Effects of key enzyme activities and microbial communities in a flocculent-granular hybrid complete autotrophic nitrogen removal over nitrite reactor under mainstream conditions.

Graphical abstract Highlights • Highest nitrogen removal was achieved at 20 °C in the hybrid CANON reactor. • Performance was not always deteriorated with temperature reduction. • Activities of key enzymes were analyzed with temperature reduction. • The factors for maintaining nitrogen removal performance varied with temperature. Abstract Recently, a flocculent-granular hybrid reactor was reported as a novel nitrogen removal system; however, the mechanisms of stable operation in the system remain unclear. In this study, the mechanisms of the stable nitrogen removal performance in a flocculent-granular hybrid system were investigated with temperature reduction. The operational period was divided into three phases with different temperatures ranges. In phase I, the nitrogen removal efficiency was stabilized at about 90% with nitrogen removal load maintained at approximately 0.28 kg N/(m3·day). In phase II, while decreasing the temperature to 20 °C, the activities of key enzymes were reduced immediately and were then maintained at a certain level. The relative abundances of aerobic ammonium-oxidizing bacteria and anaerobic ammonium-oxidizing bacteria gradually increased at this phase. In phase III, after the temperature dropped to 15 °C, the activities of key enzymes gradually increased due to adaptation to low temperature, boosting the nitrogen removal efficiency to 83%. [ABSTRACT FROM AUTHOR]