Regardless of the bioreactor type, the presence of sufficient water is crucial. Not only is a water activity around 100% a necessity for microbial action (van Balen, 1991), but biodegradation fully depends on the availability of the components in the water phase. On the microlevel, diffusion of the adsorbed contaminants into the bulk phase is the rate-determining step (Fig. 11.4). Mathematical models focus on capturing of the different physical processes into single diffusion parameters (Wu and Geschwend, 1988). The overall diffusion process depends also on the flow conditions around the particles (Crank, 1975).
An illustration of the importance of the flow around the particles was given by Koning et al. (1998), who analyzed microbial breakdown of a petroleum-contaminat-
porous particle (clay
contaminants contaminant diffusion biomass
Fig. 11.4 Diffusion of contaminants.
ed soil. A conversion level of 80% was reached within about 10 d in a slurry reactor, but under fixed-bed conditions some 150 d were required to reach the same conversion level. They concluded that "the enhanced breakdown is based on an increasing bioavailability of the contaminant due to suspending of and the mechanical strain on the soil material in the slurry reactors". In another slurry experiment, it was shown that manipulation of reactor parameters such as the energy input result in faster breakdown (Reynaarts et al., 1990).
The reactor type, scale, energy input, and aeration rate each have an impact on the diffusion. The better these parameters can be optimized, the faster the breakdown proceeds.
Having established proper diffusion conditions, the presence of adapted biomass capable of degrading the desorbed contaminants is essential. Organisms that are already present in the waste are almost always the most practical source of adapted biomass.
Of the three different bioreactors, the slurry system offers the best features to substantially influence the diffusion rate. Intense multiphase mixing allows for the exchange of desorbed contaminants, nutrients, and biomass through the medium. In addition, particle deagglomeration has been measured (Oostenbrink et al., 1995); this results in smaller particles and a faster diffusion rate (smaller particles mean shorter diffusion distances).
For solid-state processes in fixed-bed reactors there are fewer factors to influence the microenvironment of the pollution. Only air and liquid (nutrients) can be forced into the system while the solids are packed. On the microlevel, the rotating solids bi-oreactor is intermediate between the fixed-bed and slurry systems, and deagglomeration of the moist mass and phase exchange can take place.
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