Ultrapure water production
What is ultrapure water and how is ultrapure water produced?
Ultra-pure water, or UPW, is demineralized water that has been purified again, i.e. fully demineralized water in which the ingredients have been further reduced to the chemical-physical minimum of 0.056 µS/cm or 18.18 MΩ-cm (at 25°C). Even calibrated measuring devices sometimes show even lower values, but this is to be regarded as a measuring error, which can sometimes occur with even the smallest air bubbles in the pipe. Ultrapure water is often required in the production of semi-finished products or in hydrogen production.
The production of ultrapure water is initially similar to the production of demineralized water. First, the feed water, usually city water, is fully demineralized using an ion exchanger via a demineralization system and demineralized water with typically 3-5 µS/cm is produced. Alternatively, this is also possible using a permeate-staged reverse osmosis system. The demineralized water is then usually passed through degassing and subsequent electrodeionization (EDI) and can then already be ultrapure water at or close to the desired conductivity for electrodeionization. Finally, fine cleaning is then carried out using mixed-bed polishers, i.e. demineralization cartridges, which are usually regenerated externally. Depending on requirements, additional filtration stages can be added, such as filters, activated carbon and pH adjustments.
Instead of the largely standardized mixed bed resins in the demineralization cartridges, these can also be replaced with so-called ultra pure water resins or ultrapure water resins in ultrapure water applications. The UPW mixed-bed resins have higher capacities than the standard mixed-bed resins and are therefore significantly more reactive, i.e. they reach the lowest possible conductivity value more quickly. Some of these mixed-bed ion exchange resins are labeled as non-regenerable, but can be regenerated in special processes.
After the immediate production of ultrapure water, however, it should be noted that this is usually not sufficient. The requirement often does not apply at the outlet of the production plant for the demineralized water, but at the point of use in the factory (see illustration). Further purification stages are therefore required, as ultrapure water has a high affinity for absorbing substances of all kinds, such as CO2 from the air, which directly lead to an increase in conductivity.
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