Filamentous bacteria are one of the main components of activated sludge microorganisms, which not only support flocculent bacteria in forming sludge flocs but also easily lead to sludge bulking. This article summarizes the high-throughput sequencing technology and database construction that integrates the FISH probe and 16s rRNA gene of filamentous bacteria. The detection frequency and abundance were statistically analyzed to determine the core filamentous bacteria. The filamentous bacteria searchable in the NCBI database belong to six phyla and 42 genera, with a total of 94 species. The core filamentous bacteria in the expanded activated sludge include Microthrix parvicella (M. parvicella), type 0092, Mycobacterium fortuitum, etc. The physiological ecology of the core filamentous bacteria is discussed in detail, aiming to establish targeted sludge expansion control strategies, reduce the incidence and harm of sludge expansion, and propose that future research needs to promote further development in the field of environmental microbiology.

Activated sludge microorganisms in sewage treatment plants are crucial for controlling water pollution and protecting public health and the ecological environment. Activated sludge must have biodegradation, easy sedimentation, and separation functions. Filamentous bacteria play an essential role in floc formation and structure. However, low temperature, low load and low dissolved oxygen (DO) will destroy the balance between beneficial structural action and harmful overgrowth. In this study, the high-throughput sequencing (HTS) dataset of 16s rRNA gene sequence V3–V4 amplicons from 30 activated sludge samples from the Chuanhu Sewage Treatment Plant in Changchun was analyzed to investigate the abundance distribution of filamentous bacteria and further determine the main operating parameters and environmental factors. The experimental results showed that the filamentous bacterial community accounted for a large part of the entire microbial community, with the total filamentous bacterial percentage in each sample ranging from 7.32% to 56.81%, with large fluctuations in abundance and consistent with the SVI value. Although most of them were in flocs, they occasionally caused sedimentation problems when the water temperature was low. With 14 species of filamentous bacteria detected, the population structure of filamentous bacteria in the thermophilic, variable-temperature and low-temperature periods was universal and specific. The groups with a detection frequency of 100%, high abundance, and significant fluctuations in distribution were Microthrix parvicella and Nostocoida limicola I. The Pearson correlation analysis showed that the total abundance of filamentous bacteria and the fluctuation distribution of dominant filamentous bacteria abundance were significantly correlated with water temperature, sludge load, sludge age, and mixed liquid suspended solids (MLSS).

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.