APPLICATION OF MASS TRANSFER TO ROTATING BIOLOGICAL CONTACTORS

Abstract

For the analysis of most biological waste treatment systems, biological removal kinetics are combined with the proper hydraulic regime to yield equations describing system performance. Mass transfer resistances as sociated with the biomass are normally neglected. In suspended growth systems such as the activated sludge process, biological re action kinetics normally overshadow mass transfer effects because of the relatively small size of biological floe particles; thus the above approach is valid. However, in fixed film processes, this is not the case. Relatively thick biofilms are normally employed with penetration from only one direction. Mass transfer resistances associated with both the liquid phase and biofilm result in significant concentration gradients from bulk liquid to reaction sites, and generally control system performance. To design fixed film biological treatment systems, laboratory and/or pilot scale data are normally collected using reactors closely simu lating field units. The resulting empirical cor relations or kinetic equations intrinsically in corporate the mass transfer resistances of the small-scale units and are adequate for design providing similar resistances occur in the field. It is often necessary to obtain data over numer ous combinations of operating conditions to generate reliable empirical correlations. These testing requirements can be significantly re duced, with consequent saving of time and money, by including the effects of mass trans fer as well as biological kinetics in the data analysis. The purpose of this study was to develop and verify a model which could be used for the above. The challenge was to incorporate mass transfer and biological reaction kinetics into a framework which adequately described the system, but contained sufficient simplicity to allow parameter evaluation and application to system design. The rotating biological con tactor (bbc) process was chosen for this task because of its recent emergence as a viable waste treatment process and the availability of data for verification purposes.