Water distillation is a centuries-old purification technique that mimics the natural hydrological cycle to separate pure water from its contaminants. The process fundamentally relies on a physical change of state, transforming liquid water into a gas and then back into a liquid again. This simple phase transition is highly effective because it exploits the vast differences in the boiling points of water and the substances dissolved or suspended within it.
The Three Stages of Thermal Separation
The core mechanism of water purification by distillation is a process called thermal separation, which is broken down into three distinct phases.
The first phase, vaporization, begins when the source water is heated to its boiling point, which is 100°C (212°F) at standard atmospheric pressure. As the water transitions into steam, its molecules gain enough kinetic energy to break free from the liquid state.
This vaporization process leaves impurities behind in the boiling chamber. Substances such as dissolved minerals, heavy metals, salts, and non-volatile organic compounds possess significantly higher boiling points than water. Consequently, they cannot vaporize at 100°C and remain concentrated as a residue in the initial container.
The second stage is the transfer of the purified water vapor away from the source liquid and its concentrated contaminants. The rising steam contains only water molecules, effectively excluding the non-volatile matter left below.
The third stage is condensation and collection, where the pure steam is channeled into a separate area containing a cooling surface called a condenser. Inside the condenser, the steam rapidly loses thermal energy and reverts to its liquid state. This newly formed liquid, known as the distillate, is collected in a separate receiver, resulting in highly purified water.
What Distillation Removes and Retains
Distillation is exceptionally effective at removing a wide range of inorganic contaminants and biological threats from water. The process efficiently eliminates dissolved solids, including sodium chloride, nitrates, fluoride, and hardness minerals like calcium and magnesium. Furthermore, the act of boiling the water for a sustained period inactivates and kills microorganisms such as bacteria, viruses, and protozoan cysts.
This purification method also effectively removes heavy metals like lead and iron, as well as radioactive isotopes and other particulates. These substances, being non-volatile, remain trapped in the boiling chamber as a concentrated sludge or scale. The resulting distillate typically achieves a purity level approaching 99.5 percent of the original impurities removed.
A nuance of the process involves contaminants that distillation is less effective against, specifically Volatile Organic Compounds (VOCs). These compounds, which include substances like benzene and toluene, have boiling points that are lower than or very close to water’s boiling point. Therefore, they may vaporize along with the water and travel through the system, potentially re-contaminating the purified distillate.
Common Methods and Equipment for Distilling Water
Water distillation is performed using a device known as a still, which can range from small countertop appliances to massive industrial units. A small-scale home still typically consists of three main parts: a boiling chamber with a heating element, a condensing coil, and a collection container for the final product. These units are designed for batch operation, distilling a limited amount of water at a time.
To mitigate the issue of VOCs carrying over, many commercial and home distillation units incorporate a volatile gas vent or a post-treatment activated carbon filter. The vent allows the initial, most volatile vapors to escape the system, while the carbon filter absorbs any remaining VOCs that may have condensed with the water. The boiling chamber requires periodic cleaning to remove the accumulated scale of concentrated non-volatile solids left behind.
Industrial applications, such as those in the pharmaceutical or cosmetic industries, utilize much larger and more complex systems for continuous operation. These can include Multiple Effect Distillation (MED) or Vapor Compression Distillation (VCD) units. These large-scale systems are engineered for efficiency and high capacity, often integrating pre-treatment steps like reverse osmosis to protect the equipment and improve the purity of the final product. The principle remains the same, relying on the fundamental thermal separation of water from all other substances.