Water is a universal solvent. This means that many things can be dissolved in water. A dissolved substance in water is called a solute. The solute in water may be helpful (such as some minerals). Usually though, the solutes are considered contaminants and need to be removed. Reverse Osmosis is one such method of removing contaminants.
There are several methods of purifying solutes from water. There is no one method that can remove every contaminant. Without a doubt, reverse osmosis is a popular water purification method. This method is found purifying water in wastewater treatment plants, homes and industries.
How is water purified by reverse osmosis? To understand reverse osmosis, we have to first look at the process of osmosis. Then, we will examine how water is purified by reverse osmosis. In addition to that, we will see what contaminants can be removed by this process.
What is Osmosis?
Osmosis frequently occurs in nature. By definition, osmosis is the passage of water through a semi-permeable membrane from a low solute to high solute concentration. Students often ask “What is a semi-permeable membrane?”. Interestingly, these membranes can be found in nature and even within our bodies! The inner membrane of an egg is one such example. The membranes covering our lungs is a second example.
Membranes will allow water to pass through, but prevents most dissolved solutes to pass. The size of the pores in the membrane will determine what solutes can pass through.
In the Osmosis figure, a semi-permeable membrane separates two sugar water solutions of different concentrations. Water molecules may cross the membrane, but sugar molecules may not.
Having a concentration difference across a membrane is a physically unstable situation. The drive is create equal concentrations on each side of the membrane. Importantly, water move from the lower to the higher concentrated solution. This passive movement is an effort to dilute the more concentrated solution.
Water movement will occur until the solute concentration is equal on both sides. It should be noted that although water molecules are moving in both directions, there is no net movement of water.
What is Reverse Osmosis?
Conversely, reverse osmosis moves water in a direction opposite to that of osmosis. Since this is not a natural situation, a pressure force must be applied.
As in osmosis, the membrane does not permit the passage of the dissolved sugar molecules. In contrast to osmosis, the water moves from high solute to low solute concentrations.
A similar process is used in an effort to purify water. Rather than sugar molecules dissolved in water, consider that other things have become dissolved in drinking water: nitrates; bacteria; fecal matter; pesticides and fertilizers; and any a variety of other nasty components that is discussed here: What is in your water.
How Water is Purified by Reverse Osmosis
Reverse osmosis purification systems typically have at least 2 reservoirs. One reservoir has the source water which has a higher concentration of contaminants. The second reservoir has purified water with a very low concentration of contaminants. The 2 reservoirs are separated by a series of membranes which may or may not be preceded by carbon filters. Water is purified by forcing water from the source water through the membrane into the reservoir with the purified water.
Water pressure is used to force the source water across a membrane. Very high water pressure is used in industrial settings whereas home systems use a “low pressure” system. Home units are often paired with carbon filters to provide an even greater range water purification.
Reverse Osmosis technology has advanced greatly over the past few decades. The greatest advances have been in the development of new and better synthetic membranes. The type of membrane used drives the industry.
How to Choose a Filter System
Home Reverse Osmosis units are quite varied, ranging from $30 pitchers using gravity – $2800 under cabinet units – to $18,000 whole house units. For home (or portable) use, the consumer should choose the unit based on the contaminants in the area. Home
- First, identify the contaminants in your area. If you have public water, request a water contamination report from your municipal utility company. If you are on well water, a local Health Department can provide a list of likely contaminants in your region. The EPA suggests that if you are on well water that you test yearly for nitrates and coliform bacteria.
- Second, test the water directly from your tap. As the water moves from the public water supply (or your well), lead and copper may leach from the pipes into your water. These contaminants will not show up in the other reports, making a home test important. Your local Health Department can provide a list of companies certified to test water in your region.
- Third, knowing the contaminants and their levels you can determine type of membrane needed for your reverse osmosis system.
Contaminants Removed by Reverse Osmosis
The type and level of contaminants removed by reverse osmosis is determined by the pore size of the membrane and other factors. Read the information for each treatment unit to find the manufacturer’s certification and claims. Units that are labeled either “absolute one micron filter” or “ANSI 53” provide the greatest assurance of removing the most contamination. In general, the following are removed by the reverse osmosis treatment:
- Disinfectant byproducts
- Using filters marked as “absolute one micron filters” some biological organisms such as
- Cryptosporidium parvum (8-12 micron cysts)
- Giardia lamblia (8-12 micron cysts)
- Bacteria (such as E. coli and Salmonella)
- Viruses (only a few ultra filters have pores small enough to filter viruses at 0.004 to 0.1 microns in size)
A last note on choosing filters. When considering a reverse osmosis system filter, choose a filter size that is labeled “absolute” (meaning the largest hole) rather than “nominal” (the average hole) microns. Filter holes are measured in microns. The EPA recommends the absolute one micron filter to ensure the removal of protozoan cysts such as Cryptosporidium. You can find more EPA facts here: EPA Water Health Facts.
Your Needs – Your Unit
Reverse Osmosis was originally used to desalinate water. Today, it is used in most wastewater treatment plants to remove contaminants and provide drinking water to the regions. It is also a versatile unit that can be placed in the home or taken on the road as a portable unit. Understanding how water is purified by reverse osmosis is critical in choosing the right unit for your needs.
Please let me know what you would like to know about water purification. What portable water filtration units are your favorite?