Advantages in Using Non-Isothermal Bioreactors in Agricultural Waste Waters Treatment by means of Immobilized Urease. A Study on the Influence of Spacer Length and Immobilization Method

The behaviour of three different catalytic membranes, obtained by immobilizing urease on nylon sheets chemically grafted with methyl methacrylate, was studied in a bioreactor operating under isothermal and non-isothermal conditions. Membrane activation was carried out by condensation or acyl azide reaction, and spacers of different length, such as hexamethylendiamine or hydrazine were used. Under isothermal conditions, the activities of the catalytic membranes and soluble urease were characterized as a function of pH, temperature and urea concentration. Both enzyme forms showed the same optimum pH, whereas the optimum temperature was lower for the immobilized enzymes. The spacer length appeared to determine broader pH- and temperature-activity profiles for the urease derivatives. The apparent Km values of the insoluble urease were dependent on membrane type and resulted higher than those of the soluble counterpart, thus indicating an affinity loss for urea. Under non-isothermal conditions, all membranes exhibited increase of percentage activity proportional to the applied temperature difference and decreasing with the increase of urea concentrations. A decrease of the apparent Km was also observed. These results suggest that substrate diffusion limitations due to the immobilization process can be overcome in the presence of temperature gradients. In addition, the remarkable reduction of the production times supports the use of non-isothermal bioreactors for the treatment of urea polluted waste waters.