The effect of tracer diffusion in biofilm on residence time distributions

Estimating fixed-film biological reactor characteristics is often complicated by interaction of the tracer that is used in residence time distribution (RTD) studies with the biofilm in a way that causes the RTD curve to have a long tail. This tail can result from hold-up of tracer within the biofilm. A reactor model that ignores the diffusion rate at which tracer enters and leaves the biofilm may be seriously in error. Data from RTD studies in a fluidized bed reactor are presented and a model that accounts for diffusion of tracer in the biofilm is used to explain the experimental data. A material balance on inorganic nitrogen is also given to verify the organic dye tracer results. The conditions under which diffusion of tracer will cause large distortions are defined.