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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
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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.
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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
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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.
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