LAUSR

Anaerobic ammonium oxidation (anammox) pathway is sensitive to organic matter, and its recovery requires reliable evidence regarding the dominance of anammox in N-removal. This study showed that the anammox process deteriorated, with N-removal efficiencies rapidly decreasing from 87.2 to 45.7% when reactors were exposed to COD shocks of 1.12, 2.24 and 3.36 g L −1 (COD/N ratio 2, 4 and 6). Comprehensive assessments of water quality, microbial characteristics and isotope analysis were adopted to investigate anammox recovery. Operational performance took 8–20 days to recover; anammox relative abundance recovered after 20 days, based on the results of fluorescence in situ hybridisation and quantitative PCR; and the anammox pathway contributed to 80.0–91.5% of N-loss 40 days after COD shock terminated, based on the results of the isotope labelling experiment. Therefore, a complete recovery required 40 days. The isotope labelling method supplied a reliable reference for recovery assessment of anammox system in real-world applications.