A slurry bioreactor can work properly only if these three measures are balanced. For each reactor configuration, the appropriate processing conditions depend on parameters such as the reactor scale, particle size distribution, slurry density, slurry viscosity, oxygen demand of the biomass, and the solids holdup (Kleijntjens, 1991).

For solid-state fermentations there is no need to maintain a solids-liquid suspension; a compact, moist, solid phase determines the system. Both the fixed-bed reactor and the rotating-drum bioreactor are suited to solid-state fermentation (Fig. 11.3). In

Fig. 11.2 Common configurations for slurry bioreactors.
Fig. 11.3 Bioreactors for solid-state processing: (a) fixed-bed reactor, (b) rotating-drum reactor.

the fixed-bed reactor the contaminated solids rest on a drained bottom as a stationary phase. Forced aeration and the supply of water are mostly applied as a continuous phase (Riser-Roberts, 1998). Fixed-bed reactors are mostly batch operated. Although landfarming might be considered a form of solid-state batch treatment under fixed-bed conditions (Harmsen, 1991), this technique offers limited control options (in comparison to other solid-state treatment) and is therefore not considered to be a bioreactor within the present context.

Continuous solid-state processing is possible in the rotating system. Here, the solid phase (as a compact moist material) is 'screwed and pushed' through the reactor. In line with slurry processing, energy is required to transport the solids through the system.

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