Municipal wastewater treatment plants (WWTPs) use functional microorganisms in activated sludge (AS) to reduce the environmental threat posed by wastewater. In this study, Illumina NovaSeq sequencing of 16S rRNA genes was performed to explore the microbial communities of AS at different stages of the two WWTP projects in Shenzhen, China. Results showed that Proteobacteria, Bacteroidetes, Acidobacteria, Firmicutes, and Nitrospirae were the dominant phyla in all the samples, with Proteobacteria being the most abundant and reaching a maximum proportion of 59.63%. There was no significant difference in biodiversity between the two water plants, but Stage 1 and Stage 2 were significantly different. The Mantel test indicated that nitrate, total nitrogen (TN), chemical oxygen demand (COD), and nutrients were essential factors affecting the bacterial community structure. FAPROTAX analysis emphasized that the leading functional gene families include nitrification, aerobic nitrite oxidation, human pathogens, and phototrophy. This study reveals changes in the community structure of AS in different treatment units of Banxuegang WWTP, which can help engineers to optimize the wastewater treatment process.

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Microflora immobilization is promising for nutrient removal applications in sewage; however, the metabolic and microbial mechanism needs to be further explored. Heterotrophic nitrification-aerobic denitrification (HN-AD) bacterium and efficient nitrogen (N) removal bacteria were selected and immobilized on corncob particles using alginate polymer to prepare microbe–organic complex beads. The complex beads were then added into activated sludge under a continuous-flow aerobic bioreactor with sufficient sodium acetate also applied as a simple electron donor. The role of polymer electron donors under carbon-rich conditions was then studied. Results showed that the total nitrogen removal rate improved by 8.3% (reaching 91.2%) and ammonium nitrogen removal rates were approximately 98%. Only 0.59 mg/L of nitrate nitrogen was detected in the treatment group. 16S rRNA gene sequencing results showed that bacterial richness in activated sludge within the treatment group was significantly higher than within the control group (p < 0.05), and KEGG pathways analysis indicated that carbon (C) metabolism gene and N-cycle-related genes were also improved. This suggested that polymer electron donors generated complex C sources that nourished diverse bacterial species related to N cycles so that the N removal rate could be strengthened and further improved by simple electron donors and the microflora.