Reverse osmosis (RO) is a highly effective water purification method that forces water through a semi-permeable membrane to remove dissolved solids and impurities. A common question for consumers is how this filtration process affects the water’s natural mineral content, particularly the nutritionally beneficial mineral magnesium.
The Mechanism of Reverse Osmosis Filtration
Reverse osmosis works by applying pressure to feed water, forcing it through a synthetic semi-permeable membrane against the natural osmotic gradient. This pressure must exceed the water’s osmotic pressure, allowing pure water molecules to pass through while leaving dissolved contaminants behind. The membrane has an extremely small pore size, often around 0.0001 microns.
Contaminant rejection is based on the size and electrical charge of dissolved particles, known as ions. Water molecules are small and uncharged, allowing them to diffuse easily. Dissolved inorganic contaminants, such as mineral salts, exist as charged ions and are physically blocked or repelled by the membrane barrier.
The membrane material often carries a negative charge, creating electrostatic repulsion against charged ions. This mechanism efficiently removes a wide spectrum of impurities. The purified water is called the permeate, and the concentrated stream of rejected contaminants is known as the concentrate.
Magnesium Removal Efficiency
Reverse osmosis is exceptionally effective at removing magnesium from water, consistently achieving high rejection rates. Magnesium exists as a divalent ion (\(\text{Mg}^{2+}\)), meaning it carries a double positive charge. This double charge and the ion’s hydrated size make it highly susceptible to the RO membrane’s rejection mechanisms.
Typical RO systems remove magnesium at a rate between 96% and 98%. This high removal rate results from the stronger electrostatic repulsion exerted on divalent ions compared to monovalent ions. The overall removal of total dissolved solids (TDS), including magnesium and calcium, is usually within the 95% to 99% range.
Efficiency can fluctuate based on operating conditions, such as membrane quality, water temperature, and applied pressure. Higher feed water pressure generally improves the water flow rate while maintaining a high rejection percentage. Even under varying conditions, magnesium removal remains substantial, making RO water virtually free of this mineral.
Practical Implications of Magnesium Removal
The removal of magnesium by reverse osmosis has several practical consequences. One immediate effect is on the taste of the water, as minerals contribute to the flavor profile. Water lacking these dissolved solids can often be described as having a flat or somewhat acidic taste.
Magnesium is an essential nutrient, and water can contribute a meaningful portion of a person’s daily intake. Since RO water virtually lacks this mineral, consumers must rely entirely on food and supplements for their dietary requirements. Health authorities often recommend adding minerals back into purified water.
To address both taste and nutritional concerns, consumers often employ a post-treatment process called remineralization. This involves installing a separate filter cartridge after the RO membrane containing beneficial minerals, such as calcium and magnesium. Remineralization improves the flavor and helps restore a more neutral pH balance.