REVERSE OSMOSIS
The most popular water purification process on the market today, is Reverse Osmosis. This water purification process was pioneered in the early 1950s in response to the increasing incidence of kidney disease. RO was designed to replace the filtration function of the kidney, and is still used in the dialysis units of hospitals for this purpose today.
MEMBRANE
Reverse Osmosis utilizes the properties of a membrane to prevent the passage of particles through the membrane. Any particle larger than a water molecule is prevented from passing through the membrane, while the H2O is permitted to flow through to the "pure water" side. Because of the incredibly small pores which the water passes through, pre- filtration is necessary to prevent the plugging up of the membrane pores. Pre-filtration can combine 1, 2 or even 3 pre-filters. These pre-filters are combined to remove larger particles, chlorine, and even some chemicals and pesticides, before the water is subjected to the membrane purification technology. The number and type of pre-filtration depends on the water quality and impurities which are determined to be present in the water. Common pre-filtration includes, sediment filtration, chlorine removal, and chemical absorbent. The purpose of the pre-filtration is to protect and give longer life to the expensive membrane portion of the system.
SEDIMENT is first removed by a 20 micron filter. This filter may be constructed of tight wound cotton string spun around a spool, or it may be a pleated paper filter. By the way, this dot, "." is approximately 50 microns in size. Therefore a 20 micron filter would remove any substance which is half the size of a dot such as this one ". " Sediment, might include particles released from the inside of the pipes which deliver the water to your home, iron which has come out of solution and is now visible in your water as color, or just debris from the water source where your water is drawn from.
GRANULAR ACTIVATED CARBON (GAC) is most often used as the second filter, if the water source is chlorinated, as most municipal water is. Chlorine will destroy the Composite RO membrane quickly, so the chlorine must be removed before the water reaches the RO membrane. GAC filters are simple filters which are filled with small grains of carbon which are highly absorbent and effectively remove chlorine. When the water source does not contain chlorine, the GAC is usually replaced with a sediment filter of perhaps 10 microns of effective filtration, although some GAC filters can act as both a taste and odor filter, as well as a 10 micron sediment filter. It should be noted though, that because a GAC filter does not give "absolute" filtration results for sediment, the water treatment technologist who is applying the application for water treatment should be aware of what objective is to be attained. Is the objective tighter filtration after the 20 micron by placement of a 10 micron filter, or is ithe objective taste and odor removal, such as the rotten egg smell of hydrogen sulfide (H2S)? A GAC will reduce H2S levels where as a pleated or string sediment filter will not.
CARBON BLOCK filters are usually placed as the third pre-filter in a RO filter system. Carbon block filters have the potential to filter down as small as one-half micron. This is very small. As well, carbon blocks are reasonably effective at removing most chemicals and pesticides from the water. The effect of placing the pre-filters in this arrangement contributes to both the increased life expectancy of the RO membrane, as well as reduced pre- filter maintenance. The least expensive of the filters, the 20 micron, takes out the bulky larger substances. The next filter in expense, the GAC, takes out the chlorine and more particles which the 20 micron could not capture, and the most expensive of the pre-filters, the Carbon Block, is left to just absorb the chemicals.
THE MEMBRANE is the heart of the Reverse Osmosis System. As the water is forced through the membrane by water pressure, the impurities are held back and flushed out of the system through the brine water drain line. The membrane is capable of removing upwards of 95% of all impurities, if sufficient feed water pressure is available. Higher feed water pressure increases the efficiency of the membrane. Absolute minimum feed water pressure is 40 psi. If the pressure is not high enough an electric pressure booster pump must be added before the system.
After the purified water has passed through the RO membrane, it is stored in a small storage tank, waiting to be delivered through a dedicated tap, to the consumer. The tank only holds about 6-8 liters of water at a time. This small tank is designed to fit under the kitchen sink for convenience. Larger tanks can be ordered and easily installed for consumers who desire more water available at a moments notice. It generally takes about 2 hours to fill an empty 8 liter tank. Most RO systems deliver from 35-75 gallons (160-340 liters) every 24 hours, depending on the membrane capacity which came with the RO system, and depending on the water pressure. (more pressure = more purified water)
Finally, on some systems the water may pass through a final carbon filter which is installed in the delivery line between the tank and the faucet, just as a final polish for good tasting water.
CRITICISM is often voiced about RO units because for every liter of purified water, 2-3 liters of brine water are discharged to the drain, to clean the impurities out of the water. This is true. However, this water is not gone! It is simply sent back out to where it came from, the water table. Nothing has been added to the water. The impurities it picked up as it passed through the ground, is returned to the ground.
CONCLUSION: Reverse Osmosis is the number one choice of consumers around the world who wish to provide pure, safe, good tasting drinking water for themselves and their families.
Reverse Osmosis - The World's Most Popular Drinking Water System MEMBRANE
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