How the Reverse Osmosis Process Works to Remove Salt
Reverse osmosis works by using high pressure to force water through a semi-permeable membrane. This membrane has extremely small pores, so small that they allow water molecules ($H_2O$) to pass through but block larger dissolved molecules and ions, such as those of sodium chloride (table salt). The raw, untreated water is pushed against one side of the membrane, leaving the purified water on the other side. The concentrated stream of impurities, known as brine or wastewater, is flushed away.
The Science of Salt Rejection
The efficiency of salt rejection by an RO membrane is a core aspect of its design. RO membranes are highly effective because they reject contaminants based on size and charge. Since sodium ($Na^+$) and chloride ($Cl^-$) ions are larger than water molecules and carry a charge, they are efficiently blocked. While a top-performing system can achieve 99% salt removal, a trace amount might still pass through, though the resulting level is typically negligible.
The Role of Total Dissolved Solids (TDS)
The salt content in water is measured as part of its Total Dissolved Solids (TDS) level. TDS includes inorganic salts like calcium, magnesium, and sodium, as well as organic matter and other substances. RO systems significantly reduce the TDS level of water, often bringing it down to a very low range. A typical RO system can lower TDS by 90% or more, depending on the source water quality and the system's efficiency. This reduction is what makes the water taste 'flatter' to some people, as the minerals that contribute to flavor have been removed.
Reintroducing Minerals: Why Some RO Systems Remineralize
One of the most common critiques of RO water is its potential to lack beneficial minerals. The purification process doesn't discriminate, removing both harmful and benign dissolved substances. For this reason, many modern RO systems include a remineralization stage. These post-filters add back a balanced amount of minerals, such as calcium and magnesium, for several key reasons:
- Improved Taste: The addition of minerals enhances the flavor profile of the water, preventing the flat or bland taste that can result from very low TDS.
- Health Benefits: While a balanced diet is the primary source of minerals, some people prefer to have a moderate level of beneficial minerals like calcium and magnesium in their water to support overall health.
- Corrosion Control: Water with extremely low TDS and a slightly acidic pH can be mildly corrosive to plumbing over time. Adding minerals helps to stabilize the water's pH, making it less corrosive.
RO Water vs. Other Water Sources: A Comparison
| Feature | RO Water (without remineralization) | Tap Water (Municipal) | Bottled Water (Mineral) |
|---|---|---|---|
| Salt/Sodium Content | Very low to negligible (95-99% removed). | Varies widely, depending on source and treatment. | Varies by brand; can be high or low. |
| TDS Level | Typically low (10-50 ppm), though some remineralized systems aim higher. | Can vary from 200-500 ppm or more. | Varies significantly, with some brands naturally having high TDS. |
| Beneficial Minerals | Very low, as most are removed. | Varies widely; mineral content is inconsistent. | Often contains naturally occurring minerals for flavor and health. |
| Contaminant Removal | Highly effective against a wide range of impurities, including heavy metals, pesticides, and bacteria. | Effective, but contaminants can still be present depending on local standards and pipe quality. | High-quality bottled water is filtered, but standard can vary. |
| Taste | Often described as flat or neutral due to low mineral content. | Flavor can be influenced by minerals, chlorine, and other chemicals. | Varies by brand, often with a distinct mineral taste. |
Can an RO System Be Too Effective?
It's possible for an RO system to produce water with a TDS level so low that it is virtually mineral-free. This ultra-pure water is safe for hydration, but some people find its flat taste unappealing. Additionally, long-term consumption of water with extremely low mineral content may raise concerns about the lack of dietary minerals, though the minerals from water make up a small portion of the overall intake for most individuals with a balanced diet. For those seeking the optimal balance of purity and beneficial mineral content, a system with a post-filtration remineralization stage is the ideal solution.
Conclusion: The Final Word on Salt in RO Water
Reverse osmosis systems are exceptionally effective at removing dissolved solids, including the sodium and chloride ions that constitute salt. The technology was, in fact, originally developed to desalinate seawater. A well-maintained RO system will produce water with a nearly negligible salt content, far below what is typically found in municipal tap water. For those on a sodium-restricted diet or simply seeking the purest water possible, an RO system is a highly reliable choice. However, because the process removes almost all dissolved solids, it can impact taste and remove beneficial minerals. The solution is often a post-filtration stage that strategically reintroduces essential minerals, ensuring the best of both worlds: ultra-pure water with a balanced, pleasant taste.
For more information on the process, you can explore the technical overview provided by Puretec Industrial Water.
