LAUSR

To achieve nitrification/denitrification via nitrite (NO2-N) successfully under low temperature conditions, aerobic duration (AD) control and free ammonia (FA) control strategies were carried out in the start-up period of real swine wastewater treatment by two sequencing batch reactors (SBRs) operated in parallel. Over the entire 270 days of operation, NO2-N/ (NO2-N + NO3-N) accumulation in each reactor finally reached 73.3% (AD control) and 80.4% (FA control) respectively, when the temperature decreased from 25 °C to 12 °C. Meanwhile, the chemical oxygen demand (COD) and total nitrogen (TN) removal ratios were 92.1% and 95.1% respectively under AD control, and 92.2% and 95.0% under the FA control strategy. The satisfactory performance indicated that both strategies could help activated sludge to counteract the influence of low temperature well, reflecting the fact that the microbial communities in both reactors were not overly affected. Psychrophiles belonging to Bacteroidetes, γ-Proteobacteria, and Firmicutes were dominant and identified by construction of 16 s rRNA and amoA gene clone libraries. Although microbial community formation was influenced by decreasing temperature, the different strategies led to differences in community shift approaches and structures, particularly in terms of ammonia oxidizing bacteria (AOB). Some of the genera were washed out in AD control, while operational taxonomic units (OTUs) identified as Nitrosomonas eutropha and N. nitrosa could noticeably increase and quickly outcompete others under the FA control strategy.