Mass Transfer Limitations in Substrate Removal

The existence of an intraparticle diffusional resistance in the uptake of organic substrate by particles of zoogloeal floc is established by comparing the glucose and oxygen uptake rates for large floc particles with those exhibited by the same particles after blending. A zero order effectiveness factor model predicting the magnitude of this diffusional limitation in terms of the particle size distribution, particle shape, uptake rate, substrate concentration, and effective diffusivity of the substance within the particle is developed by considering a particle of biological floc to be analogous to a porous catalyst particle. Mean values for the effective diffusivity of glucose in the zoogloeal floc material ranged from about 7% to 9% of the corresponding values for glucose in water. Subjecting activated sludge floc particles to high turbulence in a laboratory blender caused a marked increase in the oxygen uptake rate. This phenomenon was apparently due to the solubilization of some sludge components.