Biomass recycling

Continuous Stirred Tanks Without Biomass Recycle. The chemostat without biomass recycle is a classic CSTR. The reactor is started in the batch mode. 

Continuous Stirred Tanks with Biomass Recycle. When the desired product is excreted, closing the system with respect to biomass offers a substantial reduction in the cost of nutrients. The idea is to force the cells into a sustained stationary or maintenance period where there is relatively little substrate used to grow biomass and where production of the desired product is maximized. One approach is to withhold some key nutrient so that cell growth is restricted, but to supply a carbon source and other components needed for the desired product. It is sometimes possible to maintain this state for weeks or months and to achieve high-volumetric productivities. There will be spontaneous cell loss (i.e., kd > 0), and true steady-state operation requires continuous purging and makeup. The purge can be achieved by incomplete separation and recycle... 

Like enzymes, whole cells are sometime immobilized by attachment to a surface or by entrapment within a carrier material. 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 transformation of substrate. A great variety of reactor types have been proposed including packed beds, fluidized and spouted beds, and air-lift reactors. A semicommercial process for beer used an air-lift reactor to achieve reaction times of 1 day compared with 5-7 days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavour profile that was attributed to mass transfer limitations. 

Consider the CSTR with biomass recycle shown in Figure 12.6. 

Scott, C. D., Ethanol Production in a Fluidized-Bed Bioreactor Utilizing Flocculating Zymomonas Mobilis with Biomass Recycle, Biotechnol. Bioeng. Symp. Ser., 13 287 (1983)... 

The influence of toxic substances on nutrient cycles can also be important. One of the more important nutrient cycles is the carbon cycle carbon from atmospheric carbon dioxide is fixed as organic carbon in plant biomass, biomass is consumed by organisms, and carbon in decaying biomass is released back to the atmosphere as carbon dioxide, with a concurrent release of phosphate, nitrogen, and other nutrients. Toxic substances may cause perturbations in such a cycle, as has been observed when toxic heavy metals in soil have killed populations of earthworms, which are important in biomass recycling. 

Biomass recycle as sketched in Figure 30.4D is frequently used in fermentors as a way of increasing the biomass productivity. The increased biomass productivity obtained with a recycle fermentor is a function of the recycle ratio, r, and the cell concentration factor, C = XJX, achieved in the concentrator. Equations expressing the recycle system behavior are derived from material balances around the reactor. For the cell biomass balance at steady state ... 

Continuous Stirred Tanks Without Biomass Recycle... 

A.K. Krishnakumar, C.Gupta, R.R.Sinha, M.RSethuraj, S.N.Potty, T. Happen, and K. Das, Ecological impact of rubber (Hevea brasiliensis) plantations in North East India 2. Soil properties and biomass recycling, Indian Journal of Natural Rubber Research, 4(2), 131-141,1991. 

Except for the presence of the cell death/maintenance metabolism term in the present analysis and the need to identify the factor P in Figure 13.8 with the ratio of the concentrations in equation (13.3.15), the analysis above gives results similar to those for a chemostat operating in a biomass recycle mode. 

The hydraulic retention time (HRT) is defined as the theoretical amount of time that the liquid is resident within the reactor. For completely mixed systems without biomass recycling, the HRT is the same as the SRT. For systems designed to encourage greater biomass retention, the SRT is generally longer that the HRT. Separating both parameters in this manner improves process stability and efficiency. 

FIGURE 23 Bioreactor operational modes (a) a continuous flow well-mixed bioreactor, (b) a continuous flow well-mixed bioreactor with biomass recycle. 

Development of Bio-Composites Based of Polymer Matrix and Keratin Fibers Contribution to Poultry Biomass Recycling... 

Biomass recycle frequently is used in fer-mentors as a way of increasing the unit productivity. In order for the system to operate successfully, a biomass concentrator must be connected to the unit (see Fig. 24.9.). 

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