Osmosis is the tendency of a fluid to pass through a semi-permeable membrane from a high fluid concentration to a lower one to equalize the concentrations. This is a natural action, the prime mechanism for maintaining cellular fluid levels. When the
pressure is reversed mechanically, so is the process. Best of all, this is not a replacement process as with ion exchange, but a removal process that can thoroughly cleanse if the permeability of the membrane is sufficiently restricted. It is the operating principle behind dialysis, separating blood plasma from toxins when the kidneys do not function properly. It allows ships at sea a source of potable drinking water. It also allows us the purest method of cleansing water outside of the expensive
method of distillation. Because RO is a mechanical process, it does not depend on the ionization of dissolved substances to effect their removal. This means that organic compounds, such as pesticides, bacteria, fungi and even viruses can be removed if the pores of the membrane are small enough to exclude them. The RO apparatus attaches either directly to a faucet or a pressurized water line. Filtration first begins by passing the water through a 1.5 micron disposable filter designed to remove sedimentary particles from the water. This prevents the larger particles from clogging the much finer RO membrane. The water then is forced across a tightly wound spiral membrane that is semi-permeable. The water must achieve a specific minimum pressure to force a percentage of it through the membrane. Clean, unionized and filtered water exits the opposite side to a collection chamber. The contaminating elements left behind in the bulk of the water, called saline simply because of its comparatively higher salt and mineral content, pass off by a separate duct.
Two types of membranes are used in RO systems, the choice of which is largely
dependent on your local water supply and needs. Cellulose TriAcetate (CTA) films
are tolerant of chlorines and chloramines found in municipal water supplies, and are a
good choice for most of these sources. The composition of CTA films is dependent
on such antibacterial substances, as they are susceptible to breakdown by contaminating bacteria. The activated carbon filters for these types are located after the membrane and are known as post-filters. CTA membranes are not a good choice if your water is harder than 500 ppm or has a pH higher than 8.5.
The second membrane type is Thin Film Composite (TFC). TFC membranes are better able to handle harder waters and have a very high resistance to bacteria. They have to be resistant - their single drawback is an inability to tolerate chlorines. TFC
membranes are also well suited to rejecting nitrates, phosphates and silicates, and
should be considered a primary choice if your water is contaminated by agricultural run-off or obtained from non-municipal supplies. The chlorines present in municipal supplies can be removed by placing the activated carbon in front of the membrane in a pre-filter configuration. Both types of membranes require operational temperatures above freezing and below 85° F (~29° C). Care should be taken in their installation to assure they are never exposed to hot water, especially if the unit is connected to a faucet rather than a cold water line. Freezing can be avoided if installed to a line or faucet inside the house where such cold temperatures are unlikely. Both the sediment pre-filter and the activated carbon filter, whether in a pre- or post-filter configuration, will need replacement at regular intervals. However, the life-span of these replacement parts is much greater than an ion exchange resin, typically on the average of four to six months' continuous use. The single greatest replacement expense lies in the membrane itself, which will eventually become clogged in two to three years. Otherwise, the only maintenance normally required is in back-flushing the filter element in order to wash out the buildup of salts that can occur and speed the clogging of the membrane. This back-flushing is performed once each 150 hours or so of operation. Another consideration is the generation of the waste saline. RO units typically provide only one gallon of deionized, filtered water out of every five gallons through the system. Were we cleaning sea water, this would be of little consequence, but in the reality of today's world this can be wasteful when cleaning fresh water. Some "professional" installations simply run the saline straight to the drain, but a more conservative use can and should be found for this runoff. If organic contamination is not a factor, the saline can be used for African Rift Lake cichlids, who prefer the harder, more alkaline conditions. Other uses, such as garden water, can be found. Since this water tends to be only 25% more concentrated than straight tap water, it can fulfill any function that normally calls for tap water, limited only by your imagination. Unlike DI units, the membrane of the RO filter can be destroyed by allowing it to dry. This can occur intentionally, by the removal of the water from the filter system, or unintentionally from inconsistent use. If you know that the unit will not be used for a
time, the membrane can be removed and stored submersed in the refrigerator. Constant use of the unit not only ensures that the membrane remains in contact with the water, but can prolong the life of the membrane. When determining the size of the unit to purchase, keep future expansion in mind, but do not overestimate the unit's usage level to the point that it remains idle for long periods without protecting the membrane. Take care ,,,,Terry
