-
Electrodeionization (EDI)
Technology Overview
Bill Loyd
-
Similar to the manner in which RO displaced ion exchange for
primary desalination, EDI has the potential to displace mixed beds
for polishing applications.
EDI Overview
-
EDI Advantages
Eliminates the need for hazardous
regeneration/neutralization chemicals
Is a clean technology, the only consumable is electricity
Is a continuous process, there's no need for offline
regeneration
Smaller footprint than conventional DI
Situational cost advantages
3
-
Power plant water treatment
Electronics & semi-conductor Ultra Pure Water (UPW)
Pharmaceutical water treatment
Miscellaneous industrial applications
Current EDI Markets
-
Basics of EDI
What is EDI?
Using electricity to remove ions from water
Water flow Electrical flow Analogies
Basics of Electrolysis/Electrodialysis/Electrodeionization
EDI Module Construction
Plate & Frame Configuration
Spiral Wound Configuration
EDI Internal Operation
Working Bed vs. Polishing Bed
5
-
Hydraulic Analogies
Hydraulic
Flow (gal/min)
Pressure (psi)
Resistance (pressure loss)
Electrical
Current (Amps)
Voltage (Volts)
Resistance (Ohms)
-
Review: Basics of Electricity
E=Voltage
I= Amps
R=Ohms
E
I R
If voltage to an EDI system were increased would the
current increase or decrease? (same resistance)
If voltage had to be increased to keep the same
number of amps flowing what might be the
reason?
-
Voltage vs. Amps EDI-60
-
Voltage vs. Temperature
-
12 Volt DC
- + anode cathode
O2 Oxygen
+
+
+
+
+
+
+
+
+
+
+
+
+
H2
Hydrogen
ca
tho
de
2 H2O + 2e- H2 + 2OH-
2 H2O O2 + 4H+ + 4e-
an
od
e
O H H
O H H
H+
H+
H+
H+
O H
O H
-
-
4e- 2e-
-
-
-
-
-
-
-
-
-
-
-
Electrolysis
-
+
12 Volt DC
cathode -
anode
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
O2 Oxygen
H2
Hydrogen
ca
thode
an
od
e
H2O
OH-
H+
H2O H2O
H2O
OH-
OH-
OH-
H+
H+
H+
Cl-
Cl-
Na+
Na+
Electrolysis
-
+
12 Volt DC
cathode -
anode
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
O2 Oxygen
H2
Hydrogen
ca
thode
an
od
e
Cl2 Chlorine
H2O
OH-
H+
H2O H2O
H2O
OH-
OH-
OH-
H+
H+
H+
Cl-
Cl-
Na+
Na+
2 Cl- Cl2 + 2e-
Electrolysis
-
- +
12 Volt DC
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
H+
OH-
OH-
H+
H+
H+
Cation Membrane
Anion Membrane
Na+
Cl-
cathode anode
Na+
Na+
Cl-
Cl-
OH-
OH-
Na+ Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
cathode
Electrodialysis
-
- cathode
+
12 Volt DC
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
H+
OH-
OH-
H+
H+
H+
Cation Membrane
Anion Membrane
Na+
Cl-
cathode anode
Na+
Na+
Cl-
Cl-
OH-
OH-
Na+ Cl-
Na+
Cl-
Na+
Cl-
Na+
Cl-
Electrodeionization
-
- cathode
+
12 Volt DC
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
H+
OH-
OH-
H+
H+
H+
Cation Membrane
Anion Membrane
cathode anode
OH-
OH- Na+
Cl-
Na+
Cl-
D C D C D
Electrodeionization
-
Polishing Bed Polishing bed is where
water splitting (pH polarization) and CO2, silica and other
weakly charged anions are removed.
Working bed Working bed is where
majority of cations and anions are removed.
-cathode
EDI pH Polarization
+
DC Power Supply
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
H+
OH-
OH-
H+
H+
H+
Cation Membrane
Anion Membrane
cathode anode
OH-
OH-Na+
Cl-
Na+
Cl-
D C D C D
Working & Polishing Zones?
EDI pH Polarization
-
Plate and Frame Configuration
Similar in Construction
to a plate heat
exchanger
Multiple chambers
sandwiched between
electrodes that are
held in compression
-
Spiral Wound EDI
Concentrate Spacer
Anion Membrane
Cation Membrane
Concentrate Spacer
Cation Membrane
Anion Membrane
Dilute Spacer
Dilute Spacer
-
Features and Benefits
Dow Spiral Wound EDI Advantages
No Leakage: Unique spiral wound design of the DOW EDI modules
ensures reliable seals and eliminates leaks.
No Maintenance: Unlike plate and frame EDI, DOW EDI have no
compression fittings or torque requirements.
