How does reverse osmosis work?
Reverse osmosis Drinking Water, also known as “RO drinking water”, is a water treatment technique used since the 1950s to produce high-quality drinking water that is virtually free from health or aesthetic contaminants. It is commonly used to treat water supplies that have a high content of total dissolved solids (TDS), such as brackish, saline or sea waters.
Point-of-use RO systems work by forcing pressurized water molecules through a semi-permeable membrane, which acts like a filter, leaving behind larger suspended particles and most dissolved substances. Reverse Osmosis drinking water systems are effective in removing excess salt and other dissolved minerals, taste, and odor, heavy metals, microorganisms, nitrates, and pesticides.
The most important thing to understand about RO versus filtration is this: RO removes the minerals and fluoride from your drinking water, filtration does not. This means that important minerals such as calcium and magnesium, which you need for your bones and your teeth, are removed.
How Does Reverse Osmosis Work?
Science class taught us that osmosis is a natural process by which water and nutrients are supplied to living cells. The cell membrane is a natural “semi-permeable” membrane, meaning only selected materials can pass through, and others cannot. An osmotic membrane allows only water to pass through easily while restricting the passage of all kinds of contaminants.
In reverse osmosis, the opposite occurs. Pressure is applied to the solution with a higher solids concentration to cause the flow of liquid to reverse. The synthetically produced membrane allows only the water molecules with very few other molecules to pass through into a storage tank for future use. The remaining source water, containing a higher percentage of contaminants, is left to waste. The process, known as ion exclusion, occurs when ions (charged atoms) form a barrier at the membrane surface to reject contaminants. With an RO system, it can be said that water is removed from the minerals, unlike traditional systems in which minerals are removed from the water.
Semi-permeable membranes are critical for reverse osmosis to be effective. Today, the most common artificial membranes are made from cellulose acetate, cellulose triacetate or aromatic polyamide resins. These membranes are tough enough to sustain the higher water pressures needed for maximum contaminant removal efficiency. Unlike ion exchange systems that need to be regenerated often, the average RO membrane can last two or three years before replacement.
Different semi-permeable membranes also can be made for specific applications to selectively remove certain contaminants. For example, an “osmotic membrane” allows water through but not dissolved molecules like salts or sugars. A “dialysis membrane” allows salts to pass through, but not other substances. Membranes can also be electrified in a process known as “electrodialysis” to produce special removal of ions.
Despite their effectiveness, RO membranes are subject to a number of factors that make them susceptible to loss of function. The number of contaminants, size, and type of equipment and system pressure all contribute to a buildup of material on the membrane.
In addition, Reverse Osmosis drinking water systems typically include a carbon prefilter to reduce chlorine that can damage the membrane. A sediment prefilter is also used to prevent fine suspended particles from permanently clogging the membrane. Larger commercial systems sometimes soften the incoming water or add scale inhibitors to preserve membrane porosity.
While RO systems generally are reliable systems, they waste water and produce treated water much slower than filtration and other systems. Typically, about eight or nine gallons of water are wasted to produce one gallon of treated water, and it can take up to three hours to produce one gallon of usable water.
Countertop or under-the-sink systems for the home use ordinary line pressure of about 40 to 60 psi. to make reverse osmosis work. Maintenance requires replacement of prefilters as needed, which is typically one to four times per year.
Always look for NSF Standard 58 certification, when purchasing a system. Learn more about NSF.