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FAQs – Electrodeionization

Frequently Asked Questions about the operation and maintenance of Electropure EDI modules, systems, and technology. Please refer to our OEM Technical Manual for additional information.  If specific troubleshooting help is needed please complete our troubleshooting checklist.

FAQ Topics: 

FlowRecoveryPressureTemperatureCurrent & Voltage

EDI Quality – OxidationCleaning & Sanitizing CO2Installation

 

Flow

Q: What flow range is my SnowPure Electropure™ EDI module designed for?

A: Individual modules range from 10 l/h up to 8 m3/h. Each module has an engineering design flow (different for 1-pass RO or 2-pass RO), and a recommended minimum and maximum flow. Please refer to our SnowPure Product Family brochure.

Q: What is the minimum feed flow for the EDI module?

A: The minimum feed flow is approximately half of the rated EDI product design flow.

Q: Flow through my EDI is decreasing, what could be causing this?

A:  Please refer to below list of potential causes for low flow:

      1. Power applied without flow; the internal plastics melt blocking the internal flow ports.
      2. Chlorine is degrading the resin beads, turning them slowly to “oatmeal”.
        1. Carbon breakthrough
        2. MBS dosage failure
        3. Electrode waste stream mistakenly recycled (the Cl2 is difficult to remove using normal AC)
        4. Ozone O3 in the feed
        5. UV can catalyze DO into Ozone (3-O2 becomes 2-O3)
        6. Accelerated oxidation from heavy metals adsorbed by the resin
      3. TOC in the RO permeate.  We recommend that you get the permeate reviewed analytically for TOC. Check pre-treatment.
      4. Something may have blocked the flow port.  Look inside the inlet and outlet ports. If there is a block in the inlet, try to reverse the flow to see if any debris comes out and the system unblocks itself.

Q: My electrode flow is decreasing, what could be causing this?

A: The flow volume through the electrode (E) stream should not be less than half of the rated flow. If it is less than half the system should be shut down to prevent possible permanent damage.  Please provide all operating parameters on our Troubleshooting Checklist. Potential causes:

      1. If you can’t change the flow by adjusting the electrode valve something may have blocked the flow port.  Look inside the inlet and outlet ports. If there is a block in the inlet, try to reverse the flow to see if any debris comes out and the system unblocks itself.
      2. Reduced flow due to high hardness in the feed and scaling of the electrodes. Use acid cleaning of the concentrate/electrode if you have CaCO3 scaling, or Minncare cleaning and remove bacterial biofilm (all streams).
      3. Melted plastic. The power supply is coming on without flow through the module and melting the internal plastics. A flow switch needs to be installed to protect the EDI. Double-check that all of the flow and pressure switches are working and that the protection circuits are functioning properly.
      4. If there is a check valve make sure it is operating properly and not restricting the flow.
      5. Electrode should be flowing to an open drain with no back pressure.

Recovery

Q: Nominal EDI water recovery is 90%. How can I increase the recovery?

A1: With 2-pass RO as feed, hence very low FCE, you can safely decrease the concentrate flow (only) by 50%, and thereby increase the EDI system recovery from 90% to 95%.

A2: Electropure EDI has a separate stream for electrode waste so that you can recover the concentrate waste 100%. SnowPure recommends the concentrate outlet be diverted to a break tank, and then pumped to the inlet of the RO. EDI concentrate is already very good water (it is concentrated RO permeate, already pretreated). The EDI system recovery can now be raised from 90% to 99%. And the RO system recovery can be raised by another 5%. Competitor EDI technologies cannot do this!

DO NOT RECOVER ELECTRODE WASTE!

Pressure

Q: My EDI feed pressure has gone up. What might be the cause?

A: When there is oxidation, the MB resins (first the anion resins) turn to “mush”, like oatmeal, from whole beads. If we find high pressure, this is the first thing we suspect. It is generally present only at the inlet of the cells. Can be confirmed at SnowPure during a module autopsy.

Q: In our plant the EDI electrode and concentrate outlet flows must be redirected to a main draining pipe located several meters from the EDI therefore the outlet pressure of both electrode and concentrate will increase, is this a problem?

A: The key is to keep the EDI product pressure greater than the EDI concentrate and electrode outlet pressures.  If concentrate exceeds permeate pressure, lower product quality will occur.

