Which filtration process is BEST suited for removing viruses from water?

Nanofiltration is the filtration process best suited for removing viruses from water. To understand why, it's important to define several key terms. Filtration, in general, is a separation process where solids are strained from a liquid using a filter medium. The size of the pores (tiny holes) in this medium determines what can pass through and what is retained. Viruses are exceptionally small, typically ranging from 20 to 300 nanometers (nm) in diameter. A nanometer is one billionth of a meter – incredibly tiny. Traditional sand filtration, commonly used in municipal water treatment, has pore sizes much larger than viruses (typically 20-60 micrometers, or 20,000 to 60,000 nm), so it is ineffective at virus removal. Similarly, microfiltration (pore sizes around 0.1 to 10 micrometers) and ultrafiltration (pore sizes around 0.01 to 0.1 micrometers) are also too large to reliably remove viruses. Reverse osmosis, while highly effective at removing many contaminants, is primarily used for desalination (removing salts) and requires very high pressure, making it less practical and more energy-intensive for routine virus removal. Nanofiltration bridges the gap. Nanofiltration membranes have pore sizes typically ranging from 1 to 100 nanometers. This pore size range is specifically designed to capture viruses while allowing smaller molecules like minerals and some dissolved organic matter to pass through. The mechanism of virus removal in nanofiltration isn't solely based on pore size exclusion; it also involves adsorption, where viruses stick to the membrane surface. This enhances removal efficiency. For example, nanofiltration can effectively remove norovirus, a common cause of gastroenteritis, from contaminated water sources. While other methods like UV disinfection (using ultraviolet light to inactivate viruses) can also reduce viral load, they don't physically remove the virus particles, meaning they can still be present in the water, although unable to replicate. Nanofiltration provides both inactivation (to a lesser extent) and physical removal, offering a more robust solution for ensuring viral safety in water.