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ArticleTechnical Article

Reverse-Osmosis Water Filters

By HHI Staff

A reverse-osmosis (R/O) water treatment device can often remove a wide range of particulate contaminants. However, the processing of your water is relatively slow, and the cost can sometimes be relatively high. While technically a “filter,” because water must pass through a membrane, most people think of this as a unique technology.

How Reverse-Osmosis Units Work

To understand how a reverse-osmosis unit works, it’s best to understand osmosis and reverse osmosis. Osmosis is the natural process by which water spontaneously passes through a semipermeable membrane (a very fine material that allows only very small molecules to pass through it). Actually, water goes through such a membrane in only one direction: it passes from a solution containing a low concentration of dissolved substances, through the membrane, to a solution containing a higher concentration. Eventually, this results in the same concentration of water on both sides of the membrane. By the way, osmosis is the process by which water passes from the stems and leaves of plants (sites of low concentrations of dissolved substances) through the cell walls (which are semipermeable membranes) into the interiors of the cells (sites of high concentrations of dissolved substances).


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Reverse osmosis, on the other hand, is not a naturally occurring process. Instead, it requires the addition of pressure to create the opposite effect from that of osmosis. In other words, in reverse osmosis, water is forcibly pushed through a semipermeable membrane so that the water goes from a solution containing a high concentration of dissolved substances to a solution with a much lower concentration. The result is a different water concentration on each side of the membrane. The intent of reverse osmosis is to separate water from dissolved substances.

Through this process, reverse-osmosis (R/O) units are capable of effectively treating home-water supplies. In practice, your home’s water pressure (the pressure in your water pipes), and sometimes additional pressure, drives a stream of water through a special membrane. If your water supply is chlorinated, the membrane might be made of cellulose acetate or cellulose triacetate. On the other hand, polymid resin film might be used with unchlorinated water (other membrane types are also available).

Interestingly, reverse-osmosis membranes aren’t all capable of removing the same-size particulates. In fact, the pores (tiny openings) in various R/O membranes are rated as to the largest matter that can pass through them. For example, a typical membrane used in residential units might have a rating of 0.01 microns. Although water molecules under pressure are able to pass through these minute openings, the holes are just too small to let solids over 0.01 microns pass through. (In reality, a membrane will not have all pores of the same size. The pores in a membrane with a rating of 0.01 microns might range from 0.001 up to 0.01 microns.) When water passes through an R/O membrane, a great deal of the dissolved minerals, heavy metals, asbestos, dirt particles, radioactive particles, fluoride and bacteria will be left behind. R/O units can be so capable that they’re usually able to remove an estimated 80-95% of all dissolved solids from the water they treat.

By the way, after the water in a reverse-osmosis unit has passed through the membrane, it then drips into a storage tank and is considered finished, treated water. When this tank is full, some units will automatically shut off and stop any further processing, until you use some of the water and the level in the tank goes down. The treated water is often dispensed from the storage tank through a special faucet mounted next to your sink. To keep the R/O process working efficiently, many R/O units have a flushing cycle in which the semipermeable membrane is routinely rinsed clean with a stream of water. Afterwards, this rinse water is drained away.

Reverse-osmosis units are available in point-of-use units (for mounting above or below a countertop) or as whole-house devices. Certain R/O units now feature automatic monitors that are designed to check the actual quality of the water (this is more common on whole-house units). Some of these monitors check both the original, untreated water as well as the finished, treated water. If any problem is detected, the R/O unit simply shuts off, thus helping to guarantee consistently good-quality water.

Reverse-Osmosis Unit Concerns

Although most reverse-osmosis (R/O) units produce fairly clean water and are more convenient to use than a water distiller, they aren’t perfect. As with other treatment strategies, reverse osmosis is not capable of removing everything you might like to eliminate from your water supply. For example, reverse osmosis can’t separate out any dissolved gases such as chlorine or radioactive radon (although it can usually remove the “daughter” particles that are emitted by gaseous radon). In addition, reverse-osmosis membranes can’t completely remove all biological contaminants (although certain models are much more capable of removing microbes than others). Yet, most R/O units should be able to remove protozoan cysts.


Interestingly, if you have a chlorinated water supply that runs through galvanized steel pipes (steel pipes coated with zinc to prevent them from rusting), you may encounter an unexpected problem with bacteria. It seems that irregular areas can develop on the interiors of these pipes because of chlorine reacting with the metal. Certain strains of bacteria can thrive in these rough spots. Although they may not be harmful to humans, they can destroy R/O membranes. Even if you don’t have chlorinated water and galvanized pipes, bacterial contamination of a reverse-osmosis membrane can occur.

