What Are the Reasons for the Decrease in Resistivity in Ultrapure Water Equipment?

The decrease in resistivity of ultrapure water equipment during operation is related to many factors. Below are some reasons for this decrease:

I. Problems with System Current Control

Increasing the operating current initially improves the quality of the ultrapure water produced. However, if the current is increased further after reaching a high point, the excessive amount of H+ and OH- ions generated by water ionization, besides being used for resin regeneration, results in a large number of surplus ions acting as current carriers for conductivity. Simultaneously, the accumulation and blockage of these current carriers during their movement, or back diffusion, leads to a decline in the quality of the produced water.

II. Unqualified Effluent from Reverse Osmosis Equipment

If the raw water has a high salt content, it is recommended to use a bipolar RO reverse osmosis system for pre-desalination, maintaining a conductivity of 1–3 μS/cm. If the influent CO2 content is high, it is recommended to use a degassing membrane or degassing tower to remove CO2. If the pH deviates too much from neutral, pH adjustment should be used to maintain the influent pH at 7–8.

III. Iron Contamination

Iron contamination during the operation of ultrapure water equipment is a major cause of the progressive decrease in resistivity of the produced water. If ordinary steel pipes are used in the raw water and pretreatment systems without anti-corrosion treatment, the iron content in the system will increase. The iron corrodes and becomes Fe(OH)3 in suspension. Resin has a strong adsorption capacity for iron, and once adsorbed by the resin, an irreversible reaction occurs.

In anion and cation exchange water treatment, the anion and cation beds undergo regeneration or cleaning to remove most of the iron from the resin. However, in ultrapure water equipment, there is no regeneration or cleaning. Iron in the water adheres to the anion and cation resins and membranes, leading to decreased component performance, poor product water quality, and a progressive decrease in resistivity.

IV. Organic Contamination

Ultrapure water equipment feed water contains organic colloidal contaminants. Reverse osmosis can only remove organic colloids with a relative molecular weight greater than 200. Those with a molecular weight below 200 enter the system. These low-molecular-weight substances are adsorbed by the anion and cation exchange resins within the components, onto the mesh of the framework and the surfaces of the anion and cation membranes. This intercepts the anion and cation exchange reaction and the rate at which ions in the water penetrate the anion and cation membranes, resulting in decreased performance and a drop in the resistivity of the produced water.

V. PH Changes

High CO2 content in the system influent: If the CO2 content exceeds 10 ppm, the system cannot produce ultrapure water.

The above is an introduction to the reasons for the decrease in resistivity of ultrapure water equipment. During the operation of ultrapure water systems, we should try to keep the resistivity of the equipment constant to ensure stable output water quality.