Process and System Design for Reliable
Operation of RO/CEDI Systems
JONATHAN WOOD and JOE GIFFORD
USFilter, Lowell, Massachusetts
IWC-04-47
Keywords: Electrodeionization, EDI, CEDI, RO/EDI, RO/CEDI, ultrapure water, design, operation
Abstract: This report explains some of the differences between continuous electrodeionization and conventional deionization,
and focuses on designing RO/CEDI systems to ensure reliable, long-term operation. Emphasis is placed on process design,
integration of the CEDI with other unit operations, and how to avoid scaling, fouling, or degradation of the CEDI modules.
INTRODUCTION
The process of continuous electrodeionization (CEDI, a
subset of EDI) has been in commercial use for 17 years,
but is still not as well understood as more mature water
purification processes such as reverse osmosis and ion
exchange deionization. The function of the ion exchange
resin in a CEDI device is somewhat different than in a
regenerable deionizer, resulting in operational
considerations that may not be obvious even to
experienced practitioners of ion exchange. This report
explains some of these differences, and concentrates on
how to design RO/CEDI systems to ensure reliable, longterm operation. Particular emphasis is placed on process
design, proper integration of the CEDI with other unit
operations, and how to avoid scaling, fouling, or
degradation of the CEDI modules.
CEDI AND IX REMOVAL MECHANISMS
Since conventional deionization is a batch process, the ion
exchange resin must retain the ionic contaminants until it
is time to regenerate the resin. In this instance, the most
important properties of the resin are capacity and
selectivity.
High capacity is required to allow a
reasonable amount of time between regeneration. High
selectivity towards the contaminants ensures that they will
be retained by the resin during the service cycle. Resins
are regenerated with fairly concentrated solutions of acid
and base. In addition to returning the resins to their active
forms, this may also provide some cleaning of the resin.
The process of continuous electrodeionization (CEDI) is a
hybrid of electrodialysis (ED) and ion exchange (IX), but
in some ways it is also similar to Reverse Osmosis (RO).
Ion exchange resin filler is placed between the ion
exchange membranes in the diluting compartments, and in
some cases in the concentrating and electrode
compartments. The purpose of the resin filler is to
provide a medium for transport of the ions being
removed, as well as for the DC current used as the driving
force (1). The DC field causes the ions to migrate from the
diluting (or product) compartments into the concentrating
(or reject) compartments. This is where the similarity
with RO is evident – both processes have a feed, product
and reject stream. In both cases it is important to avoid
the precipitation of sparingly soluble salts in the reject
stream. Another similarity is that both RO membranes
and CEDI modules have fairly small flow channels and
should not be operated on feed waters that are high in
particulates.
In a CEDI device, the DC current is continuously causing
removal of the ionic contaminants while simultaneously
electrochemically regenerating a portion of the ion
exchange resin. The ion exchange resins and membranes
are optimized to allow rapid transport of the ions across a
continuous path of an appropriately charged ion exchange
resin, through the ion exchange membrane, and into a
concentrate compartment. For this, high selectivity is not
required, in fact if a contaminant is held too tightly by the
resin it may not migrate through it as easily. High
capacity is also not required, since the contaminants need
not remain fixed to the resin. These two factors might
explain why a particular type of CEDI device could be
unaffected by over 150 hot water sanitization cycles at
80°C (2), which would normally be thought to damage the
anion exchange resin.
It has been published that for CEDI devices to effectively
remove silica, it is necessary to use relatively low cross-
linked (high water content) ion exchange resins (3). We
believe this is due to the bulky nature of the ortho-silicic
acid molecule, with its associated waters of hydration (4).
