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Water Filter Basics

By HHI Staff

Water filters can be useful to improve water quality, but they must be chosen carefully. This is because there are different kinds of filters and they don’t all remove the same types of contaminants. Plus, some aren’t recommended with certain water supplies. For example, an activated carbon filter is not a good choice unless your water supply has been disinfected. Distillation and reverse osmosis are best used as point-of-use filters. They are usually located in the kitchen, and they both work well against a variety of water contaminates.


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Before you decide to filter your water, you first need to have some idea what contaminants are in it. Public water systems are required by law to have their water tested regularly so, if you are connected to such a system, you can ask for a water-quality analysis. If you have a private supply, such as a well, you’ll need to have your water tested yourself at a private laboratory. Because there are so many possible contaminants in water, it’s possible to spend hundreds of dollars on testing that you don’t need. So, it helps to know what kinds of contaminants are common in your area. This kind of information might be available from a neighbor, your local board of health, or from a nearby municipal water system. Many local or state boards of health maintain a list of water-testing laboratories. Some local or state boards of health will actually perform testing, either at no charge or at a reasonable cost, if it’s apparent that your health is at risk.


If a water filter is located on the incoming supply line, just after it enters the house, then all the water in the house will be filtered. This is considered a whole-house system, and it can be practical with some filtration strategies, but impractical with others. Individual filters, often called point-of-use filters, can be used at specific locations. For example, an under-sink filter in the kitchen is useful for filtering cooking or drinking water. Sometimes, a small filter is attached to a shower head to remove chlorine. Unfortunately, there is no single kind of filter that’s suitable for all situations. Some remove certain contaminants very well, but leave other pollutants in the water untouched. In the 1980s, it was discovered that one manufacturer’s portable filter cartridge was actually adding methylene chloride, a possible carcinogen, to the water.


Because water filtration can be quite a complicated subject, it’s often a good idea to consult a specialist for help in designing a system for your particular needs. Most cities have several listed under Water Treatment in the telephone book. Keep in mind the fact that many water-quality consultants make their money by selling equipment, so they may try to sell you more than you actually require. If you talk to more than one specialist, and they all know you are shopping around, they’ll be more likely to only recommend the equipment you really need.


The National Sanitation Foundation (NSF) is an independent, not-for-profit organization dedicated to public safety and protection of the environment by developing standards and certification. All NSF-certified water systems are listed free-of-charge on their website, or in a booklet you can purchase. The Water Quality Association is an international organization for the water-treatment field, and they offer consumer-oriented materials.

Sediment filters

Sediment filters are very limited in function. They can remove heavy concentrations of suspended soil, but they do nothing to combat biological contaminants, chlorine, or other volatile chemicals. A sediment filter is often combined with another filtration strategy—sometimes in the same unit.


Distillation devices boil water into steam. The steam condenses back into water in another chamber, leaving behind dissolved solids and particulates. The heating process kills microorganisms in the water. Distillers allow some volatile chemicals to escape into the air, where they can be inhaled, while other volatiles remain in the water. Some distillers use a two-stage process, whereby most of the volatile chemicals are driven off first, then the water is boiled to remove the other contaminates. For a handful of very sensitive people, even triple distillation is not enough to render water tolerable.


Distillers have the potential to remove most of the contaminates found in water. They are fairly expensive to purchase and operate, and they are not practical as a whole-house method of cleaning up a water supply. Even when used just for drinking or cooking water, they can be inconvenient because they only produce a limited amount of pure water at a time, and they need regular cleaning. The distillation process removes oxygen from the water—making it somewhat acidic—and it removes virtually all the minerals. Stainless-steel distillers can produce water with a metallic taste. This is because the aggressive water leaches chromium out of the stainless steel during the distillation process. Aluminum can be leached from aluminum parts.


Because distillers operate slowly and do not provide water at the turn of a tap, a holding tank or storage jar is generally required. Typically, several gallons of water a day can be distilled and stored for use. Because distillers kill bacteria, they can be legally classified as water purifiers, whereas most other devices can only be called filters.

