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Introduction to Electrodeionization Print E-mail

Welcome to Introduction to EDI. This section is dedicated to those who are just learning about Continuous Electrodeionization also known in generic terms as Electrodeionization (EDI).  We will provide you with a basic understanding of EDI and why it is important. It will also better prepare you for the content in the Intermediate EDI sectionActive Image

Introduction to EDI gives you the basics. This section helps you obtain information on EDI fundamentals, definitions and how EDI works. It also enables you to see the benefits of EDI, understand how to design an EDI system and watchouts during design and operation to maintain your system.

This section is broken down into the following topics:

     
  • Advantages of EDI
  • Old vs. New Technlologies
  • Development History
  • EDI or Ion Exchange?
  • Continuous Operation
  • Where EDI works
  • Comparing Old and New Technology
  • Benefits of EDI
  • Salt Movement in an Electric Field
  • Ion Exchange Membranes
  • Mechanisms of Ion Removal

Advantages of EDI

EDI utilizes chemical-free regeneration.  This means a safer workplace because there is no need to store or handle hazardous acid and caustic.  There are fewer regulation concerns due to the absence of these corrosive chemicals and there is no waste neutralization or disposal issues.

EDI is a continuous process.  The ion exchange resins are being continuously being regenerated by the DC electric field.  There is no “breakthrough" of ions as happens in conventional ion exchange operations, therefore the quality of the water remains at a constant high level of purity.  The electric field also provides a bacteriostatic environment inside of the EDI cell, inhibiting the growth of bacteria and other organisms.

EDI has significantly lower operating costs than conventional ion exchange processes.  Only a relatively small amount of electric power is needed to provide high purity water.  The lack of acid and caustic regeneration means less operator attention and lower labor costs.  Capital costs can also expected to be lower, especially because no chemical storage, pumping and neutralization equipment is required. 

EDI has a significantly smaller footprint than conventional ion exchange processes.  This means less plant space will be required to provide the same quantity of water. 

 

When to Consider Using EDI

EDI may be considered to be a competitive alternate process to:

Regenerable Mixed Bed Deionization

No acid or caustic bulk storage, pumping, waste neutralization or disposal issues.  Lower operating cost due lower manpower requirements as well as the lack of chemical regeneration.  Smaller footprint. 

Service Mixed Bed Deionization (off-site regenerated rental vessels)

No ionic breakthrough resulting in a constant high quality of water.  No rental vessels, associated freight or monthly demurrage charges.

Second pass of RO

Eliminates the need for a second bank of RO membranes and associated plumbing, pumping and control equipment. 

Typical applications for EDI
  • Providing USP (United States Pharmacopeia) grade water
  • Purified Water
  • Feed to WFI (Water For Injection) stills
  • Steam Generation / Boiler Feed
  • Microelectronics / Semiconductor makeup and rinsewaters
  • For high quality makeup demineralized water
  • General Industry
  • Surface finishing
  • Chemical manufacturing
  • Hospital & University Central Systems

 

Advantages of EDI - Pharmaceutical Applications

EDI runs in a  bacteriostatic service mode.  The electric field minimizes bacteria growth in the resin beds.  Some EDI devices hot water sanitizable, resulting in more effective sanitization, faster rinseup and more easily validated systems.  RO/EDI easily meets Stage 1 conductivity specification, while conventional 2-Pass RO (RO/RO) does not, due to CO2 and/or NH3. 

 

Advantages of EDI  - Steam Generation Applications

EDI’s continuous electrochemical regeneration provides a constant water quality with no “breakthrough" of bands of ions.  EDI is a low maintenance process requiring little operator attention.  EDI is also ideal for remote locations where you might be dependent on delivery of chemicals or DI tanks.  EDI can provide low product water conductivity, of much better quality then would be possible with 2-pass RO alone

 

Advantages of EDI - Microelectronics Applications

EDI provides a high quality water, low in particles, partiall due to the fact that there is no resin attrition from backwashing or osmotic shock, as would be the case with conventional ion exchange processes.  EDI also provide water with a lower TOC (Total Organic Carbon) content because the electric field will remove charged organic molecules.  Boron removal is better on average than mixed bed deionization, unless the mixed bed columns are regenerated very frequently.

 

Perceived Limitations of EDI

Leaks

Leaks have been completely eliminated in some modern module designs.

Sensitivity to chlorine

EDI is just as sensitive to the chlorine as thin-film reverse osmosis (RO) processes.  This is easily addressed with proper pretreatment system design.

Sensitivity to hardness

Most EDI devices have a 1 ppm hardness limit which is easily addressed with proper pretreatment system design.

Not suitable for high CO2 loads

No ion exchange process is cost-effective for removal of large amounts of CO2. There are several effective and relatively inexpensive ways to remove CO2 in the pretreatment system.

 

Removal Mechanisms

While both ion exchange and EDI use ion exchange resins, the removal mechanisms are quite different.  Conventional ion exchange utilizes chemically regenerated ion exchange resins which function in a capture (exhaustion cycle) and discharge (regeneration cycle) mode.  This results in a breakthrough of ions at the end of the service cycle and a rinseout of regenerant at the beginning of the next service cycle.  Capacity & selectivity are most important resin properties in this mode of operation.

EDI (continuous electrodeionization) utilizes a reaction/transport mechanism to remove ions (through resin under influence of DC field).  This requires continuous path of like-charge resin beads.  The transport is largely across the surface of the resin beads. Transport through resin bead (particle diffusion) can be limiting.

 

Water Treatment - Old vs. New Technologies

mixedbed.gifConventional water treatment systems rely on chemically-regenerated ion exchange resins to remove dissolved solids. Regeneration chemicals are costly, hazardous and, even though they are neutralized prior to releasing to streams and rivers, add a significant amount of dissolved solids to the waterways. These systems typically use cation exchange vessels followed by anion exchange to handle the bulk of the demineralization. Mixed bed exchangers are used in many cases to ‘polish’ the treated water to a resistivity of 18 megohms. The waste regenerants from these systems are usually combined, neutralized and released to the environment. The ion exchange systems are usually supplied in duplicate, to allow one system to provide water while the other one is being regenerated. Regenerations of ion exchangers typically takes several hours, require bulk storage and pumping facilities for regenerant chemicals, and usually require a waste neutralization tank.VNXskid.jpg

State-of-the-art water treatment systems utilize reverse osmosis (RO) membranes to do the bulk of the demineralization. RO systems do not require chemical regeneration and also remove many types of total organic carbon (TOC) which will pass through ion exchange resins. (In some cases, the ion exchange resins actually contribute to the TOC content in the water). The polishing of the RO product water is carried out by continuous electrodeionization (EDI) which is capable of producing water in excess of 18 megohm resistivity. Duplexing of large EDI systems is not required, because a single EDI module may be taken off line for maintenance or repair while the remaining modules operate at a slight increase in flow rate to maintain the required flow through the system. EDI modules can be sized to operate from a fraction of a gpm up to about 50 gpm. The EDI process is a continuous process, utilizes no chemicals for regeneration, does not pollute the environment and requires a fraction of the operator attention necessary for conventional ion exchange systems. EDI systems typically run between 90 and 95% water recovery. The reject stream is usually of better quality than the feed to the RO system, enabling the reject stream to be completely re-used by pumping it back to the pretreatment section of the RO system. RO/EDI systems may achieve overall water recoveries of greater than 90% by recycling a significant portion of the RO reject stream and all of the EDI reject stream back to the front end of the treatment system.

 
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