Reducing the cross linking of the ion exchange resin
facilitates the transport of silica (and other large
molecules) through the ion exchange resins. The same is
true of the ion exchange membranes. While lowering the
cross-linking reduces the mechanical stability of the ion
exchange resin, this is generally not an issue with CEDI
devices, since they do not subject the resin to the physical
attrition of regular backwashing or the osmotic shock of
frequent chemical regeneration. However, low crosslinked resins are sensitive to oxidants, and incomplete
removal of chlorine can have a catastrophic effect on a
CEDI module, as it can with a polyamide RO membrane.
CEDI FEED WATER REQUIREMENTS
When CEDI was first commercialized in 1987, many of
the first installations were on softened tap waters, as an
alternative to two-bed demineralization. A significant
amount of additional pretreatment was also required,
including activated carbon dechlorination, cartridge
filtration, and in some cases organic scavenging. While
some such systems have remained in service over 15
years, the use of CEDI on tap water is no longer common.
Now virtually all new CEDI systems require RO
pretreatment. This allows the CEDI device to make
mixed-bed quality water, and provides more reliable
operation of the CEDI system.
CEDI feed water requirements can be divided into two
categories. The first relates to the performance of the
module. Feed water TDS and carbon dioxide limits, as
well as minimum temperature must be specified so that
the device can achieve the desired performance.
Increasing the TDS or CO2 in the feed water would not
cause permanent damage to the module, but it may impact
the product quality. The second category includes items
that could damage the module, such as high temperature
and pressure or foulants like organics and hardness.
The feed water requirements for several different
manufacturers’ commercially available CEDI devices are
shown in Table 1. Most of these requirements can be
easily met by pretreatment with single or double-pass RO.
PROCESS DESIGN CONSIDERATIONS
Hardness
Table 1 shows that most CEDI devices can only tolerate
about 1 ppm of total hardness, and in some cases the
recommended hardness is as low as 0.1 ppm. This is
often surprising to those familiar with saturation indices
such as the Langelier Saturation Index (LSI), since in
most cases the LSI of the bulk CEDI reject stream will be
negative. However, the water splitting that is necessary
for electrochemical regeneration of the ion exchange resin
can also lead to localized pH shifts, creating regions (such
as near the ion exchange membrane surface) where the
scaling potential is greater than in the bulk solution.
TABLE 1
Feed water requirements for various CEDI devices
Parameter
A
B
C
D
RO
RO
RO
RO
Feed water
type
permeate permeate permeate permeate
Conductivity,
µS/cm
CO2, ppm
TEA (ppm as
CaCO3)
Silica (SiO2)
Fe, Mn, H2S
Total Cl2
Free Cl2
Hardness,
ppm CaCO3
TOC
pH
Temperature,
°C
Pressure,
psig
Recovery, %
DP, psid
a
b
c
d
< 40
4 – 30
< 14
< 10 a
< 25
< 1.0
< 0.01
< 0.02
< 0.5
< 0.01
≤ 25
< 0.5
< 0.01
< 0.05 b
< 0.05
< 1.0
< 0.5
≤ 10
≤ 0.5
≤ 0.01
≤ 0.05
< 1.0
c
≤ 2.0
d
< 0.5
4-11
< 0.5
5-9
< 0.5
5 – 9.5
≤ 0.5
6–9
5 - 45
4.4-38
5 – 35
5 – 38
25 - 100
45 - 100
< 60
36 – 100
90 - 95
25 - 35
90 - 95
20 - 35
10 - 30
80 – 95
22 – 36
< 5 ppm CO2 recommended
recommended non detectable
< 0.1 ppm CaCO3 recommended
recommended non detectable
There are four main system configurations used to meet
the feed water hardness limits of CEDI systems:
Softening/RO/CEDI
Antiscalant/RO/Softening/CEDI
Antiscalant/RO/CEDI
Antiscalant/RO/RO/CEDI
Which configuration is used depends on such factors as
the raw water hardness, TDS, and even silt density index
(since softening can help reduce RO membrane fouling).
