The essential role of organic loading in the formation of anaerobic granules has been recognized, i.e. a relatively high organic loading facilitates the formation of anaerobic granules in the upflow anaerobic sludge blanket (UASB) reactor (Hulshoff et al., 1988; Kosaric et al., 1990). However, this is due to the high organic loading-enhanced biogas production that results in an increased upflow liquid velocity known as the major selection pressure for anaerobic granulation in the UASB reactor (Hulshoff et al., 1988). In contrast to anaerobic granulation, the accumulated evidence shows that aerobic granules can form across an organic loading rate of 2.5-15.0 kg COD m-3 day-1 (Moy et al., 2002; Liu et al., 2003), while nitrifying granules can also be developed over a very wide range of ammonia-nitrogen loadings (Yang et al., 2003, 2004; Tsuneda et al., 2004;
Qin et al., 2004c). As noted by Liu et al. (2003), aerobic granulation in SBR is substrate concentration-independent, but the kinetics behavior of aerobic granules is related to the applied substrate loading (Moy et al., 2002; Liu et al., 2003). It seems a reasonable consideration that the effect of organic loading rate on the formation of aerobic granules is insignificant, i.e. the substrate loading in the range studied so far is not a determinant of aerobic granulation in SBR. However, the physical characteristics of aerobic granules depend on the organic loading rate (Tay et al., 2003). High loading rate leads to a weakened structure of aerobic granules (Liu et al., 2003; Tay et al., 2003). An increased organic loading rate can raise the biomass growth rate and this in turn reduces the strength of the three-dimensional structure of the microbial community (Liu et al., 2003).
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