Development of the electrochemical scale removal technique for desalination applications

Abstract

The possibility of alkaline scale precipitation and removal by electrolytic devices has long been recognized. The scale removal principle of the electrochemical technique is based on the creation of a high pH environment around the cathode by water and oxygen reduction reactions which release hydroxyl ions. The alkaline environment induces precipitation of the calcium hardness in the form of CaCO 3 and of the magnesium hardness, in the form of Mg(OH) 2. Despite the commercial availability of such equipment, the use of electrochemical scale control methods is quite limited. Currently, the main field of application of electrolytic devices is for reducing the hardness of water recirculating in cooling towers. The lack of authoritative technical information on electrochemical scale removal reflects the paucity of research and development efforts in a technology which holds considerable promise for expanding the rather limited scope of viable scale control techniques. The objectives of this research project are to evaluate the potential of the electrochemical technique for RO desalination processes in general and for increasing water recovery levels in particular. The paper summarizes results of the first phase of the research. Models describing cell resistance in the absence and in the presence of a deposit on the cathode are presented. The effects of several parameters on the deposition rate and on the electric energy consumption are investigated. Results show that the higher the water hardness, the higher the scale precipitation rate and the lower the specific energy consumption. An increase in the flow velocity augments the scale deposition rate. Analysis of the velocity effect data indicates that the scale precipitation reaction is mass transfer controlled. The main optimization parameter is the current density. As may be anticipated, a low specific electrical energy is consumed when the electrolyzed solution is exposed to a large electrode surface and a high specific energy is consumed when the solution is exposed to a small electrode surface. The energy consumption can be rather low. For instance, in the electrolysis of a typical concentrate stream of a brackish desalination plant at a current density of 25 A/m 2, the energy consumption is of the order of 4 kWh per kg of precipitated CaCO 3 and the scale precipitation rate is of the order of 25 g CaCO 3/h m 2. Finally, a flow scheme is presented indicating the possibility of beneficial increase of the water recovery level in brackish water RO desalination, by partial recycle of the concentrate after electrochemical precipitation of the scale forming ions held in solution by the anti-scalant.