SBR for cultivation of aerobic granules is operated in a sequencing cycle of feeding, aeration, settling, and discharging of supernatant. In SBR, the aeration period actually consists of two phases: a degradation phase in which the substrate is depleted to a minimum, followed by an aerobic starvation phase in which the external substrate is no longer available. Thus, it is likely that microorganisms in SBR are subjected to a periodic feast and famine regime, called periodic starvation (Tay et al., 2001a). Under the periodic feast-famine conditions, bacteria becomes more hydropho-bic and high cell hydrophobicity in turn facilitates microbial aggregation (Bossier and Verstraete, 1996; Tay et al., 2001a; Liu et al., 2004a).
When bacteria are subjected to a periodic feast-famine regime, micro-bial aggregation could be an effective strategy for cells against starvation. In fact, the periodic feast-famine regime in SBR can be regarded as a kind of microbial selection pressure that may alter the surface properties of cells. However, research also showed that aerobic granules could not be successfully developed if the settling time in SBR was not properly controlled, even though a periodic feast-famine regime was present (Qin et al., 2004a,b), while negative effects of nutrient starvation on the surface properties of aerobic granules in terms of cell hydrophobicity and the content of extracellular polysaccharides were also observed (Zhou, 2004). In addition, when the starvation time in SBR was reduced from 3 h to below 30 min, no significant impact on aerobic granules was observed. This may imply that the periodic feast-famine regime could favor aerobic granulation, but so far there is no solid experimental evidence to show that starvation acts as an inducing force of aerobic granulation in SBR.
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