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High-solid anaerobic digestion (HSAD) reject water, distinguished by elevated levels of chemical oxygen demand (COD), NH4+-N and an imbalanced COD/TIN, presents a significant challenge for treatment through conventional partial nitritation/ anammox (PN/A) process. This study introduced a revised two-stage PN/A process, namely partial nitritation/denitritation-anammox (PN-DN/A) process. Its effectiveness was investigated through both pilot-scale (12 t/d) and full-scale (400 t/d) operations, showcasing stable operation with an impressive total removal rate of over 90 % for total inorganic nitrogen (TIN) and exceeding 60 % for COD. Notably, 30 % of TIN was eliminated through heterotrophic denitritation in partial nitritation-denitritation (PN-DN) stage, while approximately 55 % of TIN removal occurred in the anammox stage with anaerobic ammonium oxidizing bacteria (AnAOB) enrichment (Candidatus Kuenenia, 25.9 % and 26.6 % relative abundance for pilot and full scale). In the PN-DN stage, aerobic-anaerobic alternation promoted organics elimination (around 50 % COD) and balanced nitrogen species. Microbial and metagenomic analysis verified the coupling between autotrophic and heterotrophic denitritation and demonstrated that PN-DN stage acted as a protective buffer for anammox stage. This comprehensive study highlights the PN-DN/A process's efficacy in stably treating HSAD reject water.