Saturday, October 10, 2009

Water Purifier: Reverse Osmosis

Reverse osmosis is a process involving a diffusion mechanism and it is working by the use of pressure forcing a solution through a membrane, to retain the solute on a side and to allow the pure solvent passing to the other side. There is a dependency on pressure, influent solute concentration and water flux rate to determine the efficiency of the separation of the diffusive mechanism. This method is reversing normal osmosis process which why it is called a reverse osmosis.

Membranes that is being used for reverse osmosis is having a layer which is dense enough in the polymer matrix where the most separation of the diffusive mechanism occurs. The membrane is being design only to allow water to be able to pass through the dense barrier layer in the polymer matrix and to prevent solutes passing through. The reverse osmosis process needs a high concentration side of the membrane to exerted by a high pressure, for brackish and fresh water 2-17 bar usually sufficient enough, but for seawater the pressure would need 40-70 bar, which natural osmotic pressure must be overcome that is around 24 bar.

The process to get fresh water by removing salt from sea water is called desalination. It is one of the processes of water purifier. Desalination also has been used for medical, domestic and industrial applications for water purification.

Water purifier systems which households around the world are using for drinking includes a reverse osmosis method. The reverse osmosis method is used to improve the quality of water to drink and cook.

Water purifier systems for drinking and cooking are typically using sediment to filter particles including calcium carbonate and rust then the water has to go through an optional second sediment filter with smaller pores for better filtration. To trap chlorine and organic chemicals, water purifier is using an activated carbon filter. A second carbon filter optionally is used to capture the chemicals which could not be removed by a thin film composite membrane, a reverse osmosis filter. The chlorine and organic chemicals which is trapped by the activated carbon filter has the ability to degrade the thin film composite membranes which is known as TFC or TFM. Then any escaping microbes from the reverse osmosis membrane filtering are going to be disinfected by a ultra-violet lamp.

Reverse osmosis for home water purifier has disadvantages, as because household have low back pressure it uses a lot of water. The result of the low back pressure, the water purifier which is using reverse osmosis method only recovers 5% to 15% of the water that enters the water purifier system. The remaining water, 85% to 95% is considered as waste water and is being discharged. Methods to recover the waste water are not considered for household systems as the waste water carries the rejected contaminants. Water purifier using reverse osmosis method is more suitable for large scale industrial system as the large scale industrial system can generate high pressure which is essential for reverse osmosis filtration and can give efficiency in the production. Mineral from boiler water at power plants can be removed by using reverse osmosis method. Reverse osmosis method is more suitable for industrial sector.