Comparison of Bioreactors

Table 11.1 shows a technological and economical characterization of the three bio-reactor systems. It is clear that composting is the 'low technology' type of bioreactor, requiring only limited technological infrastructure and investment. Slurry process-

Table 11.1 Comparison of bioreactors.

Technology Economics

Biological slurry processing

• Batch and (semi)continuous

• Water addition

• Addition of nutrients and/or biomass

• Controlled conditions in closed systems

• Substantial investment (equipment)

• Complex plant or trained operators required

• Large scale needed (economy of scale)

• Use as recycling technology

• Cost per ton determined by surface, machinery, energy, and labor costs


• Batch operation

• Limited use of technology and machinery

Low energy input

• Addition of compost to the solids

• Limited control options

• Longer treatment times

• Low investment

• Large surface required

• Use as decontamination technology only

• May be used on small scale

• Cost per ton determined by surface and labor costs

Rotating solids bioreactor

• Limited control options

• Moderate scale required

• Moderate energy input

• (Semi)continuous

• Substantial investment (equipment)

• Mobile plant on-site

• Small volumes

• Cost per ton similar to that of slurry systems ing requires a technological infrastructure, and rotating-drum systems have an intermediate position.

Experiments show that during batch processing, the microbial breakdown slows down at low concentrations and full conversion is not reached. Continuous processing significantly improves the results.

The economic feasibility of bioprocessing depends largely on the nature of the waste streams to be treated. Basically, waste streams having a large water content and a considerable fine fraction (which is predominantly contaminated with organic components) are suitable for bioprocessing.

From the economic point of view, continuous slurry processing in an off-site installation can be beneficial only on a large scale. Capacities >40000-60000 t a-1 (depending on the local conditions) are needed to benefit from the economy of scale and thus to perform at acceptable cost levels. For batch-operated composting processes, smaller volumes, handled close to or on the site, may offer solutions if sufficient area is available and time is not limited. Rotating-drum systems in which solid-state processing is achieved are considered to perform well for smaller volumes (up to 3000 t) in a mobile plant used on-site.

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