One-dimensional (1-D) biofilm models have been demonstrated reliable for specific types of biofilm reactor design. Limitations using mechanistic biofilm models for engineering design do not rely on improved biofilm models, but rely on improved biofilm reactor models. This is important when considering that biofilm reactors containing submerged, free-moving biofilm carriers are the most widely applied biofilm system(s) for municipal wastewater treatment. This paper presents a new biofilm reactor model that considers the impact of submerged free-moving biofilm carrier (Xcarrier) movement on system performance. The model accounts for a hydrodynamic condition characterized as plug flow with back mixing (to model axial dispersion). The relevance of this new biofilm reactor model to engineering situations is evaluated by applying it to relevant scenarios and comparing model results.