Capacitive deionization technologyTM development and evaluation of an industrial prototype system

Abstract

The Lawrence Livermore National Laboratory (LLNL), in Berkley, California, developed a laboratory scale non-membrane electrosorption process known as Capacitive Deionization Technology™ (CDT™) for the continuous removal of ionic impurities in water. A saline solution flows through an unrestricted capacitor type module consisting of numerous pairs of high-surface area (carbon-aerogel) electrodes. The electrode material (carbon aerogel) contains a high specific surface area (400 – 1 100 m2/g), and a very low electrical resistivety (< 40 m<font face="symbol">W</font>.cm). Anions and cations in solution are electrosorbed by the electric field upon polarization of each electrode pair by a direct current (1,4 Volt DC) power source. Testing conducted on a laboratory scale unit at LLNL has proved that CDT™ has the potential to be an alternative desalination technology (Farmer5 et al., 1995). The primary objective of this research was to continue, where the laboratory scale research ended. Thus taking CDT™ from a laboratory scale technology to an industrial scale process, by developing and evaluating an industrial CDT™ prototype system. First, a process was developed to manufacture a cost effective industrial sized CDT™ module. During this process various manufacturing techniques were evaluated to produce an optimum prototype. As part of the developmental process the prototype was tested and water treatment efficiency results were first compared to results obtained on the laboratory scale module and secondly to established desalination technologies like reverse osmosis, electrodialysis, and distillation. Due to the wide variety of potential saline feed water sources, research for this dissertation focused on brackish water applications (which includes wastewater reuse applications). After establishing a cost effective small-scale model of a potential industrial manufacturing process, the prototype was tested with regard to water treatment efficiency. Test results on brackish type waters (1 000 mg/l), indicated that the industrial CDT™ prototype had an energy requirement of 0,594 kWh/1000 liters. Research results compared well to the laboratory scale energy consumptions of 0,1 kWh/1000 liters (Farmer5 et al, 1995) and to the best available existing brackish water membrane based desalination systems with energy requirements of 1,3 to 2,03 kWh/1 000 liters (AWWA, 1999). The thermodynamic minimum energy required (due to osmotic pressure) to desalinate a 0,1% or 1 000 mg/l sodium chloride solution, is 0,0234 kWh/1 000 liters. Development and evaluation results indicated that CDT™ industrial modules could be manufactured cost effectively on a large scale and that such units have the potential to be very competitive with existing technologies with regards to overall operational and maintenance costs. Therefore Capacitive Deionization Technology™ can be viewed as a potential alternative to membrane technologies in the future. Regardless of the benefits to the potable water industry, CDT™ have the potential to incur a dramatic step reduction in the operational costs of desalination plants, which will make desalination a more viable alternative technology for large-scale agricultural and industrial uses.