Once you start the program, the main menu will offer you 14 choices. I think the best place to start is with option #4) DEFINE the process flow scheme and operating parameters. This will let you setup your process flow before you start. Now, you will notice that the default is set for 3 reactors. When you hit PgUp, you can examine all the specs for each of the reactors. Notice that only reactor #1 receives a recycle input and that there is no recirculation input into any of the reactors. You might recognize that this represents a simulation of figure 2. You can change this configuration if you want, but it is an easy to start the simulation on. Keep in mind, however, that if you do enter a recirculation stream, it must originate from a reactor downstream (e.g. The stream can originate from #3 and go to #2, but not the other way around)
Once this is done, you can customize your model to whatever situation you may be simulating. Option 2 on the main menu lets you specify the kinetic and stoichiometric parameters of the autotrophic and heterotrophic organisms. The default values are typical values for microorganisms found in American wastewater treatment plants.
Option 3 lets you enter the concentrations of the feed stream. Again, the default is set for "typical" values.
If this all looks good to you, go back to the main menu and choose option 5. This will solve for the concentrations of all the constituents in each reactor. Once you see the message "COMPUTING STEADY-STATE SOLUTION" your simulation is done. Select option 7 to see the results.