Immobilized biofilm reactor

Among the wide choice of reactor designs, the biofilm reactor is one of the best suited for azo-dye conversion as it meets two important process requisites. The first is related to the hindered growth feature of bacterial metabolism under anaerobic conditions. The second is related to the necessity to increase cell densities (see previous section) with respect to those commonly harvested in liquid broths [55, 56]. Except for bacteria that forms aggregates spontaneously, immobilization of cells on granular carriers and membrane reactor technology are the two common pathways to achieve high-density confined cell cultures in either discontinuous or flow reactors. 

FBBR, fluidized-bed biofilm reactor CFSTR, completely mixed stirred tank reactor PUR, polyurethane immobilized cells UASB, upflow anaerobic sludge blanket reactor NS, not specified. 

Corn steep liquor (CSL), a byproduct of the com wet-milling process, was used in an immobilized cell continuous biofilm reactor to replace the expensive P2 medium ingredients. The use of CSL resulted in the production of 6.29 g/L of total acetone-butanol-ethanol (ABE) as compared with 6.86 g/L in a control experiment. These studies were performed at a dilution rate of 0.32 hr1. The productivities in the control and CSL experiment were 2.19 and 2.01 g/(Lh), respectively. Although the use of CSL resulted in a 10% decrease in productivity, it is viewed that its application would be economical compared to P2 medium. Hence, CSL may be used to replace the P2 medium. It was also demonstrated that inclusion of butyrate into the feed was beneficial to the butanol fermentation. A control experiment produced 4.77 g/L of total ABE, and the experiment with supplemented sodium butyrate produced 5.70 g/L of total ABE. The butanol concentration increased from 3.14 to 4.04 g/L. Inclusion of acetate in the feed medium of the immobilized cell biofilm reactor was not found to be beneficial for the ABE fermentation, as reported for the batch ABE fermentation. 

Index Entries Immobilized cell biofilm reactor butanol com steep liquor sodium butyrate Clostridium beijerinckii BA101 sodium acetate. 

The use of continuous immobilized cell biofilm reactors eliminates downtime and hence results in superior reactor productivity (2,3). Adsorbed cell continuous biofilm reactors have been shown to favorably affect process economics (4). Application of these reactors reduces capital and operational cost, thus making the process simpler. Within these reactors, cells are immobilized by adsorption, which is a simpler technique than other techniques such as entrapment and covalent bonding (5). Adsorption is a simple technique and can be performed inside the reactors without the use of chemicals, whereas entrapment and covalent bonding are complicated techniques and require chemicals for bond formation. In anaerobic systems, such as butanol production, adsorption can be performed anaerobically within the reactor. An additional advantage of adsorption is that cells form uniform biofilm layers around the support, which lessens diffusion resistance compared to entrapped and covalently bonded cells. Hence, these reactors are called biofilm reactors. Because of reduction in diffusion resistance, the reaction rate is enhanced. For this reason, adsorption was chosen as the technique to be employed for Clostridium beijerinckii BA101 cell immobilization to produce butanol. In addition to being simple, it has the potential to be used in large-scale reactors. In the present study, clay brick was chosen as the cell adsorption support. It is available at a low cost and is easy to dispose of after use. 

Fig. 1. Schematic diagram of butanol production in an immobilized cell biofilm reactor using C. beijerinckii BA101 (A) bioreactor (B) biofilm particle of C. beijerinckii...

Effect of CSL, Butyrate (NaBu), and Acetate (NaAc) on Butanol Production in Immobilized Cell Biofilm Reactor of C. beijerinckii BA101... 

In a cell-immobilized membrane reactor, the membrane plays the role of a separator of two liquid streams, namely, the wastewater and the cell medium, and simultaneously acts as a support for biofilm growth (Chung et al., 2004 Juang et al., 2008 Li and Loh, 2007a) (Fig. 20.2). The wastewater is passed through the lumen side of the hollow-fiber membrane module and the nutrient-enriched cell medium flows across the shell side. In this way, the microbes can treat the effluent on one side and can be kept active via the nutrient stream at the other side without contamination of the effluent by the nutrient. 

Several bioreactor designs are used to produce bioproducts, and include, but are not limited to batch reactors, fed-batch reactors, continuous cultivation reactors, plug flow reactors, recycle bioreactor systems, immobilized cell reactors, biofilm reactors, packed bed reactors, fluidized-bed reactors, and dialysis cultivation reactors (Williams 2002). These reactor types can contain either mixed or pure cultures, and can stimulate heterotrophic and/or phototrophic cellular functions depending on the specific reactor design. Additionally, these reactor schemes can be used to produce products directly, or to harvest biomass or other products for downstream processes. Due to the complex nature of bioreactors, particularly anaerobic digesters, the use of metagenomics is helpful to understand the physiology of such systems. 

The immobilization of biofilms on permeable membranes, for the biodegradation of pollutants has drawn increasing interest for applications, where conventional treatment technologies are difficult to apply. As one of such examples, Hu et al. demonstrated the usefulness of membrane aerated biofilm reactor (MABR) for the wastewater treatment [16] a carbon-membrane aerated biofilm reactor (CMABR) was constmcted to remove organics and nitrogen containing compounds simultaneously in one reactor. 

The basic advantage of the suspended biofilm membrane reactors over the suspended biomass system (either with dispersed cells or with floes) is that the former are able to retain much more biomass [22]. It substantially reduces the biomass wash out and allows a more stable operation with higher biomass concentration. A moving-bed-biofilm membrane reactor is illustrated in Figure 14.5. The biomass is immobilized inside and outside of the fluidized particles. The structure of the biofilm continuously... 

Like enzymes, whole cells are sometimes immobilized by attachment to a surface or by entrapment within a support. One motivation for this is similar to the motivation for using biomass recycle in a continuous process. The cells are grown under optimal conditions for cell growth but are used at conditions optimized for production of a secondary metabolite. A hollow-liber reactor, similar to those used for cross-flow filtration, can be used to entrap the cells while allowing input of the substrate and removal of products. Attachment of the cells to a nonreactive material such as alumina allows a great variety of reactor types including packed beds, fluidized and spouted beds, and air-lift reactors. Packed beds with a biofilm on the packing are commonly used for wastewater treatment. A semicommercial process for beer used an air-lift reactor to achieve reaction times of one day compared to five to seven days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavor profile that was attributed to mass transfer limitations. 

The use of immobilized cells opens the possibility of keeping the produced biomass in the reactor and using it for prolonged periods of time, under the constraint that the product is excreted by the cells. A disadvantage is the possible occurrence of mass transport limitations inside the biomass beads or in the biofilm. Much of the technology to realize a large-scale process with immobilized cells has yet to be developed, and it might well turn out to be economically unfeasible in most cases. 

Flow of different streams and location of biofilm in a cell-immobilized hollow-fiber membrane reactor. 

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