Removal behavior, inhibition kinetics, microbial community and metabolic pathways in simultaneous anammox and denitrification under indole exposure

Simultaneous anammox and denitrification (SAD) has been proven to be one promising process to remove nitrogen and organics from wastewater. However, how indole as one of nitrogen-containing heterocyclic compounds (NHCs) affecting SAD is still unclear. The study first explored the effect of short-term indole exposure (0-150 mg/L) on SAD. The degradation rate of indole was the highest at 100 mg/L indole, while the maximum nitrogen removal efficiency occurred at 10 mg/L indole. As the indole level increased, both anammox activity and the contribution of anammox to nitrogen removal had rapid decreases, whereas the denitrification was significantly enhanced. Moreover, 50% inhibitory concentration (IC50) and substrate inhibition constant (Ki) were 147.04 mg/L and 149.22 mg/L, respectively based on inhibition models. With increasing indole, the abundance of Candidatus Brocadia and Candidatus Kuenenia sharply decreased, while indole-degraders, denitrifiers and dissimilatory nitrate reduction to ammonium (DNRA) bacteria were found. High-throughput sequencing demonstrated that metabolic pathways involving nitrogen removal and indole biotransformation could be driven via a series of key functional genes. These findings offer novel insights into SAD process under the indole stress.