Water Conditioning & PurificationM A R C H 2 0 0 7

By Chris Gallagher

Fundamentals of

Electrodeionization (EDI)Technology

Electrodeionization (EDI) has ma-tured and grown in popularitysince its first commercial introduc-

tion over 18 years ago. The technologycan replace mixed ion exchange in manyapplications using a chemical-free pro-cess to produce high-quality water; how-ever, the water quality fed to the EDIsystem is generally reverse osmosis (RO)permeate. Because of the competitive na-ture of the business, better manufactur-ing and improved designs, the price ofEDI has dropped over 60 percent in theyears since its inception, making it a com-petitive option.

Life of a water systemWith any purchase of equipment, life

cycle, maintenance and cost are impor-tant factors in the decision-making pro-cess. For EDI modules there are nomoving parts. EDI systems require powersupplies, valves and piping. The simplic-ity of EDI systems has been greatly en-hanced over the years. By maintaining thepretreatment of the water generation sys-tem, EDI can have an expected life cyclegreater than five years. (Check with indi-vidual manufacturers for warranty terms.)

EDI acts as a very sensitive indica-tor of changes in feedwater and pretreat-

ment. EDI is usually the last piece of pro-cess equipment in a water generationsystem, so if the temperature, TDS orflow changes, EDI performance willchange.

Types of units and processprinciples

Many new EDI products have en-tered the market. The first commercialEDI was a plate and frame device thathad thin purifying spacers of ~0.100inches; these systems are still offered to-day. Thick purifying cell EDI entered themarket in the mid ‘90s, offering a thick-ness of > 0.200 inches. Spiral EDI pre-miered at the same time, offering adifferent cell pair configuration outsideof the plate and frame.

Regardless of the manufacturer, thesame fundamental principles are com-mon among all suppliers. The EDI deviceuses cell pairs: one cell (chamber) is forion depletion, the purifying cell. Juxta-posed to the purifying cell is a concen-trate cell. One purifying cell and oneconcentrating cell comprise a cell pairand there are many cells in a module.Each cell is separated by alternating an-ion and cation exchange membranes. Thepurifying cell is filled with ion exchangeresin and some designs incorporate con-ductive material in the concentrate andelectrode chambers. There are two elec-trodes (where voltage is applied): an an-ode that attracts anions and a cathodethat attracts cations.

When DC voltage is applied, ions inthe purifying compartment move to theirrespective nodes. The ions move out ofthe bulk solutions and through the ionexchange membranes, where they arecaptured in the concentrating compart-ment and leave the EDI module.

One of the unique principles of EDIis when enough ions are transferred,water splitting or polarization occurs.

Quality of feedwater required, at minimum, for EDI

Supplier 1 Supplier 2 Supplier 3 Supplier 4

Source RO water RO water RO water RO water

Feed < 40 µS/cm 4-30 µS/cm < 40 µS/cm naconductivity

Concentrate > 20 µS/cm > 10 µS/cm na 250 to 1,000conductivity (varies) (varies) µS/cm

Hardness < 0.25 ppm < 1.0 ppm < 1.0 ppm < 2.0 ppmas CaCO3

Silica < 1.0 ppm < 0.5 ppm < 1.0 ppm < 1.0 ppm

TOC < 0.5 ppm < 0.5 ppm < 0.5 ppm < 0.5 ppm

Pressure 20 to 50 psi < 60 psi 20 to 100 psi 36 to 100 psi

Temperature 10 to 35°C 5 to 35°C 5 to 45°C 5 to 38°C

pH 4 to 10 5 to 9.5 4 to 11 5 to 9

Total chlorine < 0.1 ppm < 0.05 ppm < 0.02 ppm < 0.05 ppm

Fe, Mn, sulfide < 0.01 ppm < 0.01 ppm < 0.01 ppm < 0.01 ppm

CO2 < 10 ppm < 5 ppm na < 10 ppm

As the table above indicates, EDI offerings today require RO as pretreatment.When designing an EDI system, water recovery needs to be taken into consider-ation and some suppliers even break down hardness into magnesium andcalcium when evaluating hardness levels. Contrary to RO, there are isolated pHareas in all chambers of product, concentrate and electrode so LSI cannot be anaccurate measurement for the scaling index.

