A highly efficient phosphorus-accumulating organism, Cupriavidus plantarum S7–1A, which can completely remove inorganic nitrogen, was isolated from municipal activated sludge. The S7–1A generated no gaseous nitrogen, was confirmed to produce no hemolysin, and showed high susceptibility to cephalosporin and floxacin antibiotics. Optimum conditions were obtained with sodium pyruvate as a carbon source, a C/N of 18, a pH of 7–9, a temperature of 30°C, a phosphorus concentration of 16.5 mg/L, and shaking at 250 rpm. The highest nitrate, nitrite, and ammonium removal efficiencies were respectively 99.94%, 99.98%, and 99.62%, and the corresponding removal rates were 4.87, 3.84, and 6.90 mg/L/h. Over 89% of total organic carbon was removed, and phosphorus was not detected at initial concentrations below 16.5 mg/L. Genome sequencing and PCR confirmed that S7–1A has nasA, nirB, nirD, ppk, ppk2, and ppx genes. In conclusion, S7–1A proved to be a safe and efficient assimilatory nitrate reduction and phosphorus-accumulating organism (ANR-PAO).

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.