Steady-state anaerobic

Table XI. COMPARISON OF COMPOSITIONS AND SELECTED STEADY-STATE ANAEROBIC DIGESTION RESULTS...

Saccharomyces cerevisiae is anaerobically grown in a continuous culture at 30°C. Glucose is used as substrate and ammonia as nitrogen source. A mixture of glycerol and ethanol is produced. At steady-state condition mass the flow rate is stated. The following reaction is proposed for the related bioprocess 4,6... 

A steady-state kinetics study for Hod was pursued to establish the substrate binding pattern and product release, using lH-3-hydroxy-4-oxoquinoline as aromatic substrate. The reaction proceeds via a ternary complex, by an ordered-bi-bi-mechanism, in which the first to bind is the aromatic substrate then the 02 molecule, and the first to leave the enzyme-product complex is CO [359], Another related finding concerns that substrate anaerobically bound to the enzyme Qdo can easily be washed off by ultra-filtration [360] and so, the formation of a covalent acyl-enzyme intermediate seems unlikely in the... 

Looking back at Table 4.4 the anaerobic cyclic steady state is from the first product of C0(C02) reduction, formaldehyde (HCHO), to [CH2OH] . This step requires a kinetic pathway in redox potential from -0.2 to -0.6 V versus the H2/H+ potential at pH 7, where sulfur is the oxidised waste ... 

Tests were carried out at 25°C and at initial pH 6.9. Cultures in the liquid medium were incubated in 50 mL Falcon tubes, continuously shaked at 220 rpm. Each culture contained a fresh Pseudomonas sp. 0X1 colony in 10 mL of medium. The airlift with 10 g of pumice was sterilized at 121°C for 30 min and then housed in a sterile room. One-day culture was transferred to the reactor and, after a batch phase, liquid medium with phenol as the only carbon source was continuously fed. The reactor volume V was fixed at 0.13 L. Aerobic conditions were established sparging technical air. Under these conditions microorganism started to grow immobilized on the solid s support. When immobilized biomass approached steady state, cyclic operation of the airlift was started by alternating aerobic/anaerobic conditions. 

Equilibrium studies under anaerobic conditions confirmed that [Cu(HA)]+ is the major species in the Cu(II)-ascorbic acid system. However, the existence of minor polymeric, presumably dimeric, species could also be proven. This lends support to the above kinetic model. Provided that the catalytically active complex is the dimer produced in reaction (26), the chain reaction is initiated by the formation and subsequent decomposition of [Cu2(HA)2(02)]2+ into [CuA(02H)] and A -. The chain carrier is the semi-quinone radical which is consumed and regenerated in the propagation steps, Eqs. (29) and (30). The chain is terminated in Eq. (31). Applying the steady-state approximation to the concentrations of the radicals, yields a rate law which is fully consistent with the experimental observations ... 

After the addition of Cu(II), absorbance changes were identical under both anaerobic and aerobic conditions. This was taken as evidence for semi-quinone (SQ) formation, though it is not clear why the oxidation would stop at the SQ stage in the presence of excess dioxygen. It should be noted that the two-electron oxidation would also lead to the same spectral changes in the presence and absence of dioxygen provided that the spectrum of the oxidized species corresponds to that of the quinone. The dependence of the initial steady-state autoxidation rate (defined as... 

A steady state for this system is reached when the variations in Cs and Cx have ceased, i.e., when their derivatives with respect to time t are equal to zero in equations (4.23) and (4.24). If we set dCs/dt = dCx/dt = 0 in (4.23) and (4.24), we obtain two coupled steady-state equations for the anaerobic digester, namely... 

We have used a model for anaerobic fermentation in this section to simulate the oscillatory behavior of an experimental fermentor. Both the steady state and the dynamic behavior of the fermentor with Zymomonas mobilis were investigated. The four ODE model simulates the fermentor quite well. Further studies have shown that this model is suitable for scaling-up and for the design of commercial fermentors. Our model has shown the rich static and dynamic bifurcation characteristics of the system, as well as its chaotic ones. All these characteristics have been confirmed experimentally and the oscillatory/chaotic fermentor model is highly suitable for design, optimization and control purposes. 

There are many other interesting and complex dynamic phenomena besides oscillation and chaos which have been observed but not followed in depth both theoretically and experimentally. One example is the wrong directional behavior of catalytic fixed-bed reactors, for which the dynamic response to input disturbances is opposite of that suggested by the steady-state response [99, 100], This behavior is most probably connected to the instability problems in these catalytic reactors as shown crudely by Elnashaie and Cresswell [99]. Recently Elnashaie and co-workers [102-105] have also shown rich bifurcation and chaotic behavior of an anaerobic fermentor for producing ethanol. They have shown that the periodic and chaotic attractors may give higher ethanol yield and productivity than the optimal steady states. These results have been confirmed experimentally [105],... 

The secondary calorimetric steady state was reached about 3 hours after the peak of heat dissipation. The drastic decrease of the level of q was related to the rapid emergence of very different dominant strains, formed by larger cells (2.1 pm ) and characterized by very broad metabolic potentialities. This homogeneous population (the diversity was almost nil) displayed an adaptation to anaerobic conditions (75 to 100% of the population was able to reduce nitrates or ferment glucose). The cell population increased rapidly and reached values of 9 10 cells/ml in Arcachon to 10 cells/ml Roscoff. The situation is different in Arcachon, during the winter months (January and February, see figure 5). 

If DMS concentrations at the surface of the ocean are presumed to be at steady state, production must balance loss. The fate of DMS is thought to be evasion across the sea surface into the marine atmospheric boundary layer. However, since rates of DMS production are unknown, it is impossible to compare production with flux to the atmosphere, which is relatively well constrained. An alternative sink for DMS in seawater is microbial consumption. The ability of bacteria to metabolize DMS in anaerobic environments is well documented (32-341. Data for aerobic metabolism of DMS are fewer (there are at present none for marine bacteria), but Sivela and Sundman (25) and de Bont et al. (25) have described non-marine aerobic bacteria which utilize DMS as their sole source of carbon. It is likely that bacterial turnover of DMS plays a major role in the DMS cycle in seawater. 

In the above discussion, the energy yields of the different anaerobic pathways of metabolism are expressed in terms of moles of ATP per mole of substrate fermented. This is the traditional way to express the energetic efficiency of fermentations and for some purposes it is informative and useful. However, it is important to recall that in vivo these pathways are linked to ATP utilizing pathways (usually ATPases). At steady state, rates of ATP synthesis by these fermentations equal rates of ATP utilization. For classical glycolysis, for example... 

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