In this work a comprehensive study on wastewater treatment of industrial urea plants by thermal hydrolysis reactors is presented. A superior model is developed for two main co-current and counter-current modes of urea thermal hydrolysis reactor. In the proposed model the urea hydrolysis reactor is divided into several continuously stirred-tank reactors (CSTRs). The vapor-liquid equilibria are treated simultaneously with chemical reactions due to the complex features of urea hydrolysis system. The model provides temperature and concentration distributions of urea along the thermal hydrolyser. The validity of the proposed model was verified by the industrial observed data. Sensitivity analysis was carried out to investigate the effect of the various operating parameters on the performance of two types of hydrolysis reactors. Comparison of two kinds of hydrolysis reactors shows in order to observe the old environmental pollution standards, the co-current configuration of hydrolyser is more suitable than the counter-current one. However to achieve new environmental pollution standards and complete treatment, the counter-current configuration was the only acceptable practicable mode. Overall, this study results in valuable information about the performance of the co-current and counter-current modes of hydrolysis reactor in urea wastewater treatment. (C) 2010 Elsevier B.V. All rights reserved.