Light Weight: A DOW EDI weighs only about one third of what the
typical plate and frame does making them easier to handle during
skid assembly or plant change outs.
Sample Port on Each Module: Each DOW EDI module has a sample
port which simplifies system piping requirements.
-
Myth or Reality?
EDI systems are prone to frequent problems and are
difficult to operate.
True
False
-
EDI Reality
Single Pass RO is much more likely to have EDI
problemsThats the reality.
No reset button like mixed bedsin that way EDI is not
very forgiving and can be more difficult to operate.
EDI modules/stacks are not designed to be cleaned,
each cleaning shortens module life and reduces module
performance.
Two pass RO makes EDI easier and more reliable to
operate than a mixed bed.
-
RO EDI
Variations in feed water
quality result in variable
feed water going to the
EDI
99%
Single Pass RO leaves little safety margin for operating
variables or system upsets!
Softener problem
Anti-scalant problem
ORP meter fails
Single Pass RO
-
Why Two Pass RO is Optimal
24
Double Pass RO leaves a large safety margin for operating
variables or system upsets!
RO RO
Variations in feed water
quality result in variable feed
water going to the 2nd Pass
RO
99%
Softener
problem
Anti-scalant problem
ORP Meter
Fails
EDI
99%
-
Cogeneration Facility, Madison, WI
2 Pass RO (DOW EDI)
Problem-antiscalant dosing failed
First pass RO elements scaled
2nd Pass RO protected EDI
Result no noticeable change in EDI resistance.
Nelson Dewey Generating Station
1 Pass RO (Alternative supplier EDI)
Problem-Softener failed to regenerate
RO elements scaled
EDI module resistance increased ~25%
Plant continued to operate.
Field Case Examples
-
DOW EDI Warranty
-
Typical EDI Design Installations
2 Pass RO w/interpass pH adjust + EDI
Preferred (RO Direct Coupled to the EDI)
2 Pass RO w/interpass pH adjust + Break Tank + EDI
N2 blanket and 1 micron absolute filter on break tank
Softener + 1 Pass RO + EDI Dow recommends using a Mixed Bed for
Single Pass RO permeate
polishing. If EDI is used a hardness switch may be required for
warranty
coverage.
27
-
2 Pass RO Consistent, high
quality feed water
1 Pass RO
Surface Water Seasonal Variation
Ground Water Low variation
Design Issues
Operational Issues
Operational Issues
EDI Installations
Typical EDI pretreatment
-
EDI Design Issues -Single Pass RO Pretreatment
Use a Conservative Design
Account for changes in the feed water
Account for changes in RO performance
Use a permeate flush at shutdown
29
-
Hardness scaling is the most common problem with EDI.
Scaling can occur very quickly inside EDI modules
if feed water exceeds recommended levels.
If scaling is not addressed modules may become extremely
difficult or even impossible to clean.
Allowable limits
0.5 to 1.0 ppm hardness as CaCO3
Feed Water Requirements for Various EDI
Devices
Manufact
urers
A B C D
Hardness
(ppm as
CaCO3)
-
RO Projection -Convert hardness to CaCO3 Equivalence
-
Convert ppm to ppm CaCO3
0.05 mg/L magnesium
0.05 mg/L Mg/(12.15
mg/meq)*50 = 0.206
ppm as CaCO3
0.09 mg/L calcium
.05 mg/L Ca/(20.04
mg/meq)*50=0.125 Ca
as CaCO3
Total 0.331 ppm as
CaCO3
Feed Water Requirements for Various EDI Devices
Manufact
urers
A B C D
Hardness
(ppm as
CaCO3)
-
0
5
10
15
20
25
30
35
40
Te
mp
era
ture
De
gC
98.40
98.60
98.80
99.00
99.20
99.40
99.60
99.80
100.00
Temp 5 10 15 20 25 30 35
% Rej 99.8 99.8 99.7 99.6 99.5 99.3 98.9
1 2 3 4 5 6 7
Temperature Impact on RO Hardness
Rejection
-
Occurs over time in the membrane boundary layer
Salt passage increases as scaling occurs due to
increase in the concentration gradient.
RO Scaling
-
-cathode
EDI Polarization
+
DC Power Supply
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
Cation Membrane
Anion Membrane
cathode anode
Na+ Cl-
Na+Cl-
D C D C D
OH-
OH-
OH-
OH-
H+
H+
H+
H+
OH-
OH-
OH-
OH-
OH-
OH-
OH-
OH-
H+
H+
H+
H+
H+
H+
H+
H+
Migration of OH- and H+ due to water splitting creates pH
polarization on the concentrate side of the IX membranes.
Low pH ~2 on the cation membrane concentrate side.
