A two-stage denitrification-partial nitritation/anammox (DN-PN/A) process to treat high-solid anaerobic digestion (HSAD) reject water: Verification based on pilot-scale and full-scale projects

High-solid anaerobic digestion (HSAD) reject water, distinguished by elevated levels of chemical oxygen demand (COD), ammonium (NH4+-N) and an imbalanced COD/TIN, presents a significant challenge for treatment through partial nitritation/ anammox (PN/A) process. To address this challenge, this study introduced two-stage denitrification-partial nitritation/ anammox (DN-PN/A) process. The effectiveness of the DN-PN/A process was confirmed through both pilot-scale (12 t/d) and full-scale (400 t/d) operations, showcasing an impressive total removal rate of over 90% for TIN and exceeding 60% for COD. Notably, 30% TIN was eliminated through heterotrophic denitrification, while approximately 55% of TIN removal occurred in the anammox stage. In the DN-PN stage, aerobic-anaerobic alternation effectively eliminated organics (around 50% COD) and balanced nitrogen species. Microbial and metagenomic analysis verified the coupling between autotrophic and anaerobic denitrification and demonstrated that DN-PN stage scted as a buffer for anammox stage. This comprehensive study highlights the DN-PN/A process's efficacy in stably treating HSAD reject water.