The aerated bioreactor for solids processing is a three-phase (solid-liquid-gas) multiphase system. The solid phase contains the adsorbed contaminants, the liquid phase (process water) provides the medium for microbial growth, and aeration complicates the system. Nutrients and adapted biomass may be added to enhance breakdown. Furthermore, processing conditions (temperature, pH, O2 level, etc.) can be monitored and to some extent controlled.
Regarding the bioreactor configuration there are two major topics:
• physical state of the multiphase system
- bioreactors with a restricted solids holdup: slurry reactors (typical solids holdup <40 wt%)
- bioreactor with restricted humidity: solid-state fermentation (solids content >50 wt%)
• operation mode
- batch operation: no fresh material is introduced to the bioreactor during processing; the composition of the content changes continuously
- continuous operation (plug flow): fresh material is introduced and treated material removed during processing, the composition in the reactor remains unchanged with time (Levenspiel, 1972); in practice, semicontinuous operation is often used (batch-wise feeding and removal giving small fluctuations in the reactor)
Three basic reactor configurations exist:
• slurry bioreactors
• solid-state fixed-bed bioreactors
• rotating-drum dry solid bioreactors
Characteristic of all types of slurry bioreactors (Fig. 11.2) is the need for energy input to sustain a three-phase system in which the solid particles are suspended; the force of gravity acting on the solids has to be compensated for by the drag forces executed by the liquid motion (Hinze, 1959). In a properly designed slurry system, the energy input is used to establish three phenomena:
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