The main cause of EDI failure is oxidation of the module, either by chlorine. Oxidation decrosslinking also causes the resins to disintegrate which results in an increased pressure drop across the module. For this very reason, dechlorination is extremely important to put the proper measures in place in the pretreatment to both the RO and EDI system. In addition to chlorine, chloramine or ozone attacks ion-exchange resins and membrane and cause decrosslinking, which results in reduced capacity. If Chloramines are present, further methods of removal may be required. Oxidation will increase apparent TOC, and the byproducts cause fouling of the anionic resin and membrane, reducing the ion-transfer kinetics.
The ideal concentration level for oxidants is zero. RO membranes are more resistant to chlorine that EDI modules so do not measure the decrease of RO rejection as an indication that there is no chlorine present!
Rule #1: There should be no detectable chlorine in the EDI feed water. Oxidative attack on a EDI module is not recoverable by cleaning!
There are three principal means of achieving the above requirement:
- Granular activated carbon
- Injection of sodium bisulfite or sodium sulfite
- UV dechlorination
Recommendation: Activated carbon dechlorination is preferred over chemical dechlorination because it is both more reliable and also less likely to cause biological fouling problems. UV dechlorination is a relatively new process, which may have difficulty achieving complete chlorine removal, especially on chlorinated feed water.
Feed water chlorine can be measured with a test kit. The smallest increment for this test kit is 0.02 mg/l.
A) Dechlorination by activated carbon
It is the responsibility of the system supplier to ensure that the activated carbon filter is sized properly for complete removal of the feed water chlorine, taking into account the concentration of chlorine in the raw feed water. Ionpure would usually design dechlorination carbon filters for a minimum empty bed contact time of 6 minutes.
B) Dechlorination by chemical injection (reducing agent)
It is the responsibility of the system supplier to ensure that the chemical injection system is sized properly for complete removal of the feed water chlorine, and includes adequate instrumentation and control safeguard to ensure that the EDI system is not fed water containing chlorine should there be a change in the feed water conditions or an upset in the operation of the chemical injection system.
In systems that contain injection of multiple pretreatment chemicals (for example antiscalant and sodium sulfite or sodium bisulfite and sodium hydroxide) it is recommended that there be a separate injection point and static mixer for each chemical.
C) Ultraviolet Dechlorination
It is the responsibility of the system supplier to ensure that the UV dechlorination system is sized properly for complete removal of the feed water chlorine, taking into account the concentration of chlorine and chloramine in the raw feed water.
- There must be no detectable chlorine in EDI feed water!
- Chlorine damage is irreversible
- Chlorine is a common cause of EDI failure
- Take Cl2 readings at RO inlet and EDI inlet
- RO permeate can contain more free chlorine than RO feed
Suggested Test Kit: Use Cl2 – Hach Model CN-70 (#1454200)