Difference Between Deionized & Distilled Water

Pure water is a surprisingly good solvent, so it's not surprising that water is typically not pure. Water can contain myriad trace contaminants that we might want to remove for different reasons, either so we can use the water for a particular specialized process or industry, or just so that we feel better about drinking it. Distillation and deionization are two common means of purifying water, each with their own advantages and disadvantages.

Why Purify Water?

• Water from either a well or from a municipal supply will almost always contain some contaminants or additives. Common contaminants are dissolved minerals, typically calcium and magnesium. Water with a high amount of dissolved minerals is normally called "hard" water and can cause problems by clogging lines and filters and leaving residues on fittings. Iron in water can also cause staining. Some contaminants can make water unpleasant or unsafe to drink. Hydrogen sulfide in water can impart an unpleasant smell, as can too much of the additive chlorine, so some people like to remove these compounds from drinking water. Water also sometimes has to be purified to make it suitable for other uses, such as in a production process where contaminants are not acceptable, or for use in a commercial car wash where customers do not want a residue left on their cars after the water evaporates.

Distilled Water

• Distillation is a simple process in which water is heated to its boiling point and allowed to turn into water vapor, or steam. This vapor travels along a conduit to a different location where it cools and condenses back into liquid water. The result is that impurities are left behind in the original container and essentially only pure water is collected in the condensate.

Deionized Water

• Deionized water is produced by running water through a bed of a porous polymeric resin, which contains loosely bound hydrogen (H+) and hydroxide (OH-) ions. As water flows over the resin, dissolved charged chemical species in the water -- such as calcium and magnesium -- are bound to the resin, and hydrogen or hydroxide ions are released to take their place. These two ions will then combine to form water. The result is that many of the most common chemical contaminants are removed and replaced with water. The resin is regenerated occasionally to replace the lost hydrogen and hydroxide.

Comparison

• Distilled water is generally more pure, since the distillation process, theoretically, only takes pure water over into the condensate collecting vessel. Any minerals or metals will be left behind, as well as biological contaminants, such as bacteria. However, it is possible for traces of some low-boiling chemical contaminants, such as volatile organic compounds, to be carried over into the condensate. As well, the process requires specialized equipment and fairly high energy input to boil the water. Deionization gives less pure water since it removes only dissolved minerals and some charged metal compounds. It will rarely remove organic or biological contamination. Deionization does have the advantages of being quick, inexpensive and low maintenance.