Characteristics and stability of aerobic granules cultivated with different starvation time

The characteristics of aerobic granules at steady state and the effects of starvation time on the stability of aerobic granules during the long-term operation were investigated in three sequencing batch reactors (SBRs R1–R3). The SBRs were operated with a cycle time of 1.5, 4.0, and 8.0 h, respectively, which resulted in a starvation time of 0.8, 3.3, and 7.3 h in three reactors, respectively. Results showed that aerobic granules were successfully cultivated in the three reactors, but the granules in R2 with a starvation time of 3.3 h showed the highest density and the best settleability at steady state. It is obvious that the starvation time has an optimum value in terms of settleability of granules. In addition, it was found that the coexistence of a minority of fluffy granules with smooth granules was the potential unstable factor in R1 with a starvation time of 0.8 h at the steady state. The sudden dominance of fluffy granules in R1 after the 160-day operation led to the operation failure of the reactor R1, whereas the granules in R2 with a starvation time of 3.3 h and R3 with a starvation time of 7.3 h showed good stability during the long-term operation. As short starvation time leads to the instability of granules, and long starvation time is not advisable for practical application due to low efficiency, starvation time should be controlled in a reasonable range.