While the RO system alone (with antiscalant pretreatment
instead of softening) may be able to produce steady-state
permeate hardness of less than 1 ppm, proper system
design is required to ensure the hardness does not exceed
this concentration. For example, most boiler makeup
water systems operate in “start/stop” mode, only
operating when the deionized water storage tank is calling
for water. When an RO system starts up from a standby
condition, the initial slug of RO permeate can be worse
than the RO feed water (since at low salt concentration
there is little osmotic/water flow and the concentration
gradient causes salts to continue to diffuse through the
RO membrane after the permeate flow stops), and the first
few minutes of RO permeate may not meet the CEDI
system feed water specifications. This phenomenon is
illustrated for permeate conductivity in Figure 1, but the
results are similar for most ionic constituents in the feed
water, and perhaps even for TOC. In this particular
instance, it took about 2 minutes for the RO permeate to
approach steady-state. Even though the volume is
relatively small, it is important to flush this water to drain
or back to the inlet of the RO rather than send it to the
CEDI system, especially since most RO/CEDI systems do
not have a buffer tank between the RO and CEDI. An
alternative approach is to flush the RO system with
demineralized water before it shuts off.
Figure 1 Rinse-down of an RO system after standby
(raw water conductivity of 250 µS/cm)
RO Permeate, µS/cm
500
400
300
Conductivity
200
100
0
0
50
100
150
200
250
Time, seconds
In some cases the steady-state permeate hardness from an
RO system will not meet the feed specification of the
CEDI module. In this case, it is possible to install an ion
exchange softener between the RO and CEDI. This
allows the use of a smaller softener (as the RO permeate
flow is usually 75% of the RO feed flow) and it will be
removing very low levels of hardness so that regeneration
frequency will be much lower than for a softener
upstream of the RO.
Whenever using a sodium cycle softener between the RO
system and the CEDI system, it is important to thoroughly
regenerate and flush the softener resin when first putting
the system into service. This is because new cation
exchange resin contains extractables that could foul the
anion resin in the downstream CEDI system. A resin trap
between the softener and CEDI is also advisable.
Carbon Dioxide
Another significant issue affecting CEDI operation is the
presence of carbon dioxide (CO2) in the feed water. Any
CO2 in the gaseous form will not be rejected by the RO
membrane, and will impart an ionic “load” on the
downstream CEDI system. For some CEDI devices, a
concentration of 10 ppm CO2 in the CEDI feed may be
enough to prevent the system from meeting the boiler
makeup water specifications.
Therefore, RO/CEDI
systems may employ a separate means of CO2 removal.
The most common methods are forced draft
degasification, membrane degasification, or increasing the
pH before the RO to convert the CO2 to bicarbonate,
which can be rejected by the RO. Dosing caustic also
increases the LSI of the RO feed and reject water and may
only be practical if the RO feed water is softened. It is
also sometimes employed between passes of a two-pass
(product-staged) RO system (5).
Since the reject stream from a CEDI system typically
contains anywhere from one fifth to one half the salt
concentration of the raw water, recycling the CEDI reject
to the inlet of the RO is often desired. This may be
possible, but it is necessary to consider that while the
CEDI reject may be lower in salt, it typically has ten
times the CO2 of the RO feed water, as shown in Figure 2
(because the RO passes CO2 and then CEDI removes it).
In the absence of a degasification or pH adjustment step,
recycling this water could result in as high as a three-fold
increase in the CEDI feed CO2 concentration, and could
have a significant impact on the CEDI product water
quality. In most cases this would be impractical without a
CO2 removal step as part of the process train.
