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Q: What is Electrodeionization?

A: Electrodeionization is the chemical-free process in which ions are removed from water via electric potential. Ions are forced from the dilute compartments into adjacent reject compartments resulting in high purity water. This electric potential also constantly regenerates the resin in the module rather than conventional ion exchange units which use acid and caustic to regenerate. EDI is currently designed to polish RO permeate (product).

Q: What are the advantages of using EDI rather than conventional mixed bed deionization?

A: EDI is a continuous process and the only thing you need is electricity to regenerate the resin. Conventional ion exchange is a batch process which requires large vessels filled with resin, acid/caustic injection systems, a neutralization waste system and all associated controls. Such chemicals can be costly, hazardous and typically requires more space than the equivalent EDI system.

Q: Can EDI Systems handle small amounts of free chlorine the same way TFC RO membranes can?

A: Actually, neither can tolerate free chlorine. We have seen damage to both RO membranes and EDI Modules at concentrations as low as 0.05 ppm. However, it is possible for EDI Module damage to be seen before the RO membranes show signs of chlorine attack.

Q: Why are the limits for hardness concentration in the EDI system feed water so low?

A: Scale can form inside the EDI Module concentrate compartments even at relatively low hardness concentrations because of the high recovery of the EDI System and pH shifts which can occur in the module.

Q: Why are there limits on the iron and manganese concentrations going into EDI system? Aren't these materials removed by ion exchange?

A: Iron and manganese can be removed by ion exchange but can cause problems in a EDI Module because they are held tightly by the resins and may oxidize and precipitate in the resin before they can be transferred out to the waste stream.

Q: Why must the feed water temperature be at least 41oF (5oC)? Why is the minimum temperature specification even higher when silica removal is important?

A: The electrical resistance of the EDI system increases with decreasing temperature. The power supplies for EDI systems are sized to compensate for this down to a temperature of 41oF (5oC) below that temperature the performance may decline due to DC voltage limitations.

Q: Can EDI Systems be installed outdoors?

A: EDI systems must be sheltered from direct sunlight. Also, if ambient temperatures routinely exceed 95oF (35oC) or go below 41oF (5oC) it is not recommended to install the EDI system outdoors. In any outdoor installation, special control enclosures are required to protect the EDI system from humidity, excessive temperatures and other natural elements (rain, sleet, snow).

Q: Is it OK to install a EDI system in the distribution loop instead of directly after the RO system?

A: Yes, EDI systems can be and have been installed in a distribution loop. However, the reject stream from the EDI system results in 5% of the loop water being continuously drawn out of the loop. This water can be returned ahead of the system for re-use.

Q: Do EDI Systems have a difficult time removing CO2? Why is the CO2 concentration in the EDI feed water so important?

A: CO2 is removed by EDI systems just as dissolved salts such as sodium chloride are. We call particular attention to the feed water CO2 concentration for two main reasons. The first is that CO2 is weakly ionized in water so its concentration is not measured by conductivity measurements. The second reason is that CO2 is often a significant portion of the load on the EDI System because it is not removed by RO systems.

Q: Can I raise the pH of the  EDI system feed water with caustic to eliminate the CO2?

A: pH adjustment has to be done in the feed to the RO system to improve the EDI performance. Raising the pH of RO permeate will not improve the EDI product water quality. Raising the pH before the RO allows the RO to remove the CO2 as bicarbonate, preventing the CO2 from reaching the EDI.

Q: How much back pressure can a EDI module handle?

A: The back pressure on the EDI module is limited only by the requirement that the inlet pressure be 100 psi or below. So, if the EDI System is operating at a flow rate which results in a 20 psi pressure drop the maximum back pressure would be 80 psi.

Q: How long does the EDI System take to come back up to quality after being in stand-by?

A: Typically a EDI System will return to quality within a few seconds of starting up from stand-by. It takes much longer however, to come back to quality after cleaning or sanitization, several hours or overnight.

Q: What is the recommended frequency of sanitization for EDI Systems?

A: The frequency of sanitization varies greatly with the customer's bacteria specifications and the system design. Some systems are never sanitized but typical frequencies for systems producing Purified Water range from weekly to quarterly.

Q: What is the typical life of the EDI resin and membrane cell packs?

A: Three to five years is considered a normal life for the cell packs. The useful life obtained at any given site will be a function of the raw water quality, the pretreatment design and how well the system is monitored and maintained.