Modeling of biological particle mixing in a fluidized-bed biofilm reactor
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Performance of fluidized-bed biofilm reactors (FBBRs) greatly depends on the amount of biomass attached to the inert core support particles and overall bed voidage. Thus, for proper design of an FBBR, it is important to know the biological particle (bioparticle) mixing and bed expansion behavior, which in turn influence bed volume and consequently the residence time of the liquid phase. This paper first investigates the superiority of some of the predominantly used fluidization correlations for biological beds to predict bed voidage and, second, it investigates the implications of bioparticle mixing in FBBRs. The fluidization correlation suggested by Foscolo et al. (1983) was found to be superior to the more widely used Richardson and Zaki (1954) correlation to predict FBBR bed voidage. Furthermore, from the three different sets of published experimental results of bed voidage applied in this study, the fluidization correlation suggested by Foscolo et al. (1983), along with the terminal settling velocity equation suggested by Mulcahy and Shieh (1987), was found to be superior to other sets of correlations used in this study to predict bed voidage in an FBBR, having the lowest mean deviation of 12.7%. A mathematical model for fluidization of a binary bioparticle mixture is also presented in this paper to model the interaction of a variety of parameters, including upflow superficial liquid velocity, core support particle size and density, and biofilm thickness. Numerical simulations using this model revealed that, for sand-like core support particles (density ranging from 1500 to 2700 kg/m(3)), a complete segregation would likely occur in the bed with larger bioparticles at the top and smaller ones at the bottom. For charcoal- or polymer-like core support particles (density ranging from 1050 to 1300 kg/m(3)), a homogeneous bed made up of both larger and smaller bioparticles would likely occur. Comparison of the model predictions with published experimental results suggested excellent agreement.
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