Figure 2 CO2 concentrations in an RO/CEDI system
RO
[1]
CEDI
[3]
[2]
[4]
Estimated for 70% RO and 90% CDI recovery, neglecting CO2 lost by venting
Location
Initial ppm NaCl
Initial ppm CO2
Steady-state ppm NaCl
Steady-state ppm CO2
[1] Raw
water
500
5
500
5
[2] RO [3] CEDI [4] CEDI
feed
feed
reject
500
10
100
5
5
50
< 500
< 10
< 100
15.6
15.6
156
Oxidants
Oxidants such as free and total chlorine (which are found
in many raw waters), as well as ozone or hydrogen
peroxide (which may be used in a plant for sanitization),
can irreversibly damage the ion exchange resin and ion
exchange membranes in a CEDI device. Oxidative attack
can cause breakdown of the cross-linking in cation and
anion resins as well as degradation of the functional
exchange sites. This results in both poor salt removal
performance as well as physical breakdown of the resin
leading to increased pressure drop (or decreased flow)
through any resin-filled compartments.
Chlorine - or any other oxidant - should be removed to
essentially non-detectable levels before the CEDI.
Because the CEDI resin is selected for performance rather
than durability, even low concentrations of oxidant can
cause permanent damage in a short amount of time.
Examination of oxidized CEDI devices has shown that the
most obvious damage occurs to the resin in the first few
inches (inlet) of the cell. In most CEDI devices it is either
not possible or not practical to remove and replace only
the damaged resin. Therefore in most instances of
oxidation it is necessary to replace all of the ion exchange
resin. Since the oxidant may either directly or indirectly
lead to damage of the ion exchange membranes, it usually
makes sense to replace all the membranes at the same
time. Because of the potential expense and downtime
involved, it is worthwhile to take all necessary
precautions to prevent the possibility of oxidation.
There are two main methods for removal of chlorine in
process water systems – flow through a granular activated
carbon bed or injection of a reducing agent (such as
sodium sulfite or sodium bisulfite). Although it is the
more expensive option, activated carbon would normally
be preferred because it is the most reliable dechlorination
process. If breakthrough does occur, it will be relatively
gradual and normally would be detected before serious
damage to the CEDI system.
Chemical dechlorination can achieve complete reduction
of chlorine, but failure could result in an immediate
increase to the feed level. Finding the correct injection
rate is like walking a tightrope, where underdosing could
result in oxidation and overdosing could cause severe
biological fouling of the RO system. For this reason, it is
common practice to use an oxidation-reduction potential
(ORP) monitor or a residual sulfite monitor as part of the
control system for dosing of the reducing agent.
The latest development in dechlorination is the use of
ultraviolet (UV) reactors, with either low-pressure or
medium-pressure UV lamps. This is generally more
effective for destruction of free chlorine than for
chloramines. The former would require UV dosage of 1530 times the disinfection dosage of 30,000 µW/cm (6),
while the latter may require 100 times this amount. The
required dosage depends on the amount of
chlorine/chloramines in the feed water, and we have
found that it can be difficult to achieve non-detectable
chlorine through the use of UV alone.
Temperature
Temperature can have a significant effect on performance
of CEDI devices. As the feed water temperature
decreases, reaction kinetics and diffusion rates slow and
CEDI module electrical resistance increases, causing an
increase in the required voltage and perhaps a decline in
performance. Therefore, to maintain rated performance
specifications, most CEDI manufacturers recommend a
minimum temperature.
Most CEDI devices also have a maximum operating
temperature, usually determined by the materials used to
construct the flow compartments (plastics or elastomers).
While the maximum operating temperature varies with
manufacturer, it is generally in the range of 35-45°C.
Some CEDI modules are constructed to allow periodic
sanitization with hot water (65-85°C). Such sanitization
is always performed with the DC power off, and usually
at a reduced pressure of 10-30 psig.
In most applications, water temperature is addressed
during system design, by adding appropriate heating or
cooling devices. However, occasionally RO and CEDI
systems are used in recirculating loops. These loops can
operate for long periods of time with little drawoff.
Under this condition, the water can heat up due to the
recirculation pump. This can lead to unacceptably high
water temperature and should be addressed with alarms
and automatic cooling or draw-off to maintain acceptable
temperature.
