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Fact Sheet Capacitive Deionisation There are a range of thermal and non-thermal treatment processes to remove salt from water. Capacitive deionisation (CDI) is an electrically driven, non-thermal process used to remove ions from water streams. CDI is being evaluated as an alternative to electrodialysis and reverse osmosis. This fact sheet provides an overview of CDI and identifies the potential of CDI as an alternative desalting process for a range of applications. WHAT IS CAPACITIVE DEIONISATION? CDI uses an electrical potential gradient to draw anions and cations into positively and negatively charged electrodes made from electrically conducting materials with a very high internal surface area (Figure 1). Figure 1 – Schematic of CDI; Charge is applied to the electrodes to induce the ions in solution to migrate to the collection electrodes based on their respective charges. Saline water enters the flow chamber in the CDI cell and anions and cations are drawn into the cathode and anode and stored within the electrodes by electrostatic forces leaving a deionised product water stream. After all sites within the electrode are saturated, the polarity of the potential field is reversed and the ions are discharged from the electrode, similar to the discharge of current from a capacitor. Once the concentrated salt stream is flushed from the cell Water Research Australia Water for the wellbeing of all Australians the polarity is reset and the desalination process starts again. WHAT IS THE TECHNOLOGY? The electrodes play a significant role in the efficiency of the desalting process using CDI. Previous research has shown the efficiency of CDI depends strongly on the surface properties of the carbon electrodes, including their surface area and pore microstructure. The ideal electrode materials for CDI should be highly conductive, with a large surface area and suitable pore size distribution. Carbon electrodes currently available limit the desalination efficiency of CDI due to their low conductivity and non-ideal pore structure and pattern. Many kinds of carbon materials have been investigated as CDI electrodes, including carbon aerogels, activated carbon, carbon cloth, mesoporous carbons, carbon nanotubes and graphene nanosheets. More recently, membrane CDI technology has attracted increasing attention. In a standard CDI device, the salt removal efficiency would be slightly lower because of co-ion (ions of the same polarity as the electrode) effects. These co-ions are near the electrode but cannot be electro-adsorbed efficiently. To avoid this adverse effect, ion-exchange membranes have been introduced in front of the electrodes (Membrane CDI or MCDI). Specifically, a cation-exchange membrane is placed in front of the electrode that is negatively polarised, and an anion-exchange membrane is placed in front of the positive electrode. Counter-ions can then move freely into and out of the electrode while co-ion transport is restricted, thus improving the performance. TYPICAL DESALINATION METHODS AND THEIR ISSUES Typically reverse osmosis and multistage flash distillation are used in 90% of desalination plants worldwide. RO desalination requires extensive use of energy, resulting in high operational costs, discharges large volumes of RO reject and is attended by long term membrane replacement costs. November 2013 Page 1 Silica fouling of membranes is also a major challenge for RO in regional Australia due to naturally high levels of silica present in groundwater. It is extremely difficult to remove once formed due to the polymerisation reaction that takes place on the membrane surface. This leads to a reduction in the quality and quantity of treated water and causes premature membrane failure. Addition of anti-scalants and pre-treatment are used to maintain membrane integrity. However, this increases chemical costs and reduces the recovery efficiency of treated water. ion exchange mainly due to the absence of scaling on the electrodes; • Reduction in environmental pollution. LIMITATIONS OF CDI FOR COMMERCIAL USE Limitations that a commercial CDI system would have to consider are: • The CDI process is heavily dependent on the ionadsorption capability of the capacitive electrode. At present, it is more suitable for brackish water desalination, and not suitable for high TDS water such as seawater; • The salt removal efficiencies decrease as the solution temperature and flow rate increase; • TDS removal efficiency decreases at higher concentrations of TDS in the initial feed; • At this stage, CDI is a less mature technology, full-scale testing is required. POTENTIAL APPLICATIONS Remote communities reliant upon groundwater and brackish water typically experience salt concentrations of 1500 to 2000 mg/L, which suits the operational capability of a commercial CDI system. Figure 2 – CDI Unit used in the project This is particularly applicable to remote area communities where building large treatment plants may not be practical. Furthermore, the low operational requirements and low energy consumption may suit remote communities with limited support infrastructure. WHAT ARE THE BENEFITS OF CDI? Potential benefits identified at bench scale and in-situ testing include: • CDI may be integrated with a solar power system, making it an attractive solution for remote locations; • CDI is a simple operation, which makes it attractive to remote communities with limited support; • Reduction of the levels for non-metal ions; • Reduction of water hardness to acceptable levels; • Performance of salinity reduction was consistent without any electrode deterioration observed. CDI is not subject to silica fouling - minimising system maintenance; • Reduction in energy to support desalination. Higher flow rates decreased the overall total dissolved solids (TDS) removal efficiency, but increased the energy efficiency of the system; • Non charged species, such as silica, do not accumulate and foul the electrodes; • Chemical-free process compared with other conventional methods such as electrodialysis, RO and Water Research Australia Water for the wellbeing of all Australians Information in this fact sheet was derived from WaterRA project (No. 1025-09) titled ‘Capacitive deionisation for high recovery and low energy desalination of brackish water supplies’ by Professor Linda Zou et al. This project was funded by WaterRA and University of South Australia. A similar project involving in-situ testing was performed at Wilora, Northern Territory using a commercially available CDI unit, funded by the National Centre of Excellence in Desalination and WaterRA (project No 1047-11). This series of fact sheets was first produced by the Cooperative Research Centre for Water Quality and Treatment (CRC WQT) in 2003. The fact sheets are largely based on research carried out in the CRC WQT and its successors, WQRA (2008-2013) and Water Research Australia (WaterRA, 2013 - ). Since 2008 the research has been funded entirely by the Members of WaterRA, who comprise Australian water utilities, universities, engineering and consulting companies and government agencies. Our Members provide annual contributions in the form of money, time, expertise and other in-kind contributions. The research conducted investigates issues of concern to the water industry and will benefit all Australians through improved water treatment and quality. Visit our website: waterra.com.au November 2013 Page 2