New Processes in Seawater Desalination

Aiman   E.   Al-Rawajfeh; Mohamed   A.   Zarooni

Abstract

Desalination refers to water treatment processes that remove salts from seawater, brackish water or industrial brine solutions. This work reviews potential processes for low cost seawater desalination processes including coprecipitation, capacitive deionization (CDI), forward osmosis (FO) and gas hydrates (GH). Co-precipitation process deals with supersaturating the saline solution, up to 80%, by addition of chemical reagents which results in the precipitation of some of the salts in seawater. The capacitive deionization (CDI) process works on the basis of electrostatic adsorption of ions (e.g. Na+, Cl-, etc.), from the seawater, on a charged pair of high surface carbon electrodes. Carbon electrodes are used due to its conductivity and large surface area that offers a high capacitance and holds electrostatic charge. For high salinity applications, a charge barrier has been added and the process has been given a stage-wise configuration to reduce costing. The forward osmosis (FO) process draws water from the saline feed water by using a highly concentrated draw solution. The draw solution must have high osmotic pressure, contain solutes which are too large to pass through the pores of the membrane and simple and economic to remove and recycle. In gas hydrates desalination, a gas or mixture of gases is brought into contact with the seawater under appropriate conditions of pressure and temperature to form hydrate. The hydrates are then brought to a region of higher temperature and lower pressure, where it dissociates to release fresh water and the hydrate-forming gas. These processes (if scalable) could be cheaper than currently used technologies. The average energy consumption of the capacitive deionization, forward osmosis and gas hydrate processes are 2.5, 1.2 and 27 kWh/m3 which correspond to averages costs of 0.1, 0.6 and 3.5 $/ m3, respectively, which represents a significant reduction in the total desalination cost in the case of CDI and FO processes. The present article deals with new patents in seawater desalination process.

Keywords: Seawater desalination, thermal processes, membrane processes, co-precipitation, capacitive deionization, flow through capacitor, forward osmosis, gas hydrates