The aim of this paper is to propose a step by step method to establish and validate a relevant, dynamical model of the activated sludge process for paper mill industry. The model is established by considering the specificities of the effluent to be treated. It uses the Biochemical Oxygen Demand (BOD) measurement, which allows a direct correlation between the state variables of the model and the measurements. This model is validated on a pilot plant of semi-industrial size fed with industrial paper mill effluents. The parameter estimation is based on an analysis of the sensitivities. Theresults are compared with a model based on Chemical Oxygen Demand (COD) partitioning which is issued from the simplification of the Activated Sludge Model n°1 (ASM1) model. Finally, the model dealing with BOD measurements not only shows the best performances but is also the easiest to implement, overcoming one main obstacle to efficient monitoring and control in paper mill wastewater treatment plants.