Demineralized water, often called demin water, is water treated to remove dissolved ionic mineral content. This process is necessary because the presence of these charged particles can interfere with sensitive industrial, laboratory, or mechanical operations by causing scale buildup or corrosion. The amount of mineral content is measured by Total Dissolved Solids (TDS), which is the collective weight of all dissolved substances. A lower TDS level indicates higher purity and less mineral interference for a specific application.
Demineralization Through Distillation
Distillation involves heating water until it vaporizes into steam, leaving all non-volatile solids and dissolved minerals behind in the boiling vessel. The steam is then collected and cooled, returning it to a liquid state as demineralized water.
This method is highly effective at removing mineral ions, most bacteria, and organic materials because they do not vaporize at water’s boiling point. However, the process is notably energy-intensive, requiring significant heat to convert the entire volume of water into steam. Distillation also may not remove certain volatile organic compounds or dissolved gases, which can vaporize along with the water.
The Reverse Osmosis Process
Reverse Osmosis (RO) is a popular demineralization technique that uses high pressure to force water molecules through a semi-permeable membrane. This pressure overcomes the natural osmotic flow, pushing the water through the membrane while rejecting larger dissolved solids and mineral ions.
The membrane blocks most dissolved salts, heavy metals, and many organic contaminants, resulting in a rejection rate typically ranging from 95% to 99%. Most RO systems require pre-filtration to remove sediment and chlorine, protecting the membrane from clogging or degradation. Although highly effective, RO water usually contains residual ions, meaning it does not reach ultra-purity levels unless combined with further treatment steps.
Ion Exchange and Deionization
Deionization (DI) is a chemical process that uses specialized synthetic resin beds to remove nearly all remaining charged ions from water. This involves exchanging undesirable mineral ions for hydrogen and hydroxyl ions. The resin beads are designed as tiny polymers with charged sites that attract and hold the contaminant ions.
Water first passes through a cation exchange resin, which swaps positively charged ions like calcium and sodium for hydrogen ions. Next, the water moves through an anion exchange resin, which replaces negatively charged ions like chloride and sulfate with hydroxyl ions. The released hydrogen and hydroxyl ions then combine to form pure water, resulting in extremely low Total Dissolved Solids.
Using a mixed-bed system, where both resin types are in a single vessel, can produce ultra-pure water. This process excels at removing ions, but it does not effectively remove non-ionic contaminants such as bacteria or non-charged organic molecules.
Common Uses and Purity Comparison
Demineralized water is necessary for applications where mineral buildup could cause damage or interfere with chemical processes. Common industrial uses include preventing scale in high-pressure boilers and cooling systems, serving as an ingredient in cosmetics and pharmaceuticals, and providing a clean rinsing agent in electronics manufacturing. It is also widely used to top off car batteries and in humidifiers to prevent the release of white mineral dust into the air.
Deionization typically yields the highest purity water, but requires the costly and frequent regeneration or replacement of resin beds. Distillation also produces very high-purity water, including the removal of some non-ionic contaminants, but it is slow and has the highest energy consumption. Reverse Osmosis provides moderate to high purity at a lower ongoing maintenance cost and is faster than distillation, making it the most common method for general commercial and residential use.