A new step aeration approach towards the improvement of nitrogen removal in a full scale Carrousel oxidation ditch

[Display omitted] •A new step aeration mode was proposed in oxidation ditch with microporous aeration.•The aeration modes resulted in different microbial community.•Step aeration mode was advantageous for the denitrifying bacteria enrichment.•Step aeration mode showed the higher nitrogen removal performance. Two aeration modes, step aeration and point aeration, were used in a full-scale Carrousel oxidation ditch with microporous aeration. The nitrogen removal performance and mechanism were analyzed. With the same total aeration input, both aeration modes demonstrated good nitrification outcomes with the average efficiency in removing NH4+-N of more than 98%. However, the average removal efficiencies for total nitrogen were 89.3% and 77.6% under step aeration and point aeration, respectively. The results indicated that an extended aerobic zone followed the aeration zones could affect the proportion of anoxic and oxic zones. The step aeration with larger anoxic zones indicated better TN removal efficiency. More importantly, step aeration provided the suitable environment for both nitrifiers and denitrifiers. The diversity and relative abundance of denitrifying bacteria under the step aeration (1.55%) was higher than that under the point aeration (1.12%), which resulted in an overall higher TN removal efficiency.