Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 246

13.8 FERMENTATION PROCESSES AND BIOREACTOR DESIGN REVOLUTIONIZED
13.8.1 Increased productivity lowers the cost of bioreactor construction
and downstream processes
Traditionally, industrial microbiology has been aiming for the applications with CSTR's (Contin-
uous Stirred Tank Reactor) (Lakatos et al. , 2011). In these systems, the raw material, or substrate,
addition is carried out without interruption and the products are removed in an equally contin-
uous liquid suspension flow. In a properly functioning continuous fermentation, chemostat, the
specific growth rate of the microbial culture equals the dilution rate. The end-product inhibi-
tion is avoided with the removal of product substances. Alternatively, the fermentations could be
arranged as batch or fed-batch processes. In the latter ones, the substrate is added in intervals, and
this approach has become rather popular in the bioindustries. It is often easier to run a fed-batch
system than a bioreactor with continuous flow. Many fermentation products are results from the
secondary metabolism, which is switched on after some more easily metabolizable substrates
have been exhausted from the batch. This natural division of metabolic functions requires multi-
phased arrangement in the fermentation process. In a fed-batch mode, the remaining microbial
mass in the bioreactor serves as inoculum, or starter, for the next round of production. Another
possibility for arranging the bioprocess initiation could be a feedback loop in the process.
Some drawbacks of the bioreactions accomplished in the stirred tank reactors are related with
the shear stress in the liquid (Bhojwani and Razdan, 1996). The stirring equipment is causing
material flows and pressures on the biocatalyst cells or enzymes. This physical stress is lowering
the production by simply breaking up the cells or molecular structures. These undesired effects
are often avoided by immobilizing the biocatalysts (Tischer and Wedekind, 1999). This approach
is also eliminating at least partially such phenomena as the end-product inhibition. In addition, the
downstream processing is simplified if the biocatalysts are closed within their own compartments.
Regardless of the processing difficulties caused by the shear forces in the liquids, it is of utmost
importance for the outcome of the fermentation to have some kinds of techniques for overcoming
the so-called diffusion limitation, which is lowering the reaction speed (Zyskin et al. , 2007).
Alternative bioreactor modes instead of the tank reactors are e.g. airlift and hollow-fiber fer-
menters (Shkilnyy et al. , 2012). In these facilities, the biocatalysts are often immobilized within
some matrices. In order to speed up the diffusion, moderate pressures could be attempted. Thus,
the accessibility of the substrate is increased, and the product removal eased up by more rapid
flow of the processed fluids or materials. The more concentrated the product is, the more effective
(and less costly) is its collection and purification,
13.8.2 PMEU (Portable Microbe Enrichment Unit) used for process simulation
The PMEU system was originally developed for the rapid detection of microbial strains in hospi-
tals, industries, and in the environment (Hakalehto, 2009, 2010). It has been used for the detection
of salmonellas (Hakalehto et al. , 2007), Campylobacteria (Pitkänen et al. , 2009), intestinal enter-
obacterial strains (Pesola et al. , 2009; Heitto et al. , 2009) and forest industry bacilli (Mentu et al. ,
2009). The basic idea behind the use of the PMEU equipment is called “enhanced enrichment.”
In different PMEU versions, the verification of bacterial growth is based on the optical sensing in
the PMEU Spectrion® (Fig. 13.3) and on the detection of volatile organic compounds (VOC) in
the PMEU Scentrion®. The rapid growth of Escherichia coli with various gas flow parameters
is demonstrated in the Figure 13.4. In the monitoring of the hospital pathogens with the PMEU
Scentrion®, very low bacterial concentrations were detected in blood samples of neonatal septic
patients (Hakalehto et al. , 2009). In bioreactor applications the PMEU versions could be used for
simulating the microbial growth, interactions and product formation.
The effect of different gas flow conditions on otherwise identical cultures in the selective
medium has been studied in the PMEU. The cultivation took place in specific PMEU cultivation
syringes. Nitrogen-gas was led to syringe 1, and the outcome gas from syringe 1 was then led to