startup with sustained long-term full-scale operation while consistently maintaining effluent NH 4 + -N below 0.5 mg/L (while the discharge limit is 1 mg/L) and total nitrogen below 3 mg/L. Step-feed redistribution enabled controlled NH₄ + -N bleed-through from the secondary process without advanced aeration control, while methanol-driven partial denitrification occurred primarily via a rate-differential mechanism that generated stable NO 2 − -N, together facilitating anammox enrichment and robust PdNA performance. Compared to historical conventional nitrification–denitrification (NdN) operation, PdNA reduced methanol demand by 25% and aeration energy by 12%, but led to further deterioration of already suboptimal sludge settleability. To address this challenge, effective sludge densification was achieved solely through optimized biological selection by increasing the step-feed fraction of primary effluent directed to upstream reactor passes, creating strong feast-to-famine conditions. Sludge volume index decreased from approximately 110–120 mL/g to as low as 50 mL/g without hydrocyclones or other dedicated physical selectors, and sludge densification was confirmed through particle size distribution and microscopy analyses. Operational data demonstrated strong synergy between PdNA and sludge densification. These results establish a practical and scalable framework for intensified nutrient removal without complex controls or major infrastructure modifications.