Diffusion into microbial aggregates

Abstract

Theoretical work in the biological waste treatment field has been directed at modeling substrate removal processes in fluidized and fixed film microbial systems in terms of the basic rate processes. Much of the research has been directed at delineating the rate limiting steps to simplify the problem. Various researchers have shown that the rate limiting step can be mass transfer through the microbial aggregate to the active sites at the cells. Therefore, any mechanistic model that incorporates mass transfer must be sensitive to variations in the reactant diffusion coefficient through floc material. A direct measure of mass flux has been developed to determine the variations in the diffusion coefficients of glucose and oxygen through microbial aggregates grown under various experimental conditions. A factorial analysis indicated significant changes in the molecular diffusion coefficient with variations in sludge age and carbon-nitrogen ratio in the growth media. Oxygen diffusivity varied from 20 to 100% of its value in water, glucose from 30 to 50%. A simple zero order diffusion-reaction kinetic model for spherical floc was constructed. It indicated that oxygen diffusion limitations are possible in the high rate activated sludge processes with large floc particles.