[Effect of Two-Stage Aeration on Nitrogen Removal Performance of Aerobic Granular Sludge]

Four mini experiments were conducted at different conditions. The heterotrophic microorganisms on the aerobic granular sludge surface consumed organic compounds at the initial stage of aeration. The denitrification rate and the efficiency of NO₂^--N and NO₃^--N removal were relatively low. Therefore, under the normal temperature conditions (20-23℃), aerobic granular sludge sequencing batch reactor (SBR) was operated in the two-stage aeration mode(first in low aeration then in high aeration mode). The low aeration time were carried out at 1, 2 and 3 hours stages respectively, and the characteristics of the granular sludge and its effects on microorganisms were analyzed by scanning electron microscopy (SEM) and fluorescence in situ hybridization (FISH) technique. The results show that the increase in the aerobic granular sludge (AGS) particle size improved the denitrification capacity; the denitrification rate of NO₂^--N was the highest at low aeration mode with 2 h and reached 9.66 mg·(g·h)^-1. The accumulation rate of nitrite increased to 77.84% and the total nitrogen removal rate to 70%. The bacterial count inside the granular sludge increased and they were mainly cocci, bacillus, and ellipsoidal bacteria. Moreover, the proportion of ammonia-oxidizing bacteria in total bacterial count increased from 13.70% to 15.40%. Therefore, the two-stage aeration process achieved shortened simultaneous nitrification and denitrification processes and showed a good denitrification performance.