Thermo-economic assessment of forward osmosis as pretreatment to boost the performance and sustainability of multi-effect distillation for seawater desalination

This work presents the thermodynamic and techno-economic assessment of a forward osmosis – multi-effect distillation (FO-MED) system able to improve the thermal performance of the MED seawater desalination process and reduce its environmental impact. Energy, exergy and techno-economic analyses have been carried out to identify the best boundary conditions that enhance the process efficiency and water cost, predicting the scale formation with the Ryznar index. Results show that using FO pretreatment in a 100 m3/d MED plant of 8 effects at 65 °C allows increasing the heating steam temperature up to 100 °C and the number of effects to 16 without increasing the risk of scaling. This results in 44 % reduction of the specific thermal energy consumption and 21 % decrease in the specific heat transfer area. The volume of saline water rejected to the sea is reduced 40 % and the water footprint 36 %. The exergy analysis reveals that the MED is the component with the highest exergy destruction. Finally, the levelized cost of water of the FO-MED is higher (7.6 $/m3) than the standalone MED process (4.6 $/m3). If high-performing and cost-effective membranes are used, the product cost could be reduced 33 % (5.1 $/m3).