My take on it There is one ...

My take on it

There is one open source program for wasteawter treatment. This is to use OpenModelica with the Modelica Wasteater Treatment Library. This gives you conventional activated sludge systems, but not SBRs.

If you look hard you may find an old copy of SeTS, which runs under Scilab. This also does not have SBRs.

Finally, in SourceForge, there is the uncompleted Killarney project, which would again not have SBRs. Killarney was being developed in C++.

If you want non-opensource, but free, then look at WRc STOAT. I help develop this, so naturally am rather biased. It does have SBRs.

Then, as you wrote, are the commercial programs: Hydromants' GPS-X, Envirosim's Biowin, Ifak's SIMBA, DHI's WEST and (coming soon) Dynamita's SUMO. For your needs - SBR and activated sludge - all are equivalent, and your choice, if you go down that route, would be based on criteria other than are they appropriate for your immediate problem.