At a previous job, I did a lot of PFDs with mass balance. We simply used Excel for the calcs and it's inbuilt drawing tools for the diagrams we displayed lots of numbers in the diagrams and went through many design iterations with multiple stakeholders, so it was important for us to have the numbers always coupled with the diagram.
If you use Excel for mass balances with circular flows, the solver plugin is important & I don't know if this is available in the OS alternatives.
If you don't need to couple the calculation to the graphics, try MS Visio (is often used professionally for PFD and P&ID) or Diag (OS clone of visio, has ASME complient P&ID symbols but not).
I've heard of application specific tools for proces imulation and mass balancing (e.g. SIMBA for wastewater treatment), I've never heard of one for mechanicl waste treatment.
While my suggestions certainly work, I would advise you to wait a bit with accepting this answer, there may be far better tools and you don't want to discourage other answers too fast!