Evaluation of anammox pathway recovery after high COD loading using water quality, molecular biology and isotope...

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.