RO is a highly effective water purification method that pushes water through a semi-permeable membrane to remove dissolved solids and contaminants. Water naturally contains dissolved substances, including calcium and magnesium, which are the components of “hard water.” This article addresses how reverse osmosis technology interacts with and removes calcium from drinking water.
Calcium Removal Effectiveness
Reverse osmosis is highly effective at removing calcium, one of the primary ions responsible for water hardness. Typical RO systems reject between 95% and 99% of calcium and magnesium ions from the feed water. This high rejection rate results from the physical and electrical properties of the calcium ion compared to water molecules.
Calcium exists in water as a charged ion, surrounded by a shell of water molecules called a hydration shell. This combined structure is significantly larger than a single water molecule, allowing it to be easily rejected by the membrane’s microscopic pores. The electrical charge of the divalent calcium ion further enhances its rejection due to electrostatic interactions with the RO membrane’s surface.
The practical result of this high removal rate is the elimination of hard water issues. Consumers notice a substantial reduction or complete absence of the white, chalky scale buildup on faucets and appliances. This also prevents the formation of soap scum, which occurs when calcium and magnesium ions react with soap.
The Mechanics of Reverse Osmosis Filtration
Reverse osmosis fundamentally works by overcoming the natural process of osmosis. Osmosis is the movement of water across a semi-permeable membrane from low solute concentration to high solute concentration. The RO process reverses this flow by applying external pressure to the water that is greater than the natural osmotic pressure.
This applied force pushes the water against the semi-permeable membrane, which has an extremely fine pore size, often around 0.0001 microns. Only small water molecules are forced through the microscopic openings to the clean side of the system. All larger dissolved solids, known as Total Dissolved Solids (TDS), are left behind and flushed away in a concentrated stream.
The physical exclusion of particles by the membrane is the main principle allowing for the removal of calcium and other contaminants. Particles with a molecular weight greater than 200 are rejected at levels greater than 99%. Since the calcium ion, surrounded by its hydration shell, is much larger than the water molecule, it cannot physically pass through the membrane structure. The process separates the water into a purified stream and a waste stream containing the concentrated impurities.
Managing Mineral Content After Filtration
The high effectiveness of reverse osmosis at removing calcium and other minerals means the resulting water is demineralized. This purified water has a very low Total Dissolved Solids level, sometimes referred to as “hungry water” due to its tendency to leach minerals. The absence of minerals like calcium and magnesium can affect the water’s taste, often described as “flat.”
Experts and health organizations have raised concerns about the long-term consumption of water stripped of these minerals. Studies suggest that minerals in water are more readily absorbed by the body than those from food. A lack of these minerals in drinking water may have implications for bone strength and cardiovascular health.
For these reasons, many RO systems now incorporate a final stage known as a remineralization filter. These filters reintroduce a balanced level of beneficial minerals, specifically calcium and magnesium, back into the purified water. This post-filtration step serves two purposes: it improves the taste and alkalinity of the water, and it restores a moderate mineral content. This allows consumers to benefit from the high purity of RO water while ensuring a balanced intake of desirable minerals.