Computational fluid dynamics (CFD) was used to investigate how residence time analysis based on passive tracer transport in environmental and chemical engineering fluid systems can be significantly affected by flow or tracer release unsteadiness. The unsteadiness may be caused by intermittent influent flow, time-varying operations, and nonuniform tracer release profiles. Oftentimes these unsteady conditions are considered negligible and their impact on residence time analysis is not considered. Systems analyzed included a full-scale stabilization pond, a full-scale oxidation ditch, and full-scale and lab-scale baffled tanks. In the case of the stabilization pond, it was found that time-dependent inflow based on realistic water consumption pattern does not significantly affect mean residence time (MRT) but does have a nonnegligible effect on baffling factor θ10 relative to constant inflow.
In the case of a surface aerated oxidation ditch, aerator speed was found to have a nonnegligible impact on MRT, and thus it is recommended that CFD tracer studies should consider changes in aerator speed during daily cycles of operation. The slug and step methods were considered for tracer analysis in baffled contactors. It was found that the tracer release time in the slug method should be kept at less than 5% of the theoretical residence time in order to obtain accurate measurements of residence time characteristics. Furthermore, residence time distribution obtained from tracer analysis based on the step method can be affected by the initial transient and subsequent fluctuations in the time series of injected tracer concentration that often characterizes physical experiments.