Previously collected data describing aerated synthetic waste water, treated in a continuous stirred-tank reactor, are analyzed to understand better the dynamic response to step changes in the dilution rate, D. Comparing a change in D between steady states leads to hysteresis trajectories on both the graph of specific growth rate, mu, vs limiting substrate level (S) and the graph of (S) vs the cell level (X). Qualitative differences between the three different monitored cases will be compared to simulations of simple models at various dilution level changes in order to gain understanding of the dynamics of the process.