The dynamic performance of an immobilised-urease bioreactor in a recycle loop

Use of immobilised urease is a promising alternative for the removal of urea from blood plasma in association with an artificial kidney device. In such a device, carrier particles containing the immobilised enzyme are retained within an extracorporeal vessel through which blood perfuses. During the operation of the system, urea diffuses into the immobilisation matrix where it is hydrolysed by urease. This system is intrinsically dynamic, since the urea concentration changes continuously with time as the perfusing blood is progressively cleared of urea. Its design and optimisation is therefore a significant technical challenge. This paper presents a model for and a simulation study of the continuous operation of an immobilised urease artificial kidney device operating, in fluidized bed mode, in a recycle loop. The partial differential equations that describe the system account for axial backmixing, intraparticle and external mass transfer resistances and intraparticle urea hydrolysis. The performed simulations reveal the effect of key parameters, such as the liquid recirculation rate and the size of the enzyme carriers on the performance of the system. Based on those, optimum operating conditions for maximum efficiency have been determined. The presented mathematical model and methodology is of general nature and thus suitable for the design and optimisation of a variety of dynamic (batch or semi-batch) biochemical systems.List of Symbols B _m dimensionless number defined as B _m=k _eR/D_eff - C _L urea concentration in the bulk liquid - C _R urea concentration at the particle surface - C _R ^L urea concentration at the inner side of the stagnant film surrounding the particle; C _R=C _R ^L / - C _p intraparticle urea concentration - D _eff, D effective intraparticle diffusivity of urea - D _L axial dispersion coefficient in the bioreactor - F volumetric flowrate - k reaction rate constant - k _e external mass transfer coefficient - k _n parameter, k _n=D _eff(n/R)² - L bioreactor length - Pe Peclet number, defined as Pe=(uL/D _L) - R particle radius (2R=D _P) - R _h instantaneous urea hydrolysis rate - u axial superficial velocity in the reactor - V reactor volume - X dimensionless length Greek Letters partition coefficient - bed voidage - integration variable - dimensionless time; = tF/V     

Control of catalase production and purity by altering certain nutritional factors ofAlternaria alternata growth medium

Both activity level of catalase and presence of glucose oxidase as an impurity were controlled by the type and concentration of nitrogen and carbon source in the culture medium ofAlternaria alternata. It was possible to produce glucose oxidase-free catalase at activity levels competing favourably with those reported for other catalase hyperproducing microorganisms.     

Pressure effects in cross-flow microfiltration of suspensions of whole bacterial cells

The effect of Trans-Membrane Pressure (TMP) on permeate flux during cross-flow microfiltration of bacterial cell suspensions in tubular ceramic membranes is studied experimentally. Continuous filtration experiments with suspensions of whole bacterial cells (Mycobacterium M156) show a dramatic permeate flux decline with increasing TMP. During the very early stages of the filtration process, a linear relationship between permeate flux and TMP is observed, suggesting an initial surface sorption of cells on the membrane surface. At longer times, the permeate flux vs. TMP data exhibit a critical pressure beyond which the permeate flux declines with increasing trans-membrane pressure. This is interpreted in terms of the formation of a compressible cake, whose permeability can be described through the Carman-Kozeny equation.     

Simultaneous production of glucose oxidase and catalase by Alternaria alternata

Summary A number of factors affecting simultaneous production of cell-bound glucose oxidase and catalase by the fungus Alternaria alternata have been investigated. Consecutive optimization of the type and concentration of nitrogen and carbon source, the initial pH and growth temperature resulted in a simultaneous increase in glucose oxidase and catalase by 780% and 68% respectively. Two second-order equations, describing the combined effect of pH and temperature on the activity of each enzyme, revealed that glucose oxidase had its optima at pH 7.9 and 32.3°C and catalase at pH 8.5 and 18.1°C. Under certain growth conditions, yields as high as 23.5 and 18,100 units/g carbon source for glucose oxidase and catalase, respectively, were simultaneously obtained.Offprint requests to: B. J. Macris