Start-Up of Full-Scale Two-Stage Partial Nitritation/Anammox (PN/A) Process Treating Reject Water from Codigestion of Sludge and Food Waste

The anaerobic codigestion of municipal sludge and food waste enhances biogas production but generates reject water rich in ammonia nitrogen and organic matter, coupled with a low carbon-to-nitrogen (C/N) ratio, posing significant treatment challenges. Although the partial nitritation/anammox (PN/A) process offers an energy-efficient alternative for nitrogen removal, its application to reject water from codigestion─especially at engineering scale─has been limited due to its complex and inhibitory characteristics. This study demonstrates the successful start-up within 21 days and seven month stable operation of a fully engineered two-stage PN/A system (15 m 3 /d) treating codigestion reject water, using a three-phase strategy comprising AnAOB enrichment, microbial acclimation, and steady operation. Key findings include stable total inorganic nitrogen removal efficiency (TINRE) exceeding 83%, a total inorganic nitrogen removal loading rate (TINRR) peak of 0.85 kgN/(m 3 ·d), successful microbial community adaptation with dominant anammox bacteria ( Candidatus Brocadia ) retained under stress, functional gene shifts (e.g., amoABC, hzsABC, hdh ) indicating metabolic plasticity, and a direct treatment cost of 8.12 CNY/m 3 reject water. This study not only offers theoretical insights but also presents a practical demonstration of efficient nitrogen removal from codigestion reject water of municipal sludge and food waste, providing valuable and entirely different guidance for future full-scale applications.