The effects of peripheral velocity on glucose and oxygen diffusivities in a rotating biological film



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In a fixed film reactor the sites of biological activity are in an attached film. Nutrients (substrate) must be transferred into the film to be reacted. The mechanism of mass tranport in films is molecular diffusion; so the ability of oxygen and substrate to diffuse into the film can be very inportant in the treatment of wastewater. The proclivity to transport is indicated by the diffusion coefficient. The diffusion coefficient of glucose (a common substrate) and oxygen has been estimated by uptake experiments or stoichiometric determinations. In this work, the diffusivities were determined by direct measurement of the mass flux through inactivated film. Rotational velocity has been shown to be important in the performance of a rotating biological disc process. This has been attributed to better mixing, increased aeration and intensity of contact between the wastewater and the film. The major resistance to mass transport in these biological films is not in the bulk fluid but in the film itself. The purpose of these experiments was to determine the effect of rotation on the internal film resistance as indicated by the diffusion coefficients of flucose and oxygen. Films were grown at two peripheral velocities, lOm/min are 20m/min. In this research in addition to determining the effects of rotational velocity on 02 and glucose diffusivities, average values for the diffusion coefficients were determined. Mechanistic explanations for rotational effects were derived from density, thickness, solid content and polysaccharide measurements. The results of these experiments indicate with 90% confidence that peripheral velocity has a significant effect on the diffusivities of glucose and oxygen. Glucose diffusivities decreased by 18% in the film grown at 20m/min, apparently due to the increase in film density. Oxygen diffusivity was increased by 33% in the 20 m/min grown film possibly due to the reduction is entrained water layer thickness.