Filtration, whether through distillation or deionisation, is the demineralisation of water to achieve a pure substance. These processes both eliminate ionic impurities from water; however there are very specific instances in which one is used over another. To understand the difference, we must learn about both processes.
Distilled water is often found on the shelves of grocery stores and is best used for laboratories, batteries, small appliances, engines, and other mechanical works. It’s generally not advised to drink this type of filtered water regularly as it is demineralised, in other words, stripped of necessary components of optimal functioning in the human body. The filtration process is called distillation and is usually done by boiling the water until it becomes steam which is then condensed into water free of impurities. This process requires a lot of energy. Water that has been distilled is considered to be pure, mistakenly leading people to think of it as good drinking water.
How it works:
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The boiling point of water is 100°C or 212°F, while other substances (impurities) have lower or higher boiling points. Many of these impurities will boil off quickly, evaporating before the water. Once the water has evaporated, remaining substances of higher boiling points will follow. Theoretically, because of the varying boiling points, water is separated from impurities. This does mean, however, that certain organic substances of similar boiling points can be found in distilled water if not filtered out afterwards.
A handy tutorial for at home distillation can be found here.
Deionised water is any water (tap, spring, distilled, etc.) that is run through electrically charged resin. An ion exchange bed eliminates virtually all impurities, yielding demineralised, perfectly clean water. This type of water is a bit special; used in microelectronics (to dissolve drugs in medicine), manufacturing plants, beauty care products, washing liquids, acid batteries and laboratories.
How it Works:
To remove the total dissolved substances (TDS) from water by deionisation, an ion exchange takes place. Water passing through an ion exchange bed with both positively (cation) and negatively (anion) charged ions allows for selective replacement. When non-water ions, TDI’s, are attracted to the positively or negatively charged resin magnet, a water ion is released. That’s put simply.
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So, let’s get into it. The two types of resins used to attract TDI’s are composed differently. Positively-charged cation resin “is typically made from styrene containing negatively charged sulfonic acid groups,” drawing out calcium, magnesium, and sodium along with other impurities. Comparatively, negatively-charged anion resin is made of styrene containing “positively charged quaternary ammonium groups,” drawing out bicarbonate, chlorine, and sulphate along with other impurities. In order for the exchange of ions to be successful, the magnetic resins are loaded with hydrogen ions. Once the undesirable TDI attaches to the magnetic resin, a hydrogen ion is released to replace it. Once filtered, several following cycles are recommended to catch any TDI’s that have slipped through.
To get deionised water at home, a deionising water filter containing negatively and positively charged resins is all you’ll need.
As already stated, distilled water is not a good choice for drinking water and neither should deionised water be imbibed. Since it is corrosive, it can damage tooth enamel and soft tissues, and is not treated to remove disease-causing pathogens. The special nature of deionised water and its specific uses make it abundantly clear that it is not intended for drinking.