What is Reverse Osmosis & How Does it Work?

What is it and how does it 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. If such a membrane separates two water solutions with different concentrations, osmosis will cause water to move from the diluted solution into the more concentrated solution, as if to dilute it. 

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 (from concentrate to permeate), unlike natural osmosis. 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, or 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 water.

Because Total Dissolved Solids (TDS) cannot be removed with mechanical filtration or standard carbon filtration, a Reverse Osmosis system is one of the most effective means of filtration. With an RO system the TDS level increases on the high pressure side of the membrane as water permeates through the membrane to the low pressure side. The high TDS water is flushed down the drain, and the water that has passed through the membrane now has very little TDS and is available for immediate use. 

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. 

When is Reverse Osmosis Applied?

In foodservice applications, there are a few specific reasons that Reverse Osmosis may be applied. RO should be considered if the TDS content in the water is high enough to:

• Impart objectionable taste. 
• Cause scale buildup in equipment. 
• Cause poor quality of beverages and ice, such as weak carbonation or soft ice.

The U.S. Environmental Protection Agency (U.S. EPA) sets a secondary standard of 500 mg/l TDS in drinking water. Secondary standards are recommended guidelines for contaminants that may cause cosmetic or aesthetic effects in drinking water. Different foodservice applications require different levels of TDS. Steam applications require water with very low TDS to prevent heavy scale accumulation that could damage or destroy the equipment, while fountain beverage applications are fine with TDS of up to 500 ppm because scale typically does not pose a problem since there is no energy being applied to the water. Some Everpure RO systems provide a “blend” feature that allows raw water to bypass the RO membrane. Both the raw water and the RO water are filtered through a traditional taste & odor filter and then blended. This control of the quantity of TDS allows the system to meet specific water specifications. For example, a coffee recipe may require water with a TDS of 150 ppm. 

The Benefits of Reverse Osmosis

Reverse Osmosis is extremely effective at eliminating or substantially reducing a wide variety of contaminants, more than most all other types of treatment. Because Reverse Osmosis removes from 95%-99% of the total dissolved solids, it is often the best technology for obtaining clean water that is free of TDS and other contaminants. And because it is stripping water of damaging contaminants that can cause harm to equipment, it helps reduce operating costs. 

RO systems are effective in removing excess salt and other dissolved minerals, taste and odor, heavy metals, microorganisms, nitrates and pesticides. Water treated by reverse osmosis has a noticeably cleaner and sparkling appearance and allows subtle flavors in foods and water-based beverages to come through. 

The Drawbacks of Reverse Osmosis

Despite their effectiveness, RO membranes are subject to a number of factors that make them susceptible to loss of function. The amount of contaminants, size and type of equipment, and system pressure all can contribute to buildup of material on the membrane. In addition, disinfectant chlorine can attack some membranes. To prevent this, RO systems typically include a carbon prefilter to reduce chlorine that can damage the membrane. A sediment prefilter also is used to prevent fine suspended particles in the source water from permanently clogging the membrane. Larger commercial systems sometimes soften the incoming water or add scale inhibitors to preserve membrane porosity. 

Reverse Osmosis systems are also more expensive than traditional filtration, and require more maintenance. And, because a portion of the water supply must be used to flush the contaminants to drain, there is a waste factor that can represent a significant portion of the total water use. Another drawback to Reverse Osmosis is it produces clean water at a slow rate, and therefore requires the use of a holding tank so water is available during peak usage periods.