M A R C H 2 0 0 7Water Conditioning & Purification

This is when H2O breaks downinto H+ (acid component) andOH-(caustic component). Thepresence of H+ and OH- serves toionize weakly ionized constitu-ents such as CO2, silica and boron;then allows these weakly ionizedconstituents to be transferred fromthe bulk solution through their re-spective membranes and into theconcentrating compartment.

Electrodialysis (ED) has beencommercially available for over 50years. ED uses the same principlesas EDI; however, to overcome theconcentration polarization effectsthat occur in dilute solutions, con-ductive material (i.e., ion ex-change resin) was introduced intothe purifying chamber.

In the ‘90s, products were intro-duced where conductive material wasused in the concentrate cell and elec-trodes. This was to lower the overall elec-trical resistance of the EDI module andto use the current more efficiently.

One spiral EDI device is unique inthat the concentrate flow runs tangen-tially, or spiral to the product or purify-

ing compartment. The anode is on theouter layer and the cathode is in the cen-ter. Concentrate recirculation is recom-mended for this design. The productchamber runs parallel to the nodes.

Concentrate recirculation is used insome designs. The intent is to keep agood flow distribution in the compart-ment, support the purifying chamber andto increase water recovery. Some manu-

facturers have a targeted flow ratein the concentrate, similar to anRO reject recycle, hence requiringthe recycle. Again, these areunique designs created to makeoptimal efficiency of the current.The flow rates of the recycle de-pend on the manufacturer andwill change with the EDI supplier.

One of the first things suppli-ers strove for was to create a stan-dard for EDI design. Today, thereare standards of 50 gpm, 15 gpm,12.5 gpm, 10 gpm and less, basedon flow per cell pair; more cellpairs, the higher the flow. Havingmultiple modules allows moreflexibility to isolate problems, butmay be more costly for additional

power supplies, piping and instrumen-tation. Single stack design, or higher ca-pacity designs with fewer stacks increaserisk and the system may need to be fullyshut down for maintenance.

Many recirculation designs also usebrine injection. The brine injection is in-tended to lower the overall stack resis-tance and make better use of the appliedDC voltage.

Water Conditioning & PurificationM A R C H 2 0 0 7

Hydraulic/pressureconsiderations

When designing an EDIsystem, one must look at thewhole water generation sys-tem. Each EDI module has apressure drop across the de-vice that can range from 15 to40 psi; each supplier also hasspecifications on back pres-sure, temperature, flow andTDS. The use of break tanksmay be required when ROpermeate is being used to feedother processes onsite. If theRO is feeding directly to theEDI, one must take the pres-sure drop across the devicewhen sizing the high pressure RO pump.

In the System Design (1.) illustration,we see two different process flows for anEDI system. The first shows the rejectfrom both the RO and EDI going to thedrain. In this case, the EDI pressure dropand piping should be considered whensizing the high pressure RO pump. In thesecond design, an RO storage tank feedsthe EDI. It is recommended that a one µmfilter be used before any EDI that is notdirectly fed by the RO. The EDI is not afilter and cannot take any particulates.In many cases, the RO storage tank caninadvertently have particles in it as a re-sult of normal use.

The System Design (1.) illustrationrepresents an RO storage tank, but also abooster pump pre- and post- EDI. If theEDI storage tank is not local, this may benecessary.

The System Design (2.) illustrationshows a recycle of the EDI reject. In mostcases, the EDI reject can have a lower TDSthan the RO feed water. However,special attention needs to be givento the ions that the RO does not re-ject and the EDI does. Silica, boronand CO2 are the three major con-stituents to look for in the EDI re-ject. If they are present, there couldbe a build up in the system.

