Energy recovery from two-stage SWRO plant using PRO without external freshwater feed stream: Theoretical analysis

Research into pressure retarded osmosis (PRO) as a method to extract energy from salinity gradients is on the rise. Seawater Reverse Osmosis (SWRO) is now a leading technology in the desalination industry worldwide, in both small and large scale applications, due to the remarkable improvements in membrane performance and associated energy efficiency. Nonetheless, SWRO desalination is inherently more energy intensive when compared to conventional fresh water treatment. The integration of PRO with SWRO systems is studied in terms of energy consumption and effluent changes. For this, two novel integration designs are evaluated, with SWRO-PRO specific energy consumption being modeled using SWRO conditions at the thermodynamic restriction, and a developed PRO model. The results show lower SWRO energy consumption for both configurations, with a reduction in consumption of 12%-18%, depending on the RO recovery ratios. Lastly, the effect of the initial flow ratio on the dilution factor has been studied. To do so, the dilution was modeled and studied for different operating conditions. It was found that detrimental effects severely reduce the dilution, especially the internal concentration polarization, which induces a decrease of energy recovery when using the PRO process.