Reverse Osmosis (RO) is a water purification technology that uses pressure filtration. The process forces source water through a specialized, semi-permeable membrane. This membrane rejects nearly all non-water molecules, allowing only purified water to pass through. Calcium (Ca) and Magnesium (Mg) are the primary mineral ions responsible for water “hardness.” RO effectively separates these dissolved solids from the water, resulting in highly purified output.
How Reverse Osmosis Targets Hardness Minerals
Reverse Osmosis membranes function by physically blocking dissolved solids larger than the water molecules themselves. The rejection process is highly dependent on both the size and the electrical charge of the contaminant ion. Calcium and magnesium ions are characterized as divalent cations, meaning they carry a charge of positive two (Ca²⁺ and Mg²⁺).
This higher electrical charge is a significant factor in the filtration process. Divalent ions are more effectively repelled by the membrane surface compared to monovalent ions, such as sodium or chloride. This enhanced repulsion, combined with their slightly larger hydrated size, ensures a very high rate of rejection. Standard RO membranes are engineered to remove calcium and magnesium at rates typically falling between 93% and 98%.
The electrical interaction with the charged ions makes the removal of hardness minerals efficient. While some monovalent ions might occasionally pass through, the strong dielectric interaction ensures that the majority of divalent ions are retained. This high rejection rate confirms that RO is extremely effective at removing the components of water hardness.
Implications of Removing Calcium and Magnesium
The removal of calcium and magnesium effectively eliminates the common problems associated with hard water. When these minerals are present in high concentrations, they react poorly with heat and soap. The removal of these divalent ions transforms the water into soft water.
One immediate benefit is the prevention of scale formation. When hard water is heated, calcium carbonate precipitates out to form a hard, crusty deposit known as scale in appliances like water heaters and coffee makers. This mineral buildup reduces appliance efficiency, restricts water flow in pipes, and can lead to costly maintenance and early equipment failure.
Another major implication is the enhanced performance of cleaning agents. Calcium and magnesium ions react with soap to form an insoluble residue, commonly known as soap scum. RO water allows soaps and detergents to lather fully and clean more effectively. This reduction in mineral interference means less product is needed for washing dishes, clothes, and bodies, saving money and eliminating residue.
Understanding the Resulting Water Quality
The removal of nearly all dissolved ions results in specific chemical characteristics. The most notable change is a dramatic reduction in the Total Dissolved Solids (TDS) count, which measures the concentration of all dissolved substances in the water. An RO system typically reduces the TDS level by 90% to 99%, yielding water with very few remaining impurities.
This purified water tends to have a slightly acidic pH, typically ranging between 5 and 7. This slight acidity is a natural consequence of mineral removal. Calcium and magnesium act as natural buffering agents in source water, and without them, the water can no longer resist changes in pH. Carbon dioxide from the air can dissolve into the mineral-free water, forming a weak carbonic acid that lowers the pH level.
Post-Filtration Remineralization
For users concerned about low mineral content or acidic pH, post-filtration options exist. Many modern RO systems include a dedicated remineralization filter as a final stage. This stage introduces small, controlled amounts of beneficial minerals, such as calcium and magnesium, back into the purified water. This process improves the final taste of the water and returns the pH to a neutral or slightly alkaline level.