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Reverse osmosis water filtration is a type of water filtration system that works by using pressure to force water through a membrane, separating the pure water from any contaminants. In normal osmosis, a solvent, such as pure water, naturally moves across a membrane from an area of low solute to an area of high solute, where there is a higher concentration of contaminants. In reverse osmosis, however, this process works exactly the other way around. Pressure is applied in order to force the water to move across the membrane from an area of high solute to an area of low solute. This effectively separates the pure water from the contaminants, which are collected on the other side of the membrane.
The membranes that are used in reverse osmosis water filtration are semi-permeable. They are designed to allow pure water to move across the membrane, but to trap most contaminants and not allow these to pass through. These membranes are usually made from a substance called polyimide, a type of plastic resin that is strong, heat resistant, and resistant to chemical change.
There are many different kinds of water filtration systems, most of which use mechanical filters of one kind or another. The unique aspect of this type of water filtration is the application of pressure to move the water across a fine membrane. Because of this, water purifiers and drinking water filter systems that use reverse osmosis typically remove many more contaminants from the treated water than most other types of filtration systems. Contaminants that are removed may include metals such as lead, chemicals like chlorine, and pathogens including viruses and bacteria. Not all substances are removed by reverse osmosis, however, and in particular, certain kinds of pesticides are often present in the filtered water.
Some of the drawbacks of reverse osmosis water filtration systems in common residential use include water wastage and slow production of treated water. For each unit of treated water produced, up to eight units of wastewater may be produced and discarded. Some systems allow the user to store the wastewater for use in the garden, washing cars, and other non-drinking usage. Water is usually purified quite slowly, and these systems often take three or four hours to produce a gallon of clean drinking water. Some critics of these filtration systems also point out that beneficial minerals are removed from the drinking water, and discarded along with the contaminants.
In addition to home filtration systems, there are a number of other applications of reverse osmosis water filtration. Industrial processes that produce wastewater containing unwanted substances, including power plants, may treat the water using this type of filtration. Reverse osmosis water filtration can also be used to turn salt water into drinking water, and to purify collected rainwater.