The role of SBR mixed liquor volume exchange ratio in aerobic granulation

Four sequencing batch reactors (SBR) were operated at different volume exchange ratios of 20–80%. Results showed that a rapid aerobic granulation could be achieved in SBR run at high volume exchange ratios, and the characteristics of mature aerobic granules in terms of mean size, SVI, granule fraction, EPS and calcium content were all closely related to the volume exchange ratio applied to SBR. To interpret the role of volume exchange ratio in aerobic granulation, a concept of minimum settling velocity of bioparticles was proposed, which is the function of both settling time and volume exchange ratio. It was further demonstrated that the effect of volume exchange ratio on aerobic granulation in SBR would be realized through selecting the minimum settling velocity of bioparticles. In fact, the minimum settling velocity would serve as a major selection pressure exerted on bioparticles. It appeared from this study that the production of EPS was stimulated significantly by high volume exchange ratio, which was associated with marketable accumulation of calcium ion in aerobic granulation. It is expected that this study may offer insights into the mechanism of aerobic granulation.