Theoretical analysis of energy harvesting from concentrated brine of Sea Water Reverse Osmosis (SWRO) system using Pressure Retarded Osmosis (PRO) is presented in this research. The mathematical model of SWRO–PRO hybrid system components such as SWRO unit, Energy Recovery Device (ERD), PRO unit and other auxiliary units were discussed. The mathematical equations were solved adapting an object oriented “Modelica language” framework in Dymola software tool. The complex flowsheet models for six different SWRO–PRO hybrid configurations were created. The performance of the SWRO–PRO hybrid system configurations was studied. The process and design parameters were optimized to reduce the Net Specific Energy Consumption (NSEC) of the system. The optimization studies were performed using SQP technique that is available in optimization library of Dymola. The possibility of using sea water (32,000 g/m3) and urban waste water (100–10,000 g/m3) as feed solution to the PRO for all the hybrid configurations were studied. Their performances were compared through simulation and optimization studies. Among the six potentially viable SWRO–PRO configurations, the one which does the direct mixing of diluted PRO draw outlet with feed water of SWRO aided to bring down the NSEC by 49% in comparison with standard SWRO desalination system. This system does not require additional ERD units and turbine at optimized process conditions, which are more expensive.