Demineralised process water – Meaning and production

Demineralised process water

In technical usage, the term “demineralised water” is used as an abbreviation for “fully demineralised water” or demineralised water. The ions and salts found in tap water (including calcium, magnesium, strontium, chloride, sodium, etc.) are not present in demineralised water. Basically, 3 water qualities can be summarised under the term “deionised process water”:

• Conductivity < 50 µS/cm: low-salt treated water, e.g. through the use of a single-stage reverse osmosis system.
• Conductivity < 1 µS/cm: ultrapure water, e.g. produced by using a two-stage reverse osmosis system and supplementary processes such as the use of mixed-bed resins, depending on the initial water quality.
• Conductivity < 0.1 µS/cm: high-purity water produced, for example, by using a process combination of reverse osmosis, electrodeionisation and mixed bed.

To increase the so-called yield of reverse osmosis, it is often preceded by a so-called softening plant.

The electrical conductivity method (e.g. Select, N-LF2000, N-LF420) is used to determine the degree of purity of deionised water. The electrical conductance (Siemens S = Ω-1) is determined by means of alternating current and Ohm’s law (G=I/R current intensity (I), voltage (U) & electrical conductance (G)) of the analysis solution or its reciprocal.

The unit of measurement is mS/m (millisiemens per metre).

Depending on the treatment and initial quality, the demineralised water may still contain additional impurities. It is also known as “distillate-like water” or “demineralised water”. In the trade, it is often offered as ironing or battery water. Distillate-like water is additionally sterilised to completely kill any bacteria it may contain.

In addition, the solubility of deionised process water is more aggressive than that of ordinary water due to the lack of salts and ions. Due to the production of deionised water (fully demineralised water) by means of membrane filtration processes and, if required, in combination with ion exchange processes, the production is significantly cheaper, as this requires much less energy and cooling water. At the same time, due to its filtration fineness, the membrane represents a germ barrier, which is comparable to the destruction of organic compounds and microorganisms that occurs during distillation. The production of deionised water is significantly cheaper than water in the distillation process. It is therefore often used as an alternative to distilled water in industrial or laboratory environments.

The practical applications of deionised water are manifold. However, the main application is in the technical field, in chemistry and biology, namely as a solvent, as a heat transfer medium in cooling circuits, or in medical facilities. Treated water plays an essential role in a large number of industrial manufacturing processes.

Process water is subject to increased demands on water quality and properties and thus also on water treatment (e.g. in the outpatient clinical area). Water contains certain substances that can have harmful effects on equipment and processes. The desired quality and the permissible residual content of remaining substances after treatment of the process water always depend on the area of application. The process water is filtered, softened or desalinated as required to meet technical standards, legal requirements or the regulations of the machine manufacturers. Process water that is not properly treated for the process can cause great damage to people and machines and have massive effects on the production lines or the entire value-added process.

For example, process water with the wrong ingredients (e.g. too high a silicate content) can damage the sterilisers and the reprocessed medical instruments in them, so that over time instruments come into circulation that are not considered to have been properly reprocessed in the sense of the MDR (Medical Device Regulation). In such a case, proper reprocessing is only possible by mechanical surface removal or the use of highly aggressive media such as hydrofluoric acid. As a result, the service life of all affected medical devices is drastically reduced and high reinvestment costs are incurred.

In the holistic approach to the process, the design of the water treatment is a decisive factor, which has a significant influence on the economic efficiency, operational safety and environmental protection as well as compliance with legal requirements. Due to the complexity, it is always advisable to consult an independent expert when designing the system.

Water treatment must take into account the desired process water quality and the required volume. The design of the various process combinations in order to comply with quality, volume and legal standards requires a high level of technical expertise.

To produce process water, water from the municipal water supply is usually used. The latter obtains the water from well and surface water (approximately 74 per cent of drinking water in Germany is obtained from groundwater) or from a combination of these water sources and treats it in accordance with legal requirements before feeding it into the supply network. In this context, it should be noted that the chemical components of the drinking water produced in this way vary greatly from catchment area to catchment area (within the legally permissible limits for drinking water) and therefore its treatment for further use must always be considered on a case-by-case basis.