The distinction is between a quality standard and a process. Purified water is a classification defining a level of quality, representing the final, measurable outcome of a treatment process. Reverse osmosis (RO) is a specific technology, a mechanical method used to achieve that defined state of purity. Understanding this difference clarifies the relationship between the final product and the manufacturing technique.
The Standard: What Defines Purified Water
“Purified water” is a quality standard specifying the maximum allowable level of contaminants remaining in the water. This classification is set by regulatory bodies, such as the U.S. Food and Drug Administration (FDA) for bottled water. It is defined by the final product’s chemical and physical characteristics. The most common metric for this standard is Total Dissolved Solids (TDS), which measures the concentration of inorganic salts, minerals, and metals.
To be classified as purified water, the TDS level must be extremely low, typically less than 10 parts per million (ppm). This low concentration indicates that almost all solids, chemicals, and particles have been removed from the source water. Achieving this purity requires the removal of contaminants, including pathogens, dissolved ions, and organic substances. The classification focuses entirely on the measurable result, not the method used to obtain it.
The Mechanism of Reverse Osmosis
Reverse osmosis is a highly effective water purification technology that operates by reversing a natural phenomenon. Osmosis naturally causes water molecules to move across a semi-permeable membrane from a low-solute area to a high-solute area. This movement equalizes the concentration of dissolved particles and creates osmotic pressure.
The RO process overcomes this natural pressure by applying external, mechanical pressure to the water on the side with the higher concentration of dissolved solids. This force pushes the water molecules in the reverse direction through a specialized, semi-permeable membrane. The membrane has pores small enough (often 0.0001 microns) to allow water molecules to pass while rejecting larger ions, salts, and most organic compounds.
A typical RO system involves multiple stages of filtration before the water reaches the membrane. Pre-filters, such as sediment and carbon filters, remove larger particles and chlorine to protect the membrane from fouling. The core RO membrane rejects 95% to 99% of dissolved salts, which are flushed away in a separate concentrate stream. This leaves the highly purified water, known as the permeate, on the other side.
Achieving Purity Through Other Methods
Reverse osmosis is only one of several methods used to achieve the high-purity standard of purified water. Distillation involves boiling water to create steam, which is then cooled and condensed back into a liquid state. Since most contaminants, including minerals and salts, have a much higher boiling point than water, they are left behind.
Deionization (DI) is another process that achieves high purity by targeting charged mineral salts, or ions. This method passes water through beds of synthetic resin beads that exchange hydrogen and hydroxyl ions for the contaminant ions. The hydrogen and hydroxyl ions then combine to form pure water. While DI excels at removing ionic impurities, it does not effectively remove non-ionic substances like bacteria or organic molecules, often necessitating pre-filtration.
Reverse Osmosis Within the Purification Landscape
Reverse osmosis is one of the most common methods globally for producing water that meets the purified water standard. Its ability to simultaneously remove dissolved salts, organic matter, and most biological contaminants in a single, continuous process makes it highly efficient. The RO process is often integrated with other stages, such as carbon filtration or deionization, to achieve ultra-pure results required for industrial or laboratory applications.
RO is a technology, while purified water is the resulting quality designation. Water treated by reverse osmosis is generally considered purified water because the process reduces Total Dissolved Solids to the required low levels. Water can also be deemed purified if produced using distillation or deionization, provided the final product meets stringent quality metrics. Therefore, all reverse osmosis water is a form of purified water, but not all purified water is necessarily produced by reverse osmosis.