A non-ideal rate-based model for thermal hydrolysis of urea has been proposed. The model incorporates reaction rate of urea hydrolysis and takes into account the effects of solution non-ideality and back-mixing on the reactor performance. The non-ideality of solution was investigated using an equilibrium-based model. The rate of reaction is written in terms of activity of reactants which are determined using a thermodynamic framework for system of NH3-CO2-H2O-urea mixture. The model provides temperature and concentration distribution of different components along the height of reactor. The predicted data of the model were consistent with the plant data indicating the validity of the model. The impact of different parameters on the performance of urea hydrolyser has been examined. The result of this work showed that an increase in inlet temperature of wastewater and steam flow rate would improve the urea removal efficiency. (C) 2003 Elsevier B.V. All rights reserved.