The paper includes the results of mass transfer influence on the overall rate of substrate biodegradation by bacterial cells immobilized in alginate gel. The overall rate of substrate biodegradation by immobilized bacteria is the function of the following parameters: ref = f (rmax, kl2, Deff). The results indicate that overall ratio of substrate biodegradation by immobilized bacteria is limited by external and internal transport. The effectiveness of the substrate biodegradation depends on the value of overall effectiveness factor. When limiting the overall rate of substrate biodegradation by the diffusion in a phase liquid-beads surface, the effectiveness factor is the function of Damkohler number. In the case of the internal diffusion limiting the overall rate of substrate biodegradation by immobilized bacteria the effectiveness factor depends on the Thiel number.