Pre-desalination with electro-membranes for SWRO

Abstract

Although seawater reverse osmosis (SWRO) is currently the only non-thermal desalination process in practical use, its characteristics make it difficult to approach the ideal reversible process. SWRO has a low water recovery (determined by the osmotic pressure) and relatively high energy consumption. A breakthrough in development of SWRO membranes can not be expected; at maximum a recovery of 60% could be obtained with membranes that can stand ultra-high pressures. In our project, an alternative development of desalination is introduced in which the osmotic pressure difference is reduced prior to SWRO with the use of electro-membranes, as in electrodialysis (ED). ED has distinctive and complementary assets when compared to SWRO. ED enables an operation close to the reversible limit, at least to the first extent of the desalination process. ED is an ideal pre-desalination step as: (i) the water recovery is not limited by a driving force (e.g., pressure), (ii) the specific energy consumption is directly proportional to the salt removal, (iii) the process economy allows low ionic fluxes and thus low irreversible losses, (iv) the system can be operated with infinitesimal changes in salinity (a pre-requisite for reversibility), and (v) the pre-treatment efforts can be kept limited. In this paper we compare a hybrid ED-SWRO scheme with state-of-the-art desalination schemes with respect to costs and energy consumption.