High pH ~12 on the anion membrane concentrate side.
Scaling occurs along the
high pH boundary layer of
the anion membrane.
Scaling Occurs Inside the Concentrate
Chambers Due to High pH
EDI Polarization
-
Feed 111 mg/l
Concentrate 156 mg/l
Permeate 2 mg/l
1 HR-13 mg/l
3 HR-45 mg/l
12 HR-53 mg/l
Feed
Ions
Concentrate
Product
Water Flow
Feed
Ions
Concentrate
Product
Water Flow
Design the RO system to do a permeate flush at shutdown
(preferred) or a dump to drain at startup to prevent a plume of
high
TDS water from entering the EDI system.
Permeate Flush to Avoid TDS Creep
-
Install before the RO
Always install a hardness switch or monitor to prevent running
past
exhaustion.
Softener EDI
RO
Hardness Monitor
Installing a Softener
-
Add a Hardness Monitor
-
#2 Avoiding Oxidation Damage
Primarily oxidation due to free chlorine in municipal water
EDI tolerance is low Damage to the cross
linking
Damage to the exchange sites
Pretreatment usually Carbon
Bisulfite injection
Feed Water Requirements for Various EDI
Devices
Manufac
turers
A B C D
Hardnes
s (ppm
as
CaCO3)
-
Dechlorination with NaHSO3
Inject before cartridge filters
Consider installing a chlorine analyzer to prevent oxidation
damage to IX resin and RO membranes.
Softener EDI
RO
Hardness Monitor
CF
Bisulfite Injection
Chlorine Analyzer
-
Dechlorination w/Activated Carbon
Preferred solution
Consider installing a chlorine analyzer to prevent oxidation
damage to IX resin and RO membranes.
Carbon
Filter Softener EDI RO
Hardness Monitor Chlorine Analyzer
-
#3 Other Design Considerations
CO2 Caustic injection
Membrane degasifier
Break tank
Avoid if possible, always direct couple
Use HEPA filter or N2 blanket, 1 micron absolute cartridge
filters
-
Why is CO2 Important?
Polishing Bed Polishing bed is where
water splitting (pH polarization) and CO2, silica and other
weakly charged anions are removed.
Working bed Working bed is where
majority of cations/anions are removed.
-cathode
EDI pH Polarization
+
DC Power Supply
anode
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
H+
OH-
OH-
H+
H+
H+
Cation Membrane
Anion Membrane
cathode anode
OH-
OH-Na+
Cl-
Na+
Cl-
D C D C D
EDI Polarization
-
CO2 Specification
RO will reject ionic impurities but CO2 will freely pass
through the membrane
CO2 competes with silica for IX sites in anion resin
and interferes with its removal
-
2 pass RO
pH adjust with caustic soda between passes
1 pass RO
Softener + pH adjust ahead of RO
RO + Membrane Contactor + EDI
Handling CO2 - pH Adjust
-
EDI Handling CO2 - pH Adjust
Softener + pH adjust
ahead of RO
pH 6.5 pH 8.5
The CO2-Water System
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00
12.00 13.00 14.00
pH
a
H2CO3
HCO3-
CO3--
-
pH Adjustment with Caustic Soda
Inject downstream of the softener
Install a hardness monitor prevent scaling the lines to the
RO
Carbon
Filter Softener
EDI RO
Hardness Monitor
Caustic Injection
Chlorine Analyzer
-
Membrane Contactors
Higher initial cost
Long operational life
when operated on RO
permeate
-
CO2 Removal with Membrane Contactor
Preferred Solution
Install down stream of the RO
Carbon
Filter Softener
EDI RO
Hardness Monitor Membrane Contactor Chlorine Analyzer
-
EDI Break Tank?
Avoid if possible
Install a HEPA filter on vent or blanket tank with N2
Install a 1 micron absolute filter after break tank
-
Optimized EDI Design -Single Pass RO - EDI
Preferred Solution!
Carbon
Filter Softener
EDI RO
Hardness Monitor Membrane
Contactor
Chlorine Analyzer
-
EDI Operational Issues -Single Pass RO Pretreatment
When is EDI a good fit for a plant?
Is the plant new to RO & EDI or do they have a lot of RO
operational problems?
A mixed bed recommended
Are the operators knowledgeable and experienced? Training at
startup
Add additional instrumentation & PLC controls
Is proper maintenance and calibration of instrumentation done?
Add to the log sheets
Does the plant track and trend data? RO and EDI normalization
programs provided for use
Consider remote monitoring
-
Thank You!
For more information please visit our web site or
contact your local Dow representative.
http://www.dowwaterandprocess.com/
TM Trademark of The Dow Chemical Company (Dow) or an affiliated
company of Dow
