Introduction

As Liu and Tay (2004) noted, many factors including substrate composition, organic loading, hydrodynamic shear force, feast-famine regime, feeding strategy, dissolved oxygen, reactor configuration, solids retention time, cycle time, settling time, and exchange ratio, would influence the formation and properties of aerobic granules developed in sequencing batch reactors (SBR). However, only those parameters associated with selection pressures on the sludge particles would contribute to the formation mechanism of aerobic granules (Liu et al., 2005a). In SBR, two major selection pressures had been identified, i.e. the settling time and the volume exchange ratio (McSwain et al., 2004; Qin et al., 2004a,b; Hu et al., 2005; Liu et al., 2005a; Wang et al., 2005a), while Arrojo et al. (2004) and Wang et al. (2005a) further showed that the discharge time of SBR would be the third important selection pressure of aerobic granulation. In fact, selection pressure has been found to be the key driving force towards successful anaerobic granulation in upflow anaerobic sludge blanket reactor (Hulshoff Pol et al., 1988; Alphenaar et al., 1993). To accelerate industrial application of the aerobic granulation technology, a sound understanding of the mechanisms behind aerobic granulation is highly desirable.

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