Temperature

Q: Is low temperature an issue for EDI?

A: Two things happen with low temperatures. First pressures rise significantly. Every 5C adds 15% to the viscosity of water therefore adds 15% to the pressure. Between 25C and 10C, the pressure drop is about 50% higher. Second, ions move more slowly and current goes down–SnowPure recommends raising voltage up 10% when temperatures are below 15C, this will help performance. We recommend running an RO performance projection at both the lowest and highest temperatures.  These two sets of data can then be entered into the SnowPure EDICAD software and compared for EDI performance and design.  See EDICAD software capabilities for more details.

Q: I have HTS type SnowPure Electropure™ EDI modules which are specially made for heat sanitization.  At what temperature should I sanitize?

A: 72C to 80C. Please refer to our technical article regarding New USP Guidelines on Heat Sanitization. USP recommends staying below 80C. SnowPure HTS modules are capable of 85C.

EDI Quality

Q: What feed quality is needed for EDI?

A: High quality RO permeate (only) must be fed to the EDI.  The EDI loading factor is measured by FCE and the main loading constituents in feed water are conductivity, CO2, and Silica.  Please see note regarding how to calculate FCE on our EDI feed water quality guidelines. EDICAD will help estimate EDI product quality at different Feed FCE.

Q: What factors contribute to EDI loading and what do these mean?

A: We consider that there are 3 different kinds of ions fed to the EDI which make-up “FCE” the measurement of loading on EDI:

      1. “Easy ions”, e.g., Na+, Cl-, etc. These make up the water’s conductivity, and provide a certain “load” on the EDI. These are removed first in sequence inside the EDI.
      2. Total CO2: CO2 and HCO3- are not as easy to remove, they are chemically interchangeable (with pH), and are removed after the easy ions. Together they add a load based on their sum.
      3. SiO2: Silica is removed third in series, and this adds a third load (often a small but important load).

Note: It is critical to include HCO3- and CO2 together. As pH shifts in the feedwater, the total stays the same but the components vary. It is rare that there is any CO3-2 with normal RO permeate. See our section on CO2 reduction and EDI feed water quality guidelines.

Q: I have high silica in my feed.  What can I do to ensure good silica removal in my RO and EDI, and prevent silica problems?

A: There are 3 keys to having high silica removal with EDI. Best case EDI removes about 97% of what the RO permeate sends to it:

      1. Ensure low conductivity feed, this gives a smaller “working bed” and a larger “polishing bed”.
      2. Low CO2 + HCO3-, since CO2 competes with SiO2. This can be done with high pH (8.4-8.7) in the RO feed, or using a membrane degasifier like Liqui-Cel (SnowPure can advise and quote.).
      3. Use the correct voltage. With the correct voltage, the “water splitting” in the polishing section of the EDI keeps the anion resins fully in the OH- form, best able to convert SiO2 to H3SiO3- for best adsorption and removal.
      4. We also sell ExcellPureRO™ high silica rejection membranes with 99.2% silica rejection. By contrast other brackish water membranes get about 98.5% rejection of SiO2 and low pressure type membranes get 95-97% rejection of SiO2.

Q: Will the EDI permeate/dilute quality be affected by temperature?

A: The quality of the RO permeate is affected by temperature/pressure.  We recommend running an RO performance projection at both the lowest and highest temperatures.  These two sets of data can then be entered into the SnowPure EDICAD software and compared for EDI performance and design. See EDICAD software capabilities for more details.

Q: What affect does TOC in the feed have on quality?

A: TOC (organics) will cause low quality by absorbing onto the anion resins and membrane. This is difficult to diagnose without a surfactant cleaning. The best way is to review the RO feed AND permeate analytically for TOC.

 Q: My quality dropped suddenly. What could cause this?

A: If quality drops dramatically there can be four causes, one is electrical, one is fouling/scaling, one is concentrate backpressure, and one is CO2 in the feed.  Please complete our troubleshooting guide for assistance.

Q: Can a low electrode flow cause quality issues?:

A: Having a blocked electrode port could cause gases to build-up in the EDI module reducing performance.  If the electrode stream is lower than half the rated flow the system should be shut-down and troubleshot to fix this issue to help prevent permanent damage to the EDI module. Try to open/close the electrode flow valves to break up any large bubbles in the electrodes, these can behave like electrical resistors.