Another cause of high temperature in a CEDI device
could be the applied DC power. Under normal operation,
the heat loss into the water due to the DC power is hardly
noticeable. However, if water flow through the module is
reduced to a low enough level, this heating can cause the
water temperature to exceed the manufacturer’s
specification and cause permanent damage to components
including the resins and membranes. For this reason, it is
imperative that controls be designed to prevent the DC
power from being on without water flow through the
CEDI module. Redundant protection is advisable.
Water Recovery
For most CEDI systems the water recovery is typically
about 90-95%. Water recovery is limited for several
reasons. Limiting the concentrations of sparingly soluble
salts helps to avoid precipitation (scaling) in the
concentrate cells. The most common types of scale
encountered in CEDI devices consist primarily of calcium
or silica. Therefore, the maximum allowable feed water
concentrations of these contaminants may be linked to the
recovery. For example, operation at 95% recovery
normally requires a feed hardness of <0.2 ppm, while at
90% recovery 1.0 ppm hardness may be allowed.
As the recovery increases, the reject salt concentration
also increases. This causes a large concentration gradient
across the membrane, leading to the potential for crossleaks or back-diffusion and reduced product quality.
Cross-leaks are normally addressed by balancing the
pressure between compartments, as discussed later. Back
diffusion is prevented by limiting the recovery.
Total Organic Carbon
Most CEDI module manufacturers have set a
specification of less than 0.5 ppm of total organic carbon
(TOC) in the feed water. This specification is empirical
based on years of operational experience. It is very
difficult to set an absolute value due to the fact that TOC
is a non-specific analysis, and the actual organic species
that make up this number vary significantly by location
and also seasonally. For the most part, RO pretreatment
will reduce TOC below the 0.5 ppm level, and typically
will remove the majority of the large organic molecules
known to be fouling to ion exchange resin.
Electrode gases
Gases are formed at the electrodes of a CEDI device, as
shown in Equations 1, 2 and 3.
At the cathode:
H2O+ e- → ½ H2 + OH-
Equation 1
At the anode:
½ H2O → ¼ O2 + H+ + eEquation 2
and
Equation 3
Cl- → ½ Cl2 + eSince CEDI devices operate at low current density
(compared to electrodialysis), the quantity of gas formed
is relatively small. At standard conditions of 25°C and 1
atmosphere, approximately 11 ml/min of gas (7.5 ml/min
H2 and 3.7 ml/min O2) are produced per amp of current.
These gases are removed by flushing water over the
surface of the electrodes during operation. Commercial
devices exist utilizing either down-flow or up-flow
flushing through the electrode compartments, so both can
be effective for gas removal.
In some CEDI devices, product water is passed over the
electrodes. In this case a gas removal step may have to be
employed prior to use of the product water. In other
devices, a small stream of water is sent to the electrodes
and then to drain or reuse in other plant applications. In
still other devices, the electrode water is combined with
the reject water inside the module. In this case, the reject
water may have to be vented before recycling.
Depending on the configuration of the electrode
compartments and the composition of the electrode feed
water, some chlorine can be formed at the anode, as
shown in Equation 3. Concentrations in the range of nondetectable to 8 ppm Cl2 have been observed (2), the higher
values in a system with concentrate salt injection. This
will also have to be taken into consideration when
recycling or reusing the electrode and/or reject water.
CEDI SYSTEM DESIGN
CEDI modules are commercially available in sizes from
less than 1 gpm to 80 gpm, in both plate-and-frame and
spiral wound configuration, and with screen-type or resinfilled concentrate and electrode compartments. The
design of the CEDI system can vary widely depending on
the size and type of CEDI module used.
Hydraulic Controls and Instrumentation
For CEDI systems using a large number of small (10-15
gpm) modules in parallel, the system design approach is
similar to an RO system, with the CEDI module
analogous to an RO element. Control and instrumentation
are provided at the system level, not the module level. On
systems with a small number of large modules it may be
practical
to
have
independent
controls
and
instrumentation for each module.
