Best pictured as a pipe where flow moves in one direction, a tracer entering a plug flow system exits completely at Volume/Flow. There is no dispersion or back-mixing to “equalize” or mix flows from multiple time periods. We often see plug flow reactor systems where influent has little variation and no toxicity/inhibition to the biomass. In practice we rarely see a true plug flow reactor as aeration systems create some degree of mixing/dispersion.
A complete mix reactor (or CSTR) is often modeled by a beaker wither a single large impeller. In this ideal example, a tracer added to the inlet is immediately dispersed evenly throughout the reactor. The tracer will appear in the effluent as the tracer/volume concentrations. A CSTR is often used in systems with influent variation and toxicity/inhibition issues caused by high concentrations of the inhibitory compounds. The immediate dispersal/dilution allows for microbial activity to commence as the inhibitory concentration levels are avoided.
In practice we rarely see a true plug flow or complete mix reactor. We often have hydraulic patterns that are somewhere in between the two ideal regimes. For example a long rectangular basin with aeration (either surface of diffusers) often has significant dispersion and back mixing so we have a lag before tracer appears at the effluent, but it appears well before time = volume/flow.
What makes hybid systems interesting is as you increase the number of cells or individual areas modeled by CSTR flows (occurs near a surface aerator), the overall tracer response appears closer to the plug flow ideal where tracer reaches the effluent near the design residence time.
The hybrid flow is used often because it is a more “natural” flow pattern and provides benefits of both ideal flow systems. To determine a working systems’ flow pattern, you can look at spill response data or run a Tracer Study where a conservative tracer is added and monitored at the effluent.