The inert nuclei model for anaerobic granulation was initially proposed by Lettinga et al. (1980). In the presence of inert microparticles in a UASB reactor, anaerobic bacteria could attach onto the particle surfaces to form initial biofilm, namely embryonic granules. Subsequently, mature granules can be further developed from the growth of these attached bacteria under given operating conditions. The inert nuclei model suggests that the presence of nuclei or microsize biocarrier for bacterial attachment is a first step towards anaerobic granulation. The inert nuclei model was supported by experimental evidence such that addition of zeolite or hydro-anthracite particles with a diameter of 100 |xm into inoculated sludge seemed to be effective in promoting the formation of anaerobic granules (Hulshoff Pol, 1989). Water absorbing polymer (WAP) particles were also used to enhance granulation (Imai, 1997). The WAP is a pulverulent resin, which swells in water and exhibits a complex network structure, which can provide more surfaces for microbial attachment and growth than other inert particles. The laboratory-scale experiments indicated that the contact between particles and biomass could be improved since the WAP has lower density than sand and other inert materials (Imai, 1997).
The basis of anaerobic granulation had been proposed as a continuous selection of sludge through washing out light and dispersed bioparticles and retaining heavier biomass in the reactors (Hulshoff Pol et al., 1988). The selection pressure model suggests that microbial aggregation in UASB reactor appears to be a protective microbial response against high selection pressures. In UASB reactors, selection pressure is created by upflow liquid flow pattern. It had been reported that under very weak hydraulic selection pressure operating conditions, no anaerobic granulation was observed (Alphenaar et al., 1993; O'Flaherty et al., 1997). Rapid development of anaerobic granules could be accomplished through a purely physical aggregation from the hydraulic stress applied on the anaerobic flocculant sludge (Noyola and Mereno, 1994). The results showed that flocculant anaerobic sludge could be converted into a relatively active granular sludge by enhancing agglomeration through only short hydraulic stress of less than 8h. Arcand et al. (1994) also reported that the liquid upflow velocity had a significant positive effect on mean granule size, but the effect on specific washout rate of smaller particles was marginal. It is very likely that relatively high selection pressure in terms of upflow liquid velocity is favorable for rapid development of anaerobic granules.
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