Water rarely exists in a perfectly pure state, often containing various dissolved and suspended substances. These impurities can affect water’s taste, odor, and suitability for diverse applications, from drinking to industrial processes and scientific research. Water purification aims to remove these contaminants, enhancing water quality for specific uses.
How Reverse Osmosis Works
Reverse osmosis (RO) is a water purification technology that uses a semi-permeable membrane to remove dissolved solids and other impurities. External pressure is applied to a concentrated water solution, forcing water molecules through the membrane. The membrane’s microscopic pores allow water molecules to pass, but are too small for most larger dissolved molecules, ions, and suspended particles.
The membrane effectively separates purified water from contaminants. RO systems efficiently remove salts, minerals, bacteria, viruses, and many organic compounds. While RO significantly reduces the concentration of dissolved solids, it may not completely remove all dissolved gases or very small uncharged molecules. Rejected impurities are concentrated on one side of the membrane and flushed away as wastewater.
How Deionization Works
Deionization (DI) is a water purification process specifically designed to remove ionized impurities. This method utilizes ion-exchange resins: cation resins exchange positively charged ions (cations) for hydrogen ions (H+), and anion resins exchange negatively charged ions (anions) for hydroxyl ions (OH-).
As water flows through the resin beds, the released H+ and OH- ions combine to form pure water, effectively removing dissolved ionic contaminants. DI is highly effective at producing water with extremely low ion concentrations, often measured by its high electrical resistivity. However, deionization does not remove uncharged particles, such as bacteria, viruses, organic compounds, or dissolved gases.
Key Differences and Similarities
While both reverse osmosis and deionization processes purify water, they operate on distinct principles and target different types of contaminants. Reverse osmosis is primarily a physical separation process that uses pressure to push water through a semi-permeable membrane. It effectively removes a wide range of dissolved solids, including many ions, along with larger particles, bacteria, and viruses. RO typically removes approximately 90-99% of dissolved inorganic contaminants, resulting in significantly reduced total dissolved solids (TDS).
Deionization, in contrast, is a chemical process that specifically removes charged ionic impurities by exchanging them with hydrogen and hydroxyl ions using specialized resins. DI systems are capable of achieving much higher levels of ionic purity, often producing water with resistivity values approaching that of theoretically pure water. However, DI is ineffective at removing non-ionic substances, such as organic molecules, particulate matter, or microorganisms. Therefore, water that has undergone reverse osmosis is not truly deionized, as it still contains a small percentage of residual ions that RO membranes do not fully reject.
Choosing the Right Water Purification
Selecting the appropriate water purification method depends on the specific purity requirements. For many common uses, such as household drinking water, reverse osmosis provides sufficient purification by removing a significant percentage of dissolved solids and other contaminants, improving taste and safety. RO systems are also employed in various industrial settings where reduced mineral content is beneficial.
For applications demanding extremely high water purity, such as in scientific laboratories, pharmaceutical manufacturing, or electronics production, deionization is often used as a post-treatment step following reverse osmosis. This combination, known as RO/DI, leverages the strengths of both methods: RO removes the bulk of impurities, extending the life of the DI resins, and DI then polishes the water to remove nearly all remaining ions. Water purity is commonly assessed by measuring total dissolved solids (TDS) for RO water and electrical resistivity or conductivity for deionized water, with higher resistivity indicating fewer ions.