In a column SBR, wastewater is treated in successive cycles of a few hours each. At the end of a cycle, settling of the biomass takes place before the effluent is withdrawn. Sludge that cannot settle down within the given settling time could be washed out of the reactor through a fixed discharge port. Basically, a short settling time preferentially selects for the growth of good settling bioparticles. Thus, the settling time exerts a major selection pressure on the microbial community.
Qin et al. (2004a,b) studied the effect of settling time on aerobic granulation in SBR designed with a fixed discharge port, i.e. fixed exchange ratio, and found that aerobic granules were successfully cultivated and became dominant only in SBR operating at a settling time of less than 5 min, while a mixture of aerobic granules and suspended sludge developed in SBR run at longer settling times. In aerobic granulation, a short settling time has been commonly employed to enhance aerobic granulation in SBR (Jiang et al., 2002; Lin et al., 2003; Liu et al., 2003, 2005; Yang et al., 2003; McSwain et al., 2004a; Hu et al., 2005). In fact, at a long settling time, poorly settling sludge flocs cannot be withdrawn effectively; and they may in turn outcompete granule-forming bioparticles. As a result, aerobic granulation could fail in SBR run at longer settling times. This seems to indicate that aerobic granules can form only at short settling times below a critical level, i.e. settling time is a decisive factor in the formation of aerobic granules in SBR. Thus, choice of an optimal settling time is very important in aerobic granulation.
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