[Nitrogen Removal Effect and Conversion Characteristics of Nitrous Oxide in Single-stage and Multi-stage A/O...

The single-stage A/O and multi-stage A/O processes were simulated by sequencing batch reactors (SBRs) with alternate stirring and aeration. The removal efficiency of nitrogen and the release mechanism of N₂O were studied under the identical conditions of influent quality, hydraulic retention time (HRT), sludge retention time(SRT), temperature and anoxic/oxic (A/O) retention time ratio. Experimental results showed that COD or ammonia-nitrogen removal had no significant difference between the single-stage and the multi-stage A/O processes for the influent quality equivalent to municipal wastewater. However, TN removal efficiency of the former was better than the later with 72.1% and 52.2%, respectively. In the conversion of total nitrogen, during the typical cycle in the single-stage A/O and multi-stage A/O processes, the yields of N₂O were 16.95 mg and 3.95 mg, respectively. The conversion rate, which is the ratio of N₂O yield and TN removal, was respectively 11.47% and 4.11%. N₂O production and emission occurred mainly in aerobic (nitrification) phase while there was little N₂O emission in anoxic (denitrification) phase. Although the dominant species of AOB was both Nitrosomonas in the single-stage A/O and the multi-stage A/O processes under the same operating conditions, it was more conducive to the growth of nitrifying bacteria (AOB, NOB) in the single-stage A/O process with the greater abundance of Nitrosomonas. Meanwhile, the type and abundance of NOB in the single-stage A/O process were significantly more than in the multi-stage A/O process too. Therefore, it is more competitive to deal with the high-strengthening ammonia-nitrogen wastewater in the single-stage A/O process. In the actual operation of wastewater treatment, using appropriate partitions of A/O or oxygen-supplying modes can not only result in better nitrogen removal but also decrease the secondary pollution caused by N₂O to the atmosphere.