Reverse osmosis (RO) has become a popular method for purifying drinking water in homes and businesses, offering a high degree of contaminant removal. Consumers frequently use this technology to improve the taste and quality of their tap water, but a major concern remains regarding its ability to protect against biological threats. The central question for many is whether this physical filtration process can reliably eliminate waterborne pathogens, specifically bacteria and viruses. Understanding the mechanism of reverse osmosis and the microscopic scale of these contaminants is necessary to fully grasp its effectiveness as a barrier against disease-causing microorganisms.
How Reverse Osmosis Works
The reverse osmosis process is fundamentally a physical separation technique that forces water through an extremely fine membrane. This technology takes the natural process of osmosis, where water moves across a semipermeable barrier to equalize the concentration of dissolved substances, and reverses it. A high-pressure pump applies force to the contaminated side, overcoming the natural osmotic pressure and pushing the pure water through the barrier instead.
The core component of the system is the semipermeable membrane, often made of a thin-film composite (TFC) material. This membrane acts as a physical barrier, allowing only water molecules to pass through while rejecting dissolved solids and larger particles. Contaminants are blocked based on their size and, to a lesser extent, their electrical charge. The rejected material, concentrated in a waste stream, is then flushed away from the system.
The effectiveness relies on the incredibly tight structure of its membrane. Standard RO membranes typically have pore sizes in the range of 0.1 to 10 nanometers, or about 0.0001 microns. This minute pore size enables the system to physically exclude a wide range of dissolved salts, chemicals, and microorganisms. The process is highly efficient because it uses size exclusion to separate the water from the impurities.
Efficiency Against Bacteria and Viruses
The ability of a reverse osmosis system to remove biological contaminants is directly related to the size difference between the membrane pores and the pathogens. Bacteria are relatively large microorganisms, generally ranging from 0.2 to 10 micrometers (µm) in size. Since the RO membrane pore size is approximately 0.0001 µm, a correctly functioning membrane provides a complete physical block against all known waterborne bacteria, such as E. coli or Legionella. Studies consistently show that a well-maintained RO system achieves a rejection rate of over 99.9% for bacteria.
Viruses present a greater challenge because they are the smallest of all waterborne pathogens, measured in nanometers (nm). The typical size range for viruses is between 0.02 and 0.3 micrometers, or 20 to 300 nanometers. Despite their minuscule size, a standard RO membrane’s pore size is still orders of magnitude smaller. This ensures that reverse osmosis remains highly effective, with rejection rates for viruses also exceeding 99.9%.
The removal of these pathogens is a physical screening process, meaning the microorganisms are prevented from passing through the membrane. This physical exclusion principle is why reverse osmosis is rated by health authorities as one of the most efficient household water treatment technologies for pathogen reduction. The process provides a reliable barrier as long as the integrity of the filtration mechanism is maintained.
Addressing System Integrity and Supplemental Filtration
While the RO membrane is highly capable of rejecting biological threats, the performance of a real-world system depends on its physical integrity. The primary risk of contamination is a breach in the membrane or a failure in the system’s seals. A small pinhole tear in the membrane or a faulty O-ring gasket can create a bypass, allowing microorganisms to flow around the filtering surface and enter the purified water stream.
Bacteria can also lead to a condition known as biofouling, where a slimy layer of microorganisms grows on the surface of the membrane. This buildup primarily degrades the system’s performance by reducing water flow and introduces contamination risks if the system is not properly disinfected and maintained. Stagnant water within the system, such as in the storage tank or pipes, can further encourage the growth of microbes.
For guaranteed safety against biological threats, especially when using water from private wells or other non-municipal sources, supplemental treatment is often recommended. Ultraviolet (UV) light sterilization is a common post-treatment safeguard that uses light energy to scramble the DNA of any pathogens that might have bypassed the RO membrane. This inactivation step ensures that even if a system failure occurs, the water is biologically safe. Chemical disinfection, such as chlorination, can also be used as a pretreatment to kill microbes before they reach and foul the RO membrane.