Some biological and physicochemical OMWW pre-treatments have been proposed to reduce the occurrence of these pollutants in the wastewater sent to the digestor (Borja et al., 1998; Beccari et al., 1999, 2000); however, only limited improvements in the overall phenol removal were achieved. More promising results have been recently achieved on the laboratory scale by integrating the activity of conventional anaerobic digestors with that of an aerobic post-treatment employing activated sludge biomass (Beccari et al., 2002) or phenolic compound-mineralizing bacteria immobilized in packed-bed loop reactors (Bertin et al., 2001). A possible approach for mitigating the release of phenolic compounds from anaerobic digestors treating OMWW may consist of intensifying the bioremediation potential of biomass employed in the process upon its immobilization in dedicated packed-bed reactors (DeFilippi and Lupton, 1998; Rajeshwari et al., 2000). In fact, the metabolic activity and versatility of pollutant biodegrading microorganisms often significantly increases upon passive cell immobilization on porous carriers (Shreve and Vogel, 1993; Annadurai et al., 2002). In addition, the positive effects exerted by some carriers on the bioavaila-bility of pollutants and/or of microbial inhibitors often contribute to further enhance the bioremediation potential of biofilm reactors (Annadurai et al., 2002). Furthermore, the use of such non-conventional bioreactors may allow reductions in process start-up time and the occurrence of shock loading and/or washout problems, which very often compromise the productivity of dispersed growth reactors operating with low-growth-rate biomass and high and fluctuating organic loads (Rajeshwari et al., 2000), as typically happens in the anaerobic digestion of OMWW (Rajeshwari et al., 2000; Marques, 2001). Bertin et al. (2004) studied the bioremediation and biomethanization of OMWW with a GAC- and SB-packed-bed biofilm loop reactor and the conventional dispersed growth digestor operating with the same microbial inoculum and OMWW. They also determined and compared the performances and stability of the two biofilm reactors under continuous modes of operation.
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