How does Reverse Osmosis Work?
Reverse osmosis (RO) is sometimes referred to incorrectly as ultra-filtration because, like a filter, it involves the movement of water through a medium. So while the end result is similar, cleaner water, the process works on a different principle.
To understand reverse osmosis it is helpful to first look at what osmosis is. Osmosis occurs in virtually every living cell on earth – all the time. The semipermeable membrane surrounding each cell allows dissolved compounds to pass in and out of the cell. Simply put the cell membrane allows equilibrium to be reached between the dissolved compounds in the fluid outside the cell with the dissolved compounds inside the cell. So in the case of nutrients, if there are less nutrients in the cell then in the surrounding fluid the membrane allows those nutrients into the cell until they are equal on both sides.
While a filter is like a sieve that blocks anything and everything larger than the size of its “holes”, osmosis works at the chemical level and allows compounds to pass through that are of higher concentration on one side of membrane compared to the other. In nature, osmosis is a two-way street that is working to keep both sides of the street the same.
Natural osmosis works by equalizing the pressure of solvent and dissolved compounds on both sides of the membrane. We can exploit this process by artificially increasing the pressure of the solvent (in our case water) on just one side of the osmosis membrane. This increased or higher pressure allows for a higher concentration of dissolved compounds on that side of the membrane and in turn a lower concentration on the lower pressure side. The more the pressure differs between the two sides the greater the difference in the concentration of dissolved compounds. So basically the water on the low pressure side contains less dissolved compounds (impurities) or is “cleaner” than the water on the high pressure side. This difference in concentration is typically expressed as a percentage and is often called the rejection rate or rejection percentage.
The rejection rate or rejection percentage is used to characterize the effectiveness of a particular reverse osmosis set up in a specific situation. The rejection rate has many variables including solvent pressure, solvent temperature and dissolved compound concentration. In advanced applications, such as water for medical research, the desired rejection rate or “target” is established first and then a reverse osmosis system is designed based on analysis of the source water and then matching the membrane properties and operating pressures and temperatures to reach that goal. In less stringent consumer drinking water systems, rejection rates are based on average concentrations of the contaminants found in average source water with an operating pressure range of 40-70psi at room temperature. These consumer units are designed for and are very effective at reducing common contaminants in municipal water supplies. However, because every situation is unique and the effectiveness of any RO system depends on outside factors including water pressure, it is vital that the install site and source water be evaluated by a professional to insure a particular set up is proper for the intended use.
In drinking water applications rejection rates need to be high enough to reduce the contaminants in the untreated water to a safe level. To determine the needed rejection rate, it is necessary to consider the initial concentration. For example, if a water supply contains nitrates at a concentration of 20 milligrams per liter (mg/l), and the RO unit is rejecting nitrates at a rate of 85 percent at a pressure of 65psi and an operating temperature of 70 degrees Fahrenheit , that means 15 percent of the contaminant is remaining, or 3mg/l (20 times 0.15 = 3). According to EPA standards, acceptable levels of nitrates in drinking water are at or below 10mg/l. So in this example, the rejection rate of 85% for nitrates is very acceptable. Of course when one factor is changed, such as a reduction in operating pressure, the rejection rate will also be reduced. For more information about the expected rejection rates of Culligan Drinking Water Systems, please see our FAQs.
Proper maintenance is also essential to maintain the effectiveness of a reverse osmosis system. Some units are equipped with automatic membrane flushing systems to clean the membrane. In addition, most reverse osmosis membranes are part of an installation that includes one or more pre-filters. These filters are there to protect the membrane from larger sediment particles and chemicals such as chlorine which will reduce the effectiveness and life of the membrane. It is important to follow the manufacturer’s guidelines on replacement of the pre-filters to ensure continuous, safe rejection rates.
While the reverse osmosis process and simple filtering are different they are often used in conjunction with one another and that can add to mistaking the one for the other. So although it is more complex and advanced than simple filtering, reverse osmosis is really just another method available to us in the water treatment business. Every water situation is unique and every one requires a unique solution. When the situation is right and the technology is applied correctly, reverse osmosis can be a powerful and effective tool to help us reach our ultimate goal – Better water. Pure and simple.
Learn more about the Methods and Benefits of Water Treatment.