When water that is unsafe for human consumption or use is made pure through a process that uses pressure to force it through a semi-permeable membrane, it is called reverse osmosis. During this process, impurities, or contaminants, are separated from the water when the pressure forces the water from an area that has a high concentration of solutes to an area with low solute concentration. The contaminants become trapped in the membrane, while the pure water is allowed to pass.

As with most things, there are both pros and cons to when it comes to reverse osmosis. On the positive side, the membrane used for this process traps and reduces the amount of bacteria and pathogens in the water. For example, salt from saline water is trapped, making reverse osmosis a good way to desalinate water. It is also highly effective in removing certain poisons, such as arsenic, from the water supply. On the negative side, reverse osmosis creates a significant amount of waste water as it removes contaminants. As much as 50 to 90 percent of water that goes through this treatment process becomes waste water. When it comes to trapping contaminants, the membranes are not always as effective as they should be. Some types of hazardous chemicals are smaller than water and may pass through the membrane just as freely. On the other hand, the membrane may trap valuable trace minerals that are important for the proper function of the body's systems.

There are various types of water filters that are available on the market for consumers. Depending on the filter type, it may be used in addition to reverse osmosis. Activated carbon filters are useful in removing unpleasant tastes and smells from water. These types of filters use carbon to remove or reduce the amounts of contaminants such as pesticides, chlorine, sediment, and lead in the water. Carbon filters are not effective, however, in removing bacteria, nitrates, or metals from the water. Water distillers are another type of water filter that is used for the removal of inorganic contaminants. This type of filtration involves boiling and as a result kills pathogens in the water by first turning the water into steam and then condensing it. Sediment water filters are yet another type of water filter that can be be used in the home. This type of filter is good for clearing up cloudiness caused by suspended materials, such as sand or clay. It also removes manganese, suspended iron, and other materials. When water passes over the filter, contaminants are trapped on its surface or inside of the filter.

On both a small and large scale, the filtration of water is important to everyone. Three of the major reasons why it is so important include providing usable water for human consumption, reducing and/or eliminating the spread of disease, and protecting and preserving wildlife, including habitats and fisheries. Reverse osmosis is a way to successfully treat water and separate many of the impure qualities from pure water. No one treatment is 100 percent perfect, however, and other filtering solutions are often needed to make water as safe as it can be.

To read more about reverse osmosis and water filtration, click any of the following links.

  • Reverse Osmosis: The University of Arizona provides informative resources on this Web page for people who are interested in knowing about the technology of reverse osmosis. It includes the pros and cons of reverse osmosis, as well as the various types and information on issues such as costs, operation, and maintenance.

  • Reverse Osmosis in Water Treatment: This CSU Northridge page is about reverse osmosis in kitchen-based systems. It explains the concept of reverse osmosis by first explaining both the process of diffusion and the technology behind regular osmosis. Visitors will also find information on what disadvantages come with reverse osmosis water treatment systems.

  • Desalination By Reverse Osmosis: Reverse osmosis water purification as a tool for removing salt from seawater is the subject of this article by the Organization of American States. Readers can find out how this method of water treatment works as well as about its usefulness to industries and agricultural interests. Also included is a study of the costs involved in using reverse osmosis for desalinization.

  • Healthy Drinking Water for Rhode Islanders: Readers looking for general information on reverse osmosis will find this PDF by the University of Rhode Island to be highly educational. It provides a brief explanation of how reverse osmosis works, the types of membranes that it can use, maintenance issues, and questions that consumers should ask before they invest in a reverse osmosis system. Also included is a summary of what this water treatment technology can and cannot protect against.

  • Recommendations for Arsenic Removal from Private Drinking Water Wells in Oregon: This PDF fact sheet is provided by the state of Oregon, and it discusses the cost-effectiveness of reverse osmosis water treatment with regard to removing arsenic from private water supplies. It also points out both the advantages and disadvantages of this type of water purification system.

  • Selecting a Home Water Treatment System: The National Sanitation Foundation has an exhaustive list of various types of water treatment systems on this page. Readers will also find links to guides for removing specific contaminants from various water supplies.

  • Water Filters: Click on this link to learn the pros and cons of three different types of water filters. The page discusses water distillers, activated carbon filters, and reverse osmosis.

  • Drinking Water Treatment: Sediment Filtration: This guide explains how this type of filter removes sediment from water, what it removes from the water, and what it does not remove.

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