Role of pressure-retarded osmosis (PRO) in the mega-ton water project

Reverse osmosis (RO) membranes have been widely applied to seawater desalination and wastewater reclamation, and many large RO plants (>100,000m(3)/d) have been constructed since 2000. Energy efficiency is indispensable for large plants, especially for future mega-ton scale seawater reverse osmosis (SWRO) plant (1,000,000 m(3)/d). In order to reduce energy demand in RO operation, high-efficient low-pressure RO membrane, energy recovery device (ERD), and pressure-retarded osmosis (PRO) which would recover power from salinity gradient between freshwater and concentrated brine were studied in the Mega-ton Water System project. A PRO hollow fiber membrane module was newly developed, and a practical continuous operation has been examined at a prototype plant for oneyear. The maximum 13.5W/m(2) of membrane power density using 10-inch module was established at our prototype PRO plant. As the results of the plant cost estimation and the financial impacts on the energy saving operation at the mega-ton scale SWRO plants, not only 20% energy reduction by the high-efficient low-pressure RO membrane and the ERD but also further 10% energy saving was possible by using the PRO system.