The exchange ratio in SBR is defined as the liquid volume withdrawn at the end of the given settling time over the total reactor working volume. Wang et al. (2005) studied aerobic granulation in SBR run at different exchange ratios in the range of 20-80%, while the settling time was kept constant at 5 min. Thus, a larger exchange ratio is associated with a higher reactor height for discharge. The fraction of aerobic granules in the total biomass was found to be proportionally related to the exchange ratio, e.g. only in the SBR run at the higher exchange ratios of 60 and 80% were aerobic granules dominant; and a mixture of aerobic granules and suspended sludge instead of pure aerobic granules developed at the smaller exchange ratios of 40 and 20%. In addition, Zhu and Wilderer (2003) successfully developed aerobic granules at a fixed exchange ratio of 75%. These results provide experimental evidence that aerobic granulation is highly dependent on the exchange ratio of SBR.
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