Influence of the tracer characteristics on hydrodynamic models of packed-bed bioreactors

A bench-scale horizontal-flow anaerobic immobilized sludge (HAIS) reactor f illed with porous ceramic spheres (5 mm diameter) was used for evaluating t he effects of the tracer characteristics on the residence time distribution (RTD) curves and on the parameters of the hydrodynamic mathematical models . Stimulus-response assays were carried out with bromophenol blue, a dextra n blue, eosin Y, mordant violet, rhodamine WT and bromocresol green as trac ers. The reactor was operated at the hydraulic residence time (HRT) of appr oximately 2 hours and the flow characteristics were evaluated by fitting th e single-parameter models of dispersion and N-continuous stirred tank react ors (CSTR) in series to the experimental data. Tracer characteristics were found to affect deeply the form of RTD curves and the apparent degree of mi xing observed in the responses were attributed to the tracer diffusion into the porous media, except for dextran blue. The best adjustment was obtaine d for the N-CSTR in series model. Thereafter, dextran blue and rhodamine WT were used in hydrodynamic experiments in the HAIS reactor operating with a dditional residence times. Values of N ranging from 34 to 62 were obtained from the dextran blue experiments for HRT ranging from 2 to 7 hours. The ap plication of the statistical analysis ANOVA one-way method indicated that t here was no significant statistical difference in the flow-pattern within t he range of hydraulic residence times applied. The mean N value of 9 was ob tained from the experiments with rhodamine WT. Such disagreement in the res ponses was attributed to the diffusion of rhodamine WT into the porous medi a, interfering in the shape of the experimental curves.