Reverse osmosis (RO) is a highly effective water purification method that uses pressure to achieve exceptional purity. This process forces water molecules through a synthetic, semi-permeable membrane, which acts as a filter. By separating the water from nearly all dissolved impurities, the RO system delivers a level of cleanliness far beyond conventional filtration methods. Understanding the science behind this separation is key to evaluating RO water purity.
The Mechanism of Reverse Osmosis
The purity of reverse osmosis water begins with overcoming a natural phenomenon called osmosis. Normally, osmosis drives water from a less concentrated solution to a more concentrated solution across a membrane to equalize the solute levels. In a purification system, this means clean water would naturally move toward the contaminated side.
To reverse this natural flow, an external force, such as household water pressure or a pump, is applied to the contaminated water side. This pressure exceeds the natural osmotic pressure, pushing the water molecules through the membrane while leaving dissolved solids and larger particles behind. The semi-permeable membrane is engineered with microscopic pores, typically around 0.0001 micron, which permits only the small water molecules (H₂O) to pass through.
Quantifying Purity: Total Dissolved Solids
The standard way to measure the performance and purity of a reverse osmosis system is by assessing the Total Dissolved Solids (TDS). TDS represents the combined concentration of inorganic salts, metals, and organic matter dissolved in the water, and is typically measured in parts per million (ppm). Monitoring the TDS level offers a quantifiable metric for the purification system’s effectiveness.
Municipal tap water often has a TDS concentration ranging from 50 to 400 ppm, though the Environmental Protection Agency (EPA) sets a secondary guideline maximum of 500 ppm for aesthetic reasons. A high-functioning RO system routinely achieves a 90% to 99% rejection rate of these dissolved solids. This efficiency results in a highly purified water output, usually with a TDS reading between 5 and 50 ppm.
Contaminants Effectively Removed
Reverse osmosis is highly effective because its physical barrier mechanism blocks a broad spectrum of contaminants based on their size and ionic charge. The system primarily removes inorganic salts and heavy metals. The membrane successfully rejects ions of toxic metals like lead, arsenic, mercury, and hexavalent chromium.
The system also drastically reduces common contaminants such as sodium chloride (table salt), nitrates, and fluoride, which are often present in water supplies. While RO is not a dedicated microbial disinfectant, the membrane’s extremely small pore size effectively blocks larger microbiological impurities, including most bacteria and viruses. Furthermore, the multi-stage nature of an RO unit, which includes pre-filters, ensures that larger sediment like rust, dirt, and suspended solids are removed.
The Trade-Off: Mineral Stripping and Health
While reverse osmosis excels at removing harmful contaminants, its high efficiency also results in the removal of beneficial minerals like calcium and magnesium. These naturally occurring minerals are what give water its characteristic taste, and their absence can lead to water tasting “flat.” This level of demineralization has led to discussions regarding the consumption of water that is essentially devoid of minerals.
Some health organizations, including the World Health Organization (WHO), have noted that long-term consumption of highly demineralized water may be associated with negative health effects due to mineral depletion, especially if a person’s diet is already low in these elements. The highly pure water is also slightly more acidic and can seek to leach minerals from anything it contacts.
To address these concerns, the industry has developed post-filtration solutions such as remineralization filters. These filters are installed after the RO membrane to reintroduce a balanced blend of beneficial minerals, primarily calcium and magnesium, back into the purified water stream. This process restores a more natural taste, increases the water’s pH to a neutral or slightly alkaline level, and ensures the resulting product offers the benefits of high purity alongside a healthy mineral content.