Visio P&ID is to draw ...
Published by jeremy dudley, Senior engineer at WRc plc
Visio P&ID is to draw documents.
'Activated sludge expert' appears to be discontinued.
I think your best bet is the *non-free* software from EnviroSim Associates, PetWin.
Failing that you can try using the municipal wastewater treatment plant software, but be aware that they are set up for a different system - no sulphides, little handling of inhibition, ...