A mathematical model for the dynamic simulation of anaerobic bioreactors containing biofilm and degrading complex substrates is presented. The model allows to simulate different reactor configurations : single CSTR, multiple CSTRs and non-ideal reactors. Our simulator was developed on the basis of gPROMS, which allows to study a wide variety of problems such as optimal start-up, design under forced regimen and parameter estimations of the biofilm kinetic from experimental data. The digestion of cattle manure in different reactor configurations was selected as example to show the model and the simulator performances.