In a UASB reactor, upflow velocity and hydraulic retention time (HRT) is inter-related and serves as a selection pressure on microbial ecology. It has been observed that anaerobic granulation can proceed well at relatively high liquid upflow velocity, but does not occur under conditions of low hydrodynamic shear (Alphenaar et al., 1993; Arcand et al., 1994; O'Flaherty et al., 1997; Alves et al., 2000). According to Alphenaar et al. (1993), granulation in UASB reactors is favored by a combination of high upflow liquid velocity and short hydraulic retention time. Usually, the effects of upflow liquid velocity on anaerobic granulation are explained by the selection pressure theory (Hulshoff Pol et al., 1988). A long HRT accompanied with a low upflow liquid velocity may allow dispersed bacterial growth and be less favorable for microbe granulation. In contrast, a short HRT in association with a high upflow liquid velocity can lead to washout of flocculant biological solids and thus promotes sludge granulation.
Research attempts have been given to develop strategy for speed-up of granulation process by controlling hydrodynamic conditions in a UASB reactor. Noyola and Mereno (1994) conducted a series of experiments to investigate the effect of liquid upflow velocities for a rapid formation of granules through a purely physical aggregation due to the hydraulic stress applied to the anaerobic flocculant sludge with increasing upflow velocities. Experiments showed that flocculant anaerobic sludge could be converted to a relatively active anaerobic granular sludge by enhancing agglomeration with only hydraulic stress in a very short time less than 8 h, and the settleability of those anaerobic granules in terms of sludge volume index (SVI) and sludge settling velocity were significantly improved as the liquid upflow velocity increased. The increased settleability of granules in turn reduced washout of sludge from 46 to 2%.
Similarly, Arcand et al. (1994) also reported that the liquid upflow velocity had a significant positive effect on mean granule size, but its effect on the specific washout rate of the smaller particles was little. It is most likely that relatively high upflow velocity combined with a short HRT seem to be in favor of fast formation and production of anaerobic granular sludge. However, for a successful start-up and stable operation of UASB reactors, the reactor HRT would not be below a critical value, namely the minimum HRT.
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