Q: How do I measure pH accurately in pure and ultrapure water?

A: pH of RO permeate can be done, but it’s difficult. pH of ultrapure water is 7.0. Please review our technical article regarding pH.

Damage by Oxidation

Q: Can trace chlorine damage an EDI module?

A: Trace chlorine or ozone in the EDI feed will damage the EDI, and lessen its lifetime. Chlorine and ozone (O3) oxidize both EDI mixed-bed resins and membranes. Chlorine attacks the anion resin first, and often we see EDI modules with anion resin beads turned into “oatmeal” or paste at the EDI inlet, causing high pressures and low quality.

RO membranes have some chlorine tolerance but EDI does not.  It is common that chlorine oxidation shows in EDI before it shows in RO performance. SnowPure recommends NIL (not detectable) chlorine or ozone in the EDI feed.

    • Chlorine in the feed
      • Active carbon breakthrough (also unexpected high dosage from municipality)
      • Bisulfite (MBS) dosage failure, or ORP failure
      • Electrode waste stream is recycled, this has chlorine in a form that is difficult for carbon to remove. Always send electrode waste to drain.
    • Ozone in the feed
      • De-ozonation not working
      • UV after the RO. UV can convert dissolved oxygen (DO, O2) to ozone (O3). All water has some DO in it that can become O3.

Cleaning and Sanitizing

Q: How long would it take to rinse an EDI module after a chemical sanitation?

A: We recommend using 0.1% Minncare (peroxide + peracetic acid, byproduct is acetone), can be raised to 1% (maximum) if the biofilm is pernicious.  The rinse-out is quick (1-2 hours), and with the Minncare residual test strips you can confirm this.  Refer to our technical information for more details on Minncare.

Q: When would I perform a low pH (acid) cleaning?

A: Most common cleaning is to use 1) acid cleaning of the concentrate/electrode if you have CaCO3 scaling, or 2) Minncare cleaning to kill bacteria (all streams). Refer to our OEM manual for cleaning instructions.

Q: When would I perform a high pH (caustic) cleaning?

A: Normally NaOH cleaning of the resins through the Feed/Permeate ports is a last resort, this only takes care of 2 things, SiO2 scaling if your feed has 2 ppm or more, and TOC fouling which is rare if you have proper pretreatment. NaOH cleaning is difficult and often results in lower quality–only do this if you must and if you know you have SiO2 scaling or TOC fouling. Do not use NaOH in the concentrate or electrode, these never have a TOC problem and the polyester in these are damaged by high pH.  Refer to our OEM manual for more details. See below for the best TOC cleaning recommendation.

Q: Should I “brine” the EDI using salt?

A: No. Never. Salt shrinks resin beads, and the mixed bed inside the EDI can demix, causing lower quality.

Q: What if I have TOC fouling of my EDI resins? How can I best clean this?

A: Use of the NON-IONIC surfactant Triton-X 100 is ideal and easy. Refer to our OEM manual and Triton-X cleaning chemical for more details.

CO2

Q: How do I reduce CO2 in the EDI feed?

A: For a chemical-free and continuous way to reduce CO2 we recommend membrane degasification (GTM) like Liqui-Cel modules used in the RO permeate feed to EDI. SnowPure is a Liqui-Cel distributor and has a number of sizes in stock. CO2 may also be reduced by chemical injection of NaOH to the RO feed (after softening) to increase pH between 8.4 to 8.7. If the system is a double-pass RO the pH adjustment should be done between passes. Corosex in the RO pretreatment can also reduce CO2 and improve RO rejection performance.

Q: How do I measure and calculate “Total” CO2?

A: Total CO2 is made up normally of CO2 and HCO3- (bicarbonate). CO2 is measured using small test kits which are readily available. If you have 2 of the 3 values (pH, CO2, and HCO3-), then you (or we) can calculate the 3rd. At pH=6.4, if you have 2 ppm HCO3- then you have 2 ppm CO2.

Q: What does CO2 mean in an EDI system and how does it affect EDI loading?

