What are the reasons for the decline of resistivity during the operation of EDI ultrapure water equipment?

The reasons for the decline of resistivity during the operation of EDI ultrapure water equipment are related to the pollution of inlet water quality, pressure, flow, voltage and inlet water quality. The following is a brief introduction to the reasons for the decline of resistivity of EDI ultrapure water equipment.

1. The effluent of reverse osmosis equipment is unqualified (including conductivity, hardness, variable metal, etc.)

If the salt content of raw water is high, it is recommended to use bipolar RO reverse osmosis equipment as pre desalting, and its conductivity shall be maintained at 1 ~ 3 μ S / cm is preferred; The influent has high CO2 content, so it is recommended to use degassing membrane or degassing tower to remove CO2. If the pH deviates too much from neutral, adjust the pH to make the pH value of EDI influent at 7 ~ 8.

2. There is a problem with the current control of EDI system

The working current increases and the quality of produced water keeps getting better. However, if the current is increased after increasing to the high point, due to the excessive amount of H + and oh – ions produced by water ionization, a large number of surplus ions act as current carrying ions for conduction, in addition to being used for regenerating resin. At the same time, due to the accumulation, blockage and even reverse diffusion of a large number of current carrying ions during their movement, the quality of produced water is reduced.

3. PH change

The influent CO2 content of EDI system is high. If the CO2 content is greater than 10ppm, EDI system cannot prepare high-purity water.

4. Iron pollution

Iron pollution in the operation of EDI system is the main reason for the progressive decline of water production resistance. If ordinary steel pipes are used in the raw water and pretreatment system without internal anti-corrosion treatment, the iron content in the system will increase. After the iron is corroded, it will be dissolved in the water in the form of Fe (OH) 2 and further oxidized to Fe (OH) 3. Fe (OH) 2 is colloidal and Fe (OH) 3 is suspended. The resin has strong affinity for iron, which will cause irreversible reaction after being adsorbed by the resin. In the treatment of anion and cation exchange water, the anion and cation beds will be regenerated or cleaned to remove most of the iron in the resin. However, in the operation of EDI equipment, there is no regeneration and cleaning, and the trace iron in the water will adhere to the anion and cation resin and anion and cation film. Iron has strong conductivity. Before it can react with the cationic resin, it is close to the negative membrane water in the EDI module and migrates to the positive membrane under the action of high current. Pure iron ions are easy to penetrate, while colloidal iron compounds are not easy to penetrate the positive membrane. They are adsorbed on the surface of the positive membrane and pollute the negative and positive membranes, which leads to the decline of the working performance of EDI components, poor water production quality and progressive reduction of resistance value.

5. Organic pollution

The organic colloid in the water inlet of EDI pure water equipment is polluted, and reverse osmosis can only remove the organic colloid with relative molecular weight greater than 200. If the molecular weight is lower than 200, it will enter the EDI system. This part of low molecular weight material is adsorbed on the mesh of the skeleton and the surface of the anion and cation membrane by the anion and cation exchange resin in the module, which hinders the replacement reaction of anion and cation and the speed of water ions penetrating the anion and cation membrane, resulting in the decline of EDI working performance and the decline of water resistivity.

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