A deterministic model for wastewater transformations in the sewer that includes activity in the wastewater and in the sewer wall biofilm was developed. It is based on the Activated Sludge Model No. 3 (ASM3). The mass transfer processes in the biofilm were modelled with the effectiveness approach, This approach allowed for fast calculation, required only a limited number of parameters and gave good results. The ASM3 related parameters were calibrated and validated with laboratory experiments. An equation for the aeration of the wastewater through the water surface was determined with a method based on the inert gas sulphur hexafluoride. The other model parameters such as wall roughness, attachment of particles to the biofilm and biofilm erosion were calibrated and validated with field experiments in a main sewer. The resulting model described the oxygen concentration and wastewater respiration well. It can be easily linked for integrated urban hydrology modelling with the WWTP Activated Sludge Model and the newly proposed River Water Quality Model No. 1.