In this study, removal of urea, ammonia and carbon dioxide from wastewater of conventional urea plant in both high and low concentration (ppm scale) levels using a cascade of hydrolyser–desorber loops has been investigated. In conventional urea plants, wastewater treatment sections including co-current configuration of hydrolyser were designed according to the old environmental standards. Nevertheless, the amounts of urea and ammonia in outlet treated wastewater are not acceptable at present time due to new environmental restrictions, shortage of water sources and possibility of upgrading this wastewater to reuse for boiler feed water or cooling water. Therefore, a thermal hydrolyser–desorber system is proposed to add at the end of current conventional treatment section in order to decrease urea and ammonia contents in treated effluent to approach 0 ppm. A general model is developed for both types of thermal hydrolyser–desorber loops which in first loop, high concentrations and in the second loop, low concentrations of urea and ammonia are treated. The extended electrolytic UNIQUAC equation has been used to describe the non-ideality of liquid phase. The proposed model incorporates reaction rate of urea hydrolysis and takes into account the effects of solution non-ideality and back-mixing on the performance of hydrolysis reactors. The model was solved numerically and provides temperature, flow rate and concentration distributions of different components along the height of reactors and desorbers. 
The proposed model has been validated against observed data. Also the effects of key parameters on the performance of wastewater treatment process have been examined. The results of this work show that increase of inlet temperature of wastewater and steam flow rate and decrease of the reflux ratio improve the urea and ammonia removal efficiency.