This paper describes a new anaerobic digestion model for wastewater treatment systems (MantisAD). The model has been developed specifically for plant-wide wastewater treatment modelling. That is, rather than modelling nitrogen as a series of fractions of other carbonaceous state variables, this new model includes six dedicated nitrogen state variables. This structure makes this new anaerobic model easier to incorporate into plant-wide models by simplifying the aerobic/anaerobic model interfaces. The model is complete and initial success with the model has been achieved. A comprehensive description of the model including a Petersen Matrix representation of the model is included as is a case study in which the model is applied to full-scale anaerobic digesters.