Application of low-intensity ultrasound to enhance simultaneous nitrification/iron-based autotrophic denitrification.

Intermittent ultrasound with an intensity of 0.2 W/ml was applied during simultaneous nitrification/iron-based autotrophic denitrification to evaluate its impacts on total nitrogen (TN) removal efficiency and microbial characteristics during low carbon/nitrogen ratio (C/N) wastewater treatment. At an optimal dissolved oxygen (DO) concentration of 1.2 mg/L, the TN removal rate was 91 ± 4%, and the TN concentration in the effluent decreased by 31% owing to the ultrasound treatment. In addition, the number of iron-compounds that formed in the sludge and wastewater increased by 55% and 37%, respectively. Low-intensity ultrasound caused a substantial increase in ammonia monooxygenase activity. Moreover, when the DO concentration increased to 1.2 mg/L, the activities of nitrate reductase and nitrite reductase, both of which are associated with denitrification, were effectively maintained. High-throughput sequencing indicated that low-intensity ultrasound enriched ammonium oxidising bacteria (Nitrosomonas) and suppressed the growth of heterotrophic denitrifying bacteria (Zoogloea and Simplicispira). These changes benefited simultaneous nitrification and autotrophic denitrification. Thus, low-intensity ultrasound promoted the simultaneous nitrification/iron-based autotrophic denitrification process during low C/N wastewater treatment.