In either case, there is a minimum amount of system
control and instrumentation required, including the
following:
Product (dilute) flow meter
Product low flow switch
Product inlet/outlet pressure gauges
Reject (concentrate) flow meter
Reject low flow switch
Reject inlet/outlet pressure gauges
DC voltmeter
DC ammeter
Product conductivity or resistivity
Dilute inlet valve
Dilute outlet valve
Concentrate inlet valve
Concentrate outlet valve
Shutdown interlock with RO or feed pumps
In addition, concentrate recirculation requires:
Recirculation flow meter
Pump control valve or variable speed drive
Makeup valve
Bleed valve
Systems with brine injection may also need:
Brine low level alarm
Concentrate conductivity controller
Typical P&I Ds are shown in Figure 3 and Figure 4 for
two different CEDI module types. One of the main
considerations in the hydraulic controls is to provide the
ability to balance the pressure differential between the
product and reject compartments. Typically the product
outlet pressure is maintained about 2-5 psi above the
reject outlet pressure, to prevent the possibility of crossleaks from the concentrate side to the dilute side. In once
through systems this is accomplished with straightforward
adjustment of the concentrate inlet and outlet valves. It is
more complicated with concentrate recirculation.
DC Power Supply
The power supply simply converts the AC line voltage to
the DC voltage used as the driving force for the
electrodeionization process. Therefore its function is
analogous to the high pressure pump in a RO system.
There are two modes of power supply control, constant
voltage and constant current. In constant voltage mode
the DC voltage is fixed, and the DC current will vary with
the electrical resistance of the CEDI stacks. For example,
if the feed water temperature drops, the ohmic resistance
of the stacks will increase, and the amperage will go
down. Many CEDI systems successfully employ this
mode of power supply operation, but it may require
occasional manual adjustment of the voltage to maintain
the desired DC current and product water quality.
Constant current operation is when the power supply is
designed to automatically adjust the DC voltage as
necessary in order to maintain the DC amperage at a preset value. This avoids the need for manual adjustment in
response to changes in stack resistance, but does not
necessarily eliminate operator intervention, since the
amount of current required by the CEDI system varies
with the product flow rate and the ionic load in the feed
water.
The ideal power supply design would provide a separate
rectifier for each CEDI module in the system. Then if one
rectifier fails, the rest of the system can remain in
operation while the damaged rectifier is being repaired or
replaced. Having individual rectifiers also offers some
degree of flexibility in monitoring, control and
optimization of the DC power applied to each module.
On very large systems individual DC power supplies may
not be practical, so some CEDI systems with many small
modules employ a single large DC power supply. In this
instance all the modules operate at the same applied
voltage, and the resulting current is divided up among the
individual modules according to their electrical resistance.
It is still possible to use constant-current operation, but it
is the DC current of the entire system that is controlled to
a setpoint, and the current through the individual modules
can vary somewhat. This can result in some performance
variation between modules, so such systems should be
evaluated on the overall system performance rather than
that of individual modules.
One of the most important operating parameters of a
CEDI system is the electrical resistance of the module(s)
– the applied DC voltage divided by the resulting DC
current. An increase in electrical resistance may indicate
a problem such as the precipitation of hardness on the
surface of the ion exchange membrane. The sooner such
a problem is identified and corrected, the greater the
chance that aggressive maintenance can be avoided.
CEDI MODULE DESIGN
In addition to the process issues already discussed, there
are some CEDI module construction features that can
have an impact on module longevity and maintenance
requirements.
External Leaks
During the first decade of continuous electrodeionization,
the stacks were not leak-free, and in many cases were
provided with a drip pan to collect the small amount of
water weeping from the cells. This did not compromise
salt removal, and many such devices have been in
operation for over a dozen years. More recently, end
users have been asking for devices that are capable of
higher operating pressure (up to 100 psig) with zero
external leakage. Two approaches to attaining this goal
have been the use of thick cells or putting the cells in a
housing.