As a result, to control bacteria and other types of contaminants that reverse osmosis can’t eliminate on it own, R/O units are often sold as a part of a combined water-treatment system. Therefore, you might find a reverse-osmosis model combined with an ultraviolet-light purifier and/or an activated-charcoal prefilter or postfilter. Another R/O system you might come across is one using a KDF filter for pretreatment. In this case, the KDF media not only is able to remove the chlorine, but has an innate bacteriostatic action capable of controlling bacterial growth on itself. Some reverse-osmosis units may also have a micro-pore prefilter, often primarily to remove membrane-damaging sediment.


It should also be mentioned that some individuals are concerned that reverse-osmosis units are actually too effective in removing dissolved minerals from the water. It seems that this demineralized water can be somewhat flat-tasting. Interestingly, it’s also called aggressive water. Aggressive water will seek out and dissolve substances with which it comes into contact. As a result, some people believe that, if you drink such demineralized water, it could seek out and dissolve minerals that are in your body — minerals that your system needs for optimal functioning.

Wasting Water?

There is one problem with R/O units on which everyone usually agrees — reverse-osmosis treatment requires the use of many gallons of untreated water to create just one gallon of treated water. While the amount varies with different models, as many as ten gallons of water may be flushed down the drain for every gallon of water that is filtered (commonly, the ratio is about three or four to one).

The major reason for this “wasted” water is that the very process of reverse osmosis creates water on one side of the membrane with a very high concentration of dissolved minerals. If the concentration gets too high, the water may not be able to hold any more minerals in a dissolved state. Therefore, the minerals begin precipitating (coming out of solution) and form crystals. So, before the mineral-rich water gets too concentrated for crystallization to occur, it’s drained away. Then, too, there’s a periodic flushing of the R/O membrane to keep it clean, which uses more water. If you’re interested, one way to minimize the “wasting” of water is to purchase a unit with a tank having an automatic shut-off valve. This will stop the R/O process until additional treated water is really needed.

You should also be aware that many R/O units can be somewhat expensive, and they tend to work rather slowly to produce a limited amount of treated water. In fact, many point-of-use models only produce five to 15 gallons of treated water a day. (Of course, this is usually enough for the drinking and cooking needs in most households.) In addition, research has found that reverse-osmosis units function less efficiently if they have to treat cold water rather than warm water.

Getting Optimum Performance

Furthermore, the pressure of your water supply is a key factor in how well your R/O unit will operate. It seems that most utility water lines are under a pressure of 40 to 60 psi (pounds per square inch), a range that happens to be ideal for most reverse-osmosis units. However, if you have a water pressure reading much below 40 psi, you’ll probably have to increase the pressure. Fortunately, some reverse-osmosis units are sold with an optional pressure-boosting feature. In addition, the pH level (the degree of acidity or alkalinity) of your water supply can affect your R/O unit’s performance.

You may want to keep in mind that if your home requires whole-house R/O equipment, you’ll likely need a separate water storage tank and a repressurization pump (to raise the water pressure high enough to allow the treated water to flow from the tank through your home’s plumbing lines). And, because reverse-osmosis water treatment is usually quite slow, your whole-house model may have to work continually.

If you’re a chemically sensitive person, you may be interested in knowing that there have been reports that some point-of-use R/O units, that are made of plastic, have given the water they’ve treated a “plasticky” taste and odor. Obviously, this is more likely with brand new units. However, the use of an activated-charcoal filter after the R/O unit (as a type of postfilter) should be able to solve this problem. As your R/O unit ages, steadily lower amounts of the plastic compounds will migrate into the water, so it should become more tolerable.

Not surprisingly, reverse-osmosis membranes don’t last forever. Generally, they’ll have to be replaced every one to three years (follow the manufacturer’s guidelines). Replacement is necessary because the original pore openings will enlarge over time through simple abrasive action. Therefore, as any R/O membrane ages, it allows ever-larger contaminants to pass through it. Incidentally, water tests are available to let you know if the membrane is functioning properly or if it should be replaced.


From Creating a Healthy Household: The Ultimate Guide For Healthier, Safer, Less-Toxic Living, © 2000 by Lynn Marie Bower. Used by permission.



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Reverse-Osmosis Water Filters:  Created on February 22nd, 2007.  Last Modified on February 28th, 2011


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