Reverse osmosis (RO) water is not the same as deionized (DI) water, as they rely on fundamentally different purification processes. RO uses physical separation, while DI relies on a chemical ion exchange mechanism. Though both methods produce highly purified water, they achieve distinct final purity profiles. RO water is not considered deionized because it retains a small but measurable amount of charged mineral ions, which deionization is specifically designed to remove. These two methods are often used together in sequence to create the highest quality water.
The Fundamentals of Reverse Osmosis
Reverse osmosis is a mechanical process that forces water molecules through an extremely fine, semi-permeable membrane under high pressure. This pressure overcomes natural osmotic pressure, which is the movement of water across a membrane from low to high solute concentration. Applying external pressure reverses this flow, pushing the water against its natural gradient. The membrane acts as a physical barrier, allowing water molecules to pass while rejecting most dissolved solids and contaminants.
The pores in an RO membrane are exceptionally small, typically measuring between 0.0001 and 0.001 microns. This microscopic size allows the membrane to physically separate contaminants based on their molecular size and weight. An RO system is highly effective at removing larger particles, bacteria, pyrogens, colloids, and organic molecules, often achieving a removal rate of 95% to 99% of dissolved salts.
While effective, the physical nature of the separation means that certain very small, uncharged molecules or gases can pass through the membrane. The efficiency of rejection is influenced by the ionic charge of the contaminant. As a result, the purified water, known as the permeate, still contains trace amounts of charged ions and dissolved gases, meaning it is not completely deionized.
The Principles of Deionization
Deionization, in contrast to the physical process of RO, is a chemical purification method that specifically removes charged mineral ions from water. The process uses specialized synthetic ion exchange resins, which are tiny plastic beads with fixed electrical charges. Water passes through two types of these resins, which are either contained in separate vessels or mixed together in a single bed.
The first type, cation exchange resin, is charged with hydrogen ions (\(\text{H}^{+}\)), which it chemically trades for positively charged contaminants (cations) in the water. The second type, anion exchange resin, is charged with hydroxyl ions (\(\text{OH}^{-}\)), which it exchanges for negatively charged contaminants (anions). As the contaminants are captured by the resins, the released \(\text{H}^{+}\) and \(\text{OH}^{-}\) ions immediately combine to form a new, pure water molecule (\(\text{H}_{2}\text{O}\)).
Deionization is effective at achieving near-perfect removal of charged species, resulting in water with extremely low ionic content. However, the process is chemically selective only for ions, meaning it does not effectively remove uncharged contaminants. This includes microorganisms like bacteria and viruses, as well as non-ionic organic compounds, which must be removed by other methods.
Comparing Final Water Characteristics
The most direct way to distinguish between RO and DI water is by measuring their final water quality using different metrics. RO water quality is assessed by its Total Dissolved Solids (TDS) content, which measures the concentration of all dissolved inorganic and organic substances. Standard RO water achieves a conductivity level between 5 and 50 microSiemens per centimeter (\(\mu\text{S}/\text{cm}\)), reflecting the trace amount of remaining ions.
Deionized water quality is measured by its electrical resistivity or its inverse, conductivity, which is a highly sensitive measure of ionic purity. Because the DI process targets and removes charged ions, it produces pure water with a conductivity often below 1 \(\mu\text{S}/\text{cm}\). High-grade mixed-bed systems can reach as low as 0.055 \(\mu\text{S}/\text{cm}\). This purity means that DI water is essentially non-conductive, while RO water still conducts a small amount of electricity due to its residual ion load.
Why RO and DI Are Used Together
Many industrial and laboratory applications require a quality of water that neither RO nor DI can efficiently achieve alone, leading to the two technologies being used in tandem. Reverse osmosis is employed as the first stage, acting as a pre-treatment for the deionization system. The RO unit performs the bulk removal of contaminants, stripping out 95-99% of dissolved solids, particulates, and organic matter.
This pre-purification is financially and operationally beneficial because the DI resins are sensitive to fouling and are rapidly depleted by a heavy contaminant load. Using RO first significantly extends the lifespan of the ion exchange resins, often by a factor of 10 to 20 times, reducing replacement costs. The subsequent deionization stage acts as a final polishing step, removing the trace ions that the RO membrane missed. This combined process yields water of the highest purity, suitable for demanding industrial and laboratory applications.