A mathematical model for the flow through the siphon of a sewage purification machine is presented. The model is based on the average unidirectional flow velocities of the two phases (water and bubbles of air), the evaluation of hydrodynamic forces corresponding to the plant’s geometry, and on estimates of the mechanical interaction of the two phases. Data from experiments with an industrial machine are used to identify the model parameters and a comparison of the model’s trajectories with the experimental records is given.