Influence of starvation time on formation and stability of aerobic granules in sequencing batch reactors

Three sequencing batch reactors, R1, R2 and R3, with a 1.5-h, 4-h and 8-h cycle time, respectively, were used to cultivate aerobic granules with the same synthetic wastewater containing 1000 mg l(-1) COD. As the initial COD concentrations in the cycles were the same, three different cycle times led to three different starvation times in repeated cycles of the three reactors. It was found that 63 cycles were needed to form granules with the longest starvation time in R3 while it took 256 cycles in R1 with the shortest starvation time. However, as far as the formation time was concerned, granules were formed on day 16 with 1.5-h cycle time while on day 21 with 8-h cycle time, which indicated that a shorter cycle time with a shorter starvation time speeded up the granulation. This was mainly due to the stronger hydraulic selection pressure at shorter cycle time. However, it was found that granules formed with cycle time of 1.5h were unstable. Fluffy granules with poor settling ability were observed in R1 in the 4th month, which led to the collapse of R1 after 160-day of operation. Granules in R2 and R3 showed good stability during the long-term operation. Therefore, a reasonable starvation time was necessary to maintain the long-term stability of aerobic granules.