Deionised water, or DI water, is sometimes called demineralised water, or pure water and is used within many different industries.
Deionised water starts as ordinary tap water that gets filtered through a purification system to remove all the natural minerals. The reason for purifying water within Window Cleaning is that pure water dries to leave a completely crystal clear finish.
If you've ever thought 'Why do Window Cleaners not dry the windows?' Here's why; When applied to Window Cleaning, using pure water means that there is no need to remove excess water from the window, you can leave it wet and it will dry completely clear! This makes modern Window Cleaning generally a safer trade, as much of the work can be safely from the ground, without needing ladders.
How you actually purify water within Window Cleaning will depend on a few factors and is covered more in our Introduction to Pure Water Window Cleaning, but there is generally two methods to do so. One is preferred in hard water areas, called RO/DI filtration - this is a slower and more complicated process but it generally has the lowest running costs.
The second option is known as DI Only, which is preferred for soft water areas. This method is simple and quick, but it can have higher running costs. Both methods are covered in more detail in the article Types of filtration, RO/DI or DI Only