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What Is Demineralised Water?

Demineralised water is water that has had all the mineral elements removed.

There are selected methods that can be used to demineralise water, including deionisation, distillation and reverse osmosis (RO), however, these methods will not fully return full quality demineralised water, without a final polishing process.

The most effective demineralisation methods are deionisation (DI), which uses an ion-exchange resin to fully remove mineral elements.

Water Demineralising Process

DI is the only process to fully demineralise water, there are differences between demineralised water and deionised water.

The process necessary to produce deionised water can be achieved by ion exchange, distillation or reverse osmosis.

Ion Exchange

The ion exchange process requires for water to pass through selected ion-exchange resins referred to as cation and anion which are charged to attract positive and negative mineral ions, respectively, that are dissolved in water.

To produce fully demineralised water, a final polishing stage is required, which is referred to as a mixed bed column, which combines both cation and anion ion-exchange resins, in one vessel.

Distilled Water

Distilled water is produced by boiling water, the steam that is generated is then cooled and condensed back into water. Various minerals have different boiling points to that of water and as a result some of the volatile impurities may flash off before steam is produced and others may require a higher temperature, they may combine into the steam and remain in the condensed water after distillation. The resultant method will not produce fully demineralised water and will require mixed bed polishing as a final stage.

RO-DI Systems

Reverse osmosis (RO) is a cost-effective option to produce deionised water however, in order to achieve demineralised quality, would require for the first pass deionised water (permeate) to be passed through a second RO plant or mixed bed polisher, where the latter is the preferred option.

This treatment method produces high purity water that can be used for a range of purposes.

Demineralised water must be stored in a controlled environment as it can turn into deionised water if exposed to air.

What Are Demineralised Water Plants Used For?

The ultra-pure demineralised water is typically used to:

  • Feed high pressure boilers.
  • Wash water for the manufacture of computer chips.
  • Medical facilities typically renal dialysis or laboratories that require very pure water.
  • Used for pharmaceutical process water and optical lens manufacture.
  • Microelectronics and any other process that requires high purity water.
  • Steam driven turbines.

The type of resins or membranes selected for demineralisation plants depend on many factors, which include;

  • Water temperatures.
  • The level of treated water quality required.
  • Input water quality or issues with the input water quality.
  • Presence of organic foulants.
  • Flow through plant required.
  • Regeneration method – either counter-current or co-current regeneration (Ion exchange).