Examining the Key Denitrifying Bacterial Community Structure and Individual Proliferation of Activated Sludge in Wastewater Treatment Plants Operating at Low Temperatures

To analyze the microbiological mechanisms of biological denitrification during low-temperature operations, continuous sampling of influent and activated sludge samples was conducted at the Changchun Municipal Wastewater Treatment Plant. The relative abundance and absolute gene abundance of ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and denitrifying bacteria were determined using high-throughput sequencing technology and reverse transcription–polymerase chain reaction (RT–PCR) technology, respectively. Nitrosomonas and Nitrosospira were the dominant bacteria in ammonia-oxidizing bacteria; the detection rate was 100%; and the abundance distribution fluctuated greatly. The percentages of net proliferation rate greater than −0.05 were 75% and 62.5%, respectively, but the temperature effect was not obvious. The detection rate of Nitrosomonadaceae (norank) was 76.67%, and the percentage of net proliferation rate greater than −0.05 was 50%. The growth of ammonia-oxidizing archaea was limited at low temperature, and the abundance of most bacteria fluctuated greatly. The frequencies of net proliferation rate of Crenarchaeota (norank), Thaumarchaeota (norank), and Nitrososphaera greater than −0.05 were more than 50%. Of the 20 OUTs of denitrifying bacteria, 16 had a net increment rate greater than −0.2/d with a frequency greater than 50 per cent, of which Sinorhizobium and Alphaproteobacteria were detected with a frequency of 100% in activated sludge. The frequency of AOB and denitrifying bacteria net proliferation rate greater than zero during the low-temperature period was relatively high, which ensured the smooth progress of the denitrification process and reasonably explains the microbiological mechanism. In addition, it can be inferred that the migration of influent microorganisms can shape the population structure of denitrifying bacteria, as the net proliferation rate of most bacterial populations was less than 0.