Current efficiencyCurrent efficiency governs all

EDIs. The main factors are flowrate, total feed water equivalentsand current. Most manufacturersrecommend a nominal flow andconsistency in the feed to produceconsistent EDI water quality. Thecurrent does the work, so changesin current will directly affect thewater quality. The major factorsthat affect all EDIs are: current,temperature, TDS and fouling. Thehigher flow-through lowers the

residence time, thus decreasing the timethe fluid is in the path of the current. Thelower the amperage, the less work themodule is performing.

Variables to be monitored• Pressure• Temperature• Flow rates

— Dilute— Concentrate— Electrode

• Voltage• Amperage• Conductivity• Resistivity

Voltage and amperage should bemonitored. This can detect an increase inoverall stack resistance that can be indica-tive of fouling or of changes in the feedwater. Many thicker cell designs can bemore sensitive to changes in feed watercompared to thinner cell designs. A de-

crease in flow and an increasein pressure can be indicativeof changes in the feed sourceor fouling.

The importance of datacollection is trending. Someupsets can be seasonal or theresult of events occurring in thefeed source. If the data istrended, it can predict the per-formance of the system andgive you the information tomake informed decisions.

Normalization correctsrecorded data and assists withmaking educated compari-sons. For example, tempera-ture can greatly affect the

performance of certain EDIs. A decreaseof 10°C in temperature could increasethe overall stack resistance by threefold,hence reducing the amperage and reduc-ing the water quality; however, it doesnot mean the EDI stack may have fouledor that it needs cleaning. Temperatureeffects can be compensated by sizing thepower supply correctly to assure thereis enough voltage or by tempering thewater.

Seasonal effects can include geog-raphy, naturally occurring events andfeed water source variability (chlorine,chloramines, hardness, pH and tem-perature). Seasonal activities can alsoaffect the water generation system. De-pending on the geography, feed waterTDS can change throughout the year.One of the variabilities commonly seenis the addition of ammonia with chlo-rine to form chloramines. Total chlorine(free +chloramines) can irreversiblydamage today’s EDI modules. If a sys-

tem was designed to remove chlo-rine and not chloramines,additional processes will be re-quired. Reduced EDI performanceis due to symptoms in pretreat-ment processes.

About the author Chris Gallagher is the Founder and

President of Applied Water Solutions.He has spent over18 years managing,m a n u f a c t u r i n gand designingseparation and fil-tration systemswithin a wide rangeof industries. Inaddition to his in-depth experience with conventionaltreatment, Gallagher has specialized introubleshooting, upgrading, testingand maintaining all types of EDI sys-

zF Qf (Cd

inlet– Cdoutlet ) x 100%

N Iξ

ξ

=

Where:= current utilization efficiency, %= charge of ion= Faraday's constant, 96,485 amp-s/mol= diluate flow rate, L/s (= gpm/15.85)= diluate ED cell inlet ion concentration, mol/L= diluate ED cell outlet ion concentration, mol/L= number of cell pairs= applied current, amps

zF

Qf

Cdinlet

Cdoutlet

NI

Current efficiency: detailed equation

Flow rate (Cdinlet– Cd

outlet ) 1.31

Current (I)=

Current efficiency: simplified equation

% Efficiency

M A R C H 2 0 0 7Water Conditioning & Purification

tems. He installed some of the first commer-cial EDI systems and since then has contrib-uted to the growth and development of thetechnology. He received a BS Degree in me-chanical engineering from the Universityof Massachusetts. He earned a Masters De-gree in management technology and opera-tions from the McCallum Graduate School ofBusiness, where he is a member of the Execu-tive Board today. Gallagher is an inventorwith a number of US patents relevant to EDIand has written for many industry publica-tions.

About the company Applied Water Solutions, Inc., a global,

independent authorized RO-EDI expert, hasover 18 years of experience supporting mem-brane technology. The company is dedicatedexclusively to solving the pure water needs ofdifferent industries, with deep expertise inElectropurification (EP™), current PurewaterDesign Practices (cPDP), RO, ion exchange,WFI and cGMP environments for purifiedwater systems. Visit the company’s website:www.Applied WaterSolutions.com.

All illustrations in this article are courtesyof Applied Water Solutions, Inc.

Reprinted with permission of Water Conditioning & Purification Magazine ©2007