The most common bioreactor type used for anaerobic digestion is the continuously stirred tank reactor (CSTR). Vanderhaegen et al. (1992) cultivated anaerobic granules in (CSTR), and found that granular sludge disappeared within three weeks when the reactors were incubated statically instead of being shaken. The hydrodynamic shear force appeared to be necessary for maintaining the integrity of granular sludge, and anaerobic granulation may not be reactor type-dependent, but related to the way by which the reactor is operated. It should be pointed out that hydrody-namic flow patterns in UASB and CSTR are quite different. In CSTR, the bacteria move in a stochastic way, i.e. there is no regular liquid upflow pattern. The experiments by Vanderhaegen et al. (1992) challenged the general belief that liquid upflow pattern is essential for the development of granular methanogenic sludge.
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