A: We consider that there are 3 different kinds of ions fed to the EDI which make-up “FCE” the measurement of loading on EDI:

      1. “Easy ions”, e.g., Na+, Cl-, etc. These make up the water’s conductivity, and provide a certain “load” on the EDI. These are removed first in sequence inside the EDI.
      2. Total CO2: CO2 and HCO3- are not as easy to remove, they are chemically interchangeable (with pH), and are removed after the easy ions. Together they add a load based on their sum. CO2 competes with SiO2 and B (Boron) for removal in the EDI. So for cases were low SiO2 and low Boron are needed, we recommend removing CO2 + HCO3 to below 1 ppm if possible. Then it is easier to get SiO2 to below <5 ppb or <2 ppb as needed.
      3. SiO2: SiO2 is removed third in series, and this adds a third load (often a small but important load).
      4. B: Boron is removed fourth in series.

It is critical to include HCO3- and CO2 together. As pH shifts in the feedwater, the total stays the same but the components vary. It is rare that there is any CO3-2 with normal RO permeate. See our section on CO2 reduction and EDI feed water quality guidelines.

Current and Voltage

SnowPure highly recommends controlling VOLTAGE, unlike competitor EDIs. Read more in our article on Voltage Control.

Q: What type of power supply should I use?

A: See SnowPure’s selection of DC power supplies and power supply guidelines (link).  A high-quality DC power supply is important for EDI and the power supply must have low AC ripple and interlock capability.

Q: I have very low to zero current.  What could be causing this?

A: The voltage should be set and let the current float. If operating correctly, with 10% concentrate, you will get about 10-11x the conductivity of the feed in the concentrate. With this conductivity the stack resistance will be right and you will have a strong current to remove the ions. Current will be (roughly) proportional to the concentrate conductivity, and will reflect how many ions are being moved inside the EDI. If the concentrate conductivity is lower than 10 uS/cm you are not removing ions well. Activate the power supply and check voltage at electrodes. Check for proper polarity (DC+ and DC-) with handheld and with wiring colors. Low current can also be caused by:

      1. Fouling of the resins and membranes (toc/silica/ …)
      2. Scaling in the concentrate (CaCO3 …)
      3. High concentrate flow (low concentrate conductivity)
      4. Biofilm fouling caused by bacterial outbreaks (not normal if the EDI is running continuously, possible if the module/system is shut down for a period of time.
      5. The Current Control limit on the DC supply may have been turned down.

Q: I have high current. What could be causing this?

A: High FCE will cause a high current.  See EDI feed water recommendations (link).  Low concentrate flow (high EDI recovery) will also cause high current.

Leaking

Q: My EDI is leaking, what can I do to correct this?

A1: Refer to torqueing procedure in our OEM Technical Manual for our XL model EDI modules. If the module is EXL consult with factory. If the leak is coming from the feed/product ports and pipe sealant (dope) was used on the ports a chemical reaction could have caused the ports to crack.  Inspect the ports for possible hair-line splits on the connection.  Also check for correct installation of the O-rings in the fittings. Other considerations such as mounting and grounding are outlined in installation checklist.

A2: If both the front face and the rear face of the EDI are constrained (bolted to the frame) then the module cannot be tightened. This is a design/installation problem.

Installation

A detailed Installation Checklist is shipped with each EDI module. Contact SnowPure for another copy.

Q: Are there any special considerations for EDI piping, mounting, etc.?

A: Our connections are made from very strong engineering polymers, but it is important that a pipe (dope) sealant is not used on the EDI threaded ports as this will cause cracking of the ports.  SnowPure recommends the use of Teflon tape for XL port threads. No sealant is needed for Zap and EXL ports. The solvents in these sealants (most of them) are designed for metal threads and will lead to weakening of the plastic over time. Warnings regarding pipe sealants are sent with each module. There are other considerations such as mounting and grounding which are outlined in installation checklist. SnowPure sells an all-Teflon pipe thread sealant, contact us.

Q: Do I need to vent the electrode stream to outside?

A: As described in the OEM Technical Manual some of by-products, i.e. H2 and O2 are generated at the electrodes during operation.  Make sure the area in which the EDI is installed has adequate ventilation. If installing in a small area, such as a container or utility room, vent electrode to outside. Multiple-module systems will generate more H2 and O2 obviously, so be aware and do the calculations. Check local industrial codes. Please refer to our OEM manual and installation checklist.