Reverse osmosis (RO) is a widely recognized and effective method for purifying water, used in various settings, from homes to industrial applications. It excels at reducing a broad spectrum of contaminants. While RO systems offer comprehensive purification, they have specific limitations on the substances they can remove. Understanding these limitations is important for optimal water quality.
How Reverse Osmosis Filters Water
Reverse osmosis operates using a semi-permeable membrane and applied pressure, forcing water under pressure through this specialized membrane, which acts as a selective barrier. The membrane’s microscopic pores are designed to allow water molecules to pass through while effectively blocking larger dissolved solids, impurities, and other contaminants. This process reverses the natural osmotic flow, leaving behind unwanted substances. Filtration effectiveness is determined by the membrane’s pore size. Adequate water pressure is necessary to drive water through the membrane for efficient contaminant rejection.
Substances RO Systems May Not Remove
Reverse osmosis systems do not remove every type of contaminant. Volatile Organic Compounds (VOCs), which are chemicals that easily evaporate at room temperature, can sometimes pass through the RO membrane itself. These include substances found in paints, cleaning supplies, and fuels. While the RO membrane alone may not effectively remove VOCs, RO systems typically integrate activated carbon filters to address these compounds.
Dissolved gases, such as hydrogen sulfide, radon, carbon dioxide, oxygen, and nitrogen, are also not consistently removed by RO membranes. These gases can contribute to taste and odor issues in water. Additionally, some very small, uncharged molecules, including certain pharmaceutical residues or specific pesticides, may not be fully rejected. While RO is generally effective against many pesticides, the removal efficiency can vary depending on the specific chemical’s properties.
Although RO is highly effective at removing most bacteria and viruses, a properly functioning membrane is essential. A compromised or damaged membrane could allow microorganisms to pass through. While RO systems are designed to remove a high percentage of dissolved solids and ions, extremely small or highly soluble ions, particularly monovalent ones like sodium, may have a very low pass-through rate.
Reasons for Incomplete Contaminant Removal
The inability of reverse osmosis to remove certain substances stems from principles governing membrane filtration. Molecular size and shape play a significant role; extremely small molecules, especially those with molecular weights below approximately 150 to 250 Daltons, can sometimes pass through the membrane’s pores. The membrane’s effective pore size is critical, and molecules smaller than this threshold may not be adequately rejected.
Electrical charge also influences rejection. RO membranes are highly effective at repelling charged ions, which is why they excel at removing dissolved salts and heavy metals. However, uncharged or neutral molecules are not subject to the same electrostatic repulsion and can therefore pass through the membrane more easily. This explains why some small organic compounds or dissolved gases, which are often uncharged, are less effectively removed.
Volatility is another factor, particularly for compounds like VOCs. These compounds can vaporize and pass through the membrane as a gas rather than being filtered as a liquid solute. While RO systems are engineered for high efficiency, no membrane is absolutely perfect. Factors such as membrane age, incoming water quality, operating pressure, and water temperature can influence overall performance and rejection rates.
Enhancing Water Quality Beyond RO
To address contaminants that reverse osmosis might not fully remove, complementary filtration methods are often integrated into water treatment systems. Activated carbon filters are highly effective at adsorbing volatile organic compounds (VOCs), chlorine, and other chemicals that cause unpleasant tastes and odors. They are commonly used as pre-filters to protect the RO membrane and as post-filters to polish the water’s taste.
Ultraviolet (UV) sterilization provides an additional layer of protection against microorganisms. UV light neutralizes bacteria, viruses, and other pathogens by disrupting their DNA, preventing them from reproducing and causing illness. This method is particularly beneficial if there’s a concern about a compromised RO membrane or if the source water may contain biological contaminants.
Aeration, which involves exposing water to air, can be used to remove dissolved gases like hydrogen sulfide, which causes a rotten egg smell, and some volatile organic compounds. This process helps to strip these gases from the water. Regular maintenance and timely replacement of filters and the RO membrane are essential to ensure the system operates at its peak efficiency, preventing contaminants from bypassing the filtration stages.