The 8-10 mm resin compartments used in thick cell
devices are inherently stronger than the 3 mm spacers
used in thin cell devices. They also allow the use of true
O-ring seals, which can be used to guarantee the absence
of external leaks.
Figure 3 Typical P&ID for CEDI system using module(s) with resin-filled concentrate and electrode compartments
(once-through operation)
Figure 4 Typical P&ID for CEDI system using module(s) with screen-type concentrate and electrode compartments.
This system includes concentrate recirculation and concentrate salt injection.
There are now a few commercial CEDI modules that
contain the cells inside a cylindrical housing. This has
been done with both spiral wound and stacked-disk type
(7)
devices. The stacked disk device incorporates O-ring
seals in addition to an FRP housing, and is warranted
against external leakage. One of the spiral devices has
experienced problems of resin seepage and resin loss (8),
although these claim to have been resolved by a recent
design change.
Salt bridging
Many plate and frame CEDI devices expose the edge of
the ion exchange membrane to the ambient air. Such
devices are susceptible to the buildup of salt deposits on
the outside of the modules. This may be as a result of
water weeping out the concentrate compartment gasket, or
possibly from water “wicking” out the edge of the
membrane. When the water evaporates, any salt it
contained is left behind. If these deposits are not
removed, they can build up to the point where they form a
bridge between the cell pairs and the metallic frame of the
device. Such salt bridges can provide an alternative path
for the DC current, leading to arcing that could in turn
lead to module damage.
Salt bridging is more of an issue if the concentrate
conductivity is high. This is the case in some CEDI
systems with screen-type concentrate spacers that require
salt injection into the concentrate stream to reduce the
stack electrical resistance. Salt injection is not required in
devices with resin-filled concentrate and electrode
spacers.
It is possible to prevent the formation of salt bridging by
eliminating all external leaks (as described above) and by
preventing the loss of water out the edge of the ion
exchange membrane. The latter can be accomplished by
completely surrounding the membrane with an O-ring
seal, so that it is not exposed to the air.
Biofouling
Biological fouling has been observed in some CEDI
installations, and in the authors’ experience it usually
occurs in an area of the screen-type concentrate spacers
that is outside the electric field (the inlet section above the
electrode and the outlet section below the electrode).
Biofilm is very seldom seen in resin-filled compartments
of a CEDI device, nor is it seen in the open channels
leading into and out of the resin chambers. These
observations suggest that (1) the electric field suppresses
biological growth and (2) the geometry of the screen type
flow spacer is more susceptible to biological growth than
a more open channel.
It is also believed that recirculation of the concentrate
stream may contribute to biological fouling, as some
experts have raised these concerns regarding the use of
reject recirculation with reverse osmosis systems (9), and
fewer biofouling problems have been experienced in
CEDI systems with once-through operation.
CONCLUSIONS
CEDI technology has matured, but it is not yet as well
understood as RO. Ensuring long-term, reliable operation
of CEDI systems involves elements of module, system
and process design. It is important to properly integrate
the design and operation of the pretreatment, RO, and
CEDI systems. Issues of particular importance are
removal of hardness and chlorine. With proper system
design and operation, expected CEDI module life is over
5 years, and some systems have achieved continuous
operation of over 15 years.
REFERENCES
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High Purity Water by Electrodeionization: Performance
of the Ionpure (tm) Continuous Deionization System,
Ultrapure Water, Vol. 4, No. 3, pp. 43-50 (1987).
(2) Gifford, J and Atnoor, D. An Innovative Approach to
Continuous Electrodeionization Module and System
Design for Power Applications, International Water
Conference, October 2000.
(3) Ganzi, G. et. al., Electrodeionization Apparatus, U.S.
Patent No. 5,316,637.
(4) Iler, R.K., The Chemistry of Silica, John Wiley &
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