The Anaerobic Digestion Model No. 1 (ADM) and the Activated Sludge Model No. 2d (ASM) have been combined in a sequencing batch reactor simulation using a FORTRAN ODE solver. Special attention has been paid to the interface definition between the two models which use different biological state variables. The simulation results demonstrate the alternating treatment characteristics resulting from the sequence of anaerobic and aerobic steps. A parameter study reveals the process optimization potential of the model. Some numerical issues of the ADM are discussed in detail. The Jacobian matrix is provided, and the error handling of negative gas-rates and negative concentrations is described from a numerical point of view.