(565a) A Reverse Osmosis-Pressure Retarded Osmosis (RO-PRO) Hybrid Process for Low Energy Desalination

Reverse osmosis (RO) is a main stream technology for desalination.  In seawater RO desalination, a pressure greater than the osmotic pressure of the seawater is used to drive water through the semi-permeable membrane.  Current RO plants generate large quantities of concentrated brine, and its disposal is a major challenge due to the significant disposal cost and adverse environmental impact.  Pressure retarded osmosis (PRO) has attracted great interest in recent years for its potential of energy generation from concentrated brines.  PRO is the reverse process of RO, where the applied pressure on the concentrated brine side is less than the trans-membrane osmotic pressure difference such that water flows from a low osmotic pressure feed water to the high osmotic pressure brine.  This osmotically driven water flux is against the pressure gradient, which means that the PRO process converts the chemical potential energy (the osmotic energy) into mechanical energy.  In an RO-PRO hybrid process, the waste brine from the RO stage can be utilized for the subsequent PRO stage, and the osmotic energy harvested is utilized to offset the desalination energy demand.  The potential of this technology in desalination and its challenges will be discussed in this presentation.