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The reasons for the decrease in resistivity during the operation of EDI (Electrodeionization) ultrapure water systems are related to factors such as the quality of the incoming water, pressure, flow rate, voltage, and contamination of the feed water. Below are some of the main causes for the drop in resistivity of EDI ultrapure water systems:
RO System's Effluent Does Not Meet Standards
If the feedwater has a high salt content, it is recommended to use a bipolar RO (Reverse Osmosis) system as a pre-deionization step, keeping the conductivity between 1–3 μS/cm. If the CO2 content in the feedwater is high, it is advisable to use a degassing membrane or tower to remove CO2. For pH levels that deviate too much from neutral, pH adjustment should be used to maintain the feedwater pH between 7–8.
Issues with EDI System's Current Control
Increasing the operating current improves the water quality. However, once the current reaches its maximum and continues to increase, excess H+ and OH- ions generated by water ionization may cause ion accumulation and blockage, or even back-diffusion. This leads to a decrease in the quality of the product water.
Changes in pH
High CO2 content in the feedwater of the EDI system can negatively impact ultrapure water production. If CO2 content exceeds 10 ppm, the EDI system will not be able to produce high-purity water (this is a critical issue).
Iron Contamination
Iron contamination is one of the main reasons for the progressive decrease in resistivity in EDI systems. If ordinary steel pipes are used in the raw water and pretreatment system without internal corrosion protection, the iron content will increase. Once the iron is corroded, it dissolves in water mainly as Fe(OH)2 and further oxidizes to Fe(OH)3. Fe(OH)2 is colloidal, while Fe(OH)3 is in a suspended state. The resin in the EDI system has a strong affinity for iron, and once adsorbed, it can cause irreversible reactions. In conventional cation and anion exchange processes, regeneration or cleaning of the resin beds can remove most of the iron. However, in an EDI system, since there is no regeneration or cleaning, trace iron in the water adheres to both the cation and anion resins, as well as the membranes. Iron has strong electrical conductivity, and before it can react with the cationic resin, it migrates towards the anion membrane under the influence of high current. Pure iron ions easily pass through the membranes, but colloidal iron compounds are harder to penetrate the anion membrane and are adsorbed on its surface. This leads to contamination of both the anion and cation membranes, ultimately causing a decrease in system performance and water quality, and a progressive reduction in resistivity.
Organic Contamination
If organic contaminants are present in the feedwater, reverse osmosis can only remove organic colloids with a molecular weight greater than 200. Organic substances with a lower molecular weight (below 200) pass into the EDI system. These low-molecular-weight substances are absorbed by the cation and anion exchange resins within the components, and they adhere to the surfaces of the cation and anion membranes. This obstructs the ion exchange reactions and slows the ion penetration speed through the membranes, thereby reducing the performance of the EDI system and lowering the resistivity of the produced water.
