When considering the quality of water for home use, a common question arises about its mineral content: is reverse osmosis (RO) water hard or soft? The direct answer is that water purified by an RO system is definitively soft, falling into the category of ultra-soft water. Understanding this classification is important because the concentration of dissolved minerals impacts the taste of your drinking water and the longevity of your household plumbing and appliances. The RO process is an effective method for drastically reducing the mineral content.
Understanding Water Hardness and Softness
Water hardness is determined by the concentration of dissolved divalent cations, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions. These naturally occurring minerals dissolve into water as it travels through soil and rock, such as limestone and gypsum, causing the water to be classified as hard.
To quantify this, water professionals use standard classification scales, most commonly parts per million (PPM) or grains per gallon (GPG). Water is considered soft when its mineral concentration is below 60 PPM or 3.5 GPG. Levels between 61 and 120 PPM are classified as moderately hard, and anything above 180 PPM is considered very hard.
The distinction between hard and soft water has noticeable effects in daily life. Hard water reduces the effectiveness of soaps and detergents and leaves behind mineral deposits known as scale.
The Mechanics of Reverse Osmosis Filtration
Reverse osmosis is a purification technology that actively strips water of dissolved contaminants, including hardness minerals. The process relies on applying external pressure to force water through a semi-permeable membrane, reversing the natural osmotic flow. This pushes water from a concentrated solution to a less concentrated one.
The membrane is the defining component of an RO system, featuring extremely small pores, often around 0.0001 microns. This tiny pore size acts as a molecular filter, allowing water molecules (\(\text{H}_2\text{O}\)) to pass through while rejecting most larger molecules and ions, including the divalent calcium and magnesium ions.
The effectiveness of this process is measured by the reduction in Total Dissolved Solids (TDS), which represents the total concentration of dissolved inorganic and organic substances. An RO system is engineered to reject between 90% and 99% of these dissolved solids. The RO membrane physically blocks the vast majority of dissolved inorganic solids, removing the hardness minerals.
Classification of RO Water as Ultra-Soft
Because the reverse osmosis process efficiently removes the vast majority of hardness-causing minerals, the resulting water is classified as ultra-soft. The system strips out the \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions, ensuring the post-treatment water consistently falls far below the 60 PPM threshold that defines soft water.
The resulting Total Dissolved Solids (TDS) concentration in RO water typically ranges between 10 and 50 PPM. Some high-quality systems can produce water with a TDS level below 10 PPM. This near-total demineralization means that the RO product water registers at the bottom end of the soft water classification scale.
This low-TDS output confirms that reverse osmosis is a highly effective method for producing extremely soft water. The RO process ensures that the finished water product contains minimal traces of the substances that contribute to water hardness, making the water functionally demineralized.
Practical Implications of Using Water with Extremely Low Hardness
The use of ultra-soft RO water has several noticeable effects within a household. A primary benefit is the complete prevention of mineral scale buildup in water-using appliances, plumbing, and fixtures. This absence of calcium and magnesium deposits helps maintain appliance efficiency and extends the operational lifespan of water heaters, dishwashers, and coffee makers.
Another practical implication involves how RO water interacts with soap and detergents. Since the hardness minerals are not present to react with soap, soft water lathers effortlessly. This means you can use significantly less soap and detergent for bathing, laundry, and dishes, leaving skin and hair feeling cleaner because there is no mineral film or soap scum residue.
However, water with low mineral content is sometimes described as having a flat taste due to the lack of dissolved solids, which is why some systems include a remineralization stage. Furthermore, water that is virtually free of dissolved minerals can be slightly corrosive to metal pipes and fixtures, as it seeks to leach minerals from its surroundings. For specific applications like hydroponics or fish tanks, the extremely low mineral content often requires the addition of specific trace minerals to support biological systems.