Low-dose sodium acetate-mediated energy compensation stabilizes oxygen-limited urea hydrolysis coupled with partial nitritation/anammox for high-strength urea wastewater treatment.

The high free ammonia (FA) environment generated during high-strength urea wastewater hydrolysis can impair biological nitrogen removal, while insufficient energy supply under inorganic influent conditions may limit long-term high-load urea hydrolysis. In this study, a continuous-flow two-stage system coupling an up-flow anaerobic filter reactor (UAFR) with a partial nitritation/anammox (PN/A) reactor was developed, and the role of low-dose sodium acetate in stabilizing UAFR urea hydrolysis was investigated. The UAFR showed hydrolysis deterioration under inorganic feeding, whereas hydrolysis performance rapidly recovered after anhydrous sodium acetate equivalent to 100 mg/L chemical oxygen demand was added on day 53. When the influent urea concentration increased to 2000 mg/L, the UAFR maintained a urea removal efficiency above 99.5%, with a urea removal rate of 8.0 kg urea/(m3·d). Acetate withdrawal caused a marked increase in effluent urea, indicating that high-load hydrolysis stability was closely associated with continuous acetate supply. Microbial community and metagenomic analyses showed enrichment of fermentative bacteria such as Tissierella and community-level increases in ackA-pta and urease-related genes, suggesting enhanced acetate-associated energy metabolism and urea hydrolysis potential under high-FA stress. After PN/A treatment, the overall total nitrogen removal efficiency reached 73.4%. This study demonstrates that low-dose acetate can stabilize UAFR urea hydrolysis and enable its coupling with autotrophic PN/A for high-strength urea wastewater treatment.