Activated-carbon filters

Filters containing activated carbon are often referred to as taste-and-odor filters and they are very effective at removing volatile chemicals such as chlorine, chloroform, pesticides, etc. They will not remove dissolved solids, particulates, or kill microorganisms. In fact, bacteria can grow and multiply inside a granular-carbon filter. Chlorinated water can contain some bacteria that’s not been killed by the chlorine, and this bacteria can actually grow in the carbon itself. This is most problematic if the water sits in the carbon for an extended period of time—especially over several days. The simple act of letting the water run for a while will flush out the filter and the lines, so many people recommend running the water in the morning (after it has been sitting overnight) for a minute before drinking any water from the tap.


Activated-carbon filters that utilize a block of carbon, rather than granular carbon, are much less likely to harbor bacteria. Some manufacturers add silver to granular carbon to kill the bacteria, but there is some concern that the silver can negatively contaminate the water. If chlorinated water is run through an activated-carbon filter regularly, and the filter is changed regularly, there is usually little danger from the small amount of bacteria present. Not all bacteria are harmful. In fact, some are very beneficial to human digestion. If you are on a public water supply that has been chlorinated, harmful bacteria probably aren’t present, so activated carbon is usually a safe choice. But if you have a private water supply, harmful bacteria might be present, and the use of an activated-carbon filter can be risky.


Radon can be removed from water with the use of an activated-carbon filter, however many of the small filters on the market don’t have enough capacity to result in much reduction. To be effective, a filter should contain 1-3 cubic feet of activated carbon, depending on the radon content of the water. Radon is not a widespread problem in water, but it can be in some parts of the country. It’s been estimated that there about 200 cancer deaths a year (160 from lung cancer and 40 from stomach cancer) due to radon in drinking water.


Activated-carbon filters are very good at removing chlorine and other volatile chemicals. The quantity of volatile chemicals removed by an activated-carbon filter is directly proportional to the amount of carbon in the filter. So, a very small filter can have the carbon depleted rather quickly, while a larger model might last for a month or more. Sometimes, if an activated-carbon filter is not changed regularly, it can begin unloading pollutants back into the water supply, but this has been noted only with filters containing powdered carbon. If you have a good sense of smell, you can often tell when it’s time to change an activated-carbon filter—when the hot water starts smelling like chlorine. If your sense of taste is better than your sense of smell, you might start tasting chlorine when the filter begins to become depleted.


Activated-carbon filters are fairly inexpensive and they work well as either whole-house filters or point-of-use filters. There are no moving parts and the only maintenance involves periodic cartridge replacement. Housings are made of plastic or stainless steel. Small models are available for use on shower heads, faucets, or under-the-sink.


Ozone and UV


Ozone: Ozone kills bacteria and other microorganisms and is often used in conjunction with other filtering technologies. It is not effective in reducing levels of chemical contaminants.



UV (ultraviolet): These systems use ultraviolet light to kill bacteria and other microorganisms. They cannot remove chemical contaminants.


Source: Environmental Working Group (EWG)

KDF filters contain a specially formulated material that can remove some of the same contaminants as activated carbon. In fact, it’s been estimated that a KDF filter is ten times better at removing chlorine. A KDF filter can also remove lead from water, and it doesn’t support bacterial growth, so it can usually be used safely on a private water supply where an activated-carbon filter may not be a good idea. Sometimes, KDF and activated carbon are combined in the same filter. KDF filters can put a tiny amount of zinc and copper into the water—but the levels are within the EPA’s drinking-water standards.


Reverse-osmosis filters are fairly advanced water-cleaning devices. They operate by forcing a stream of water through a semi-permeable membrane to rid it of dissolved solids, particulates, and bacteria. This membrane must be flushed clean by a second stream of water, which then directs the contaminants down the drain. The membrane can become clogged with bacteria, so some manufacturers recommend that reverse-osmosis filters only be used with chlorinated water supplies. A sediment filter is often used to remove most of the dissolved solids so the reverse-osmosis unit is not overwhelmed. Generally, reverse-osmosis filters are very good at removing all contaminates except volatile chemicals. They are not quite as effective as distillers, but they are easier to use.


These units have a significant drawback—they waste 3-10 gallons of water for each gallon that is processed. This is because the membrane needs constant rinsing. Their effectiveness is dependent on temperature, pH, and water pressure. Usually at least 30 pounds of water pressure is required. Like distillers, they only produce a limited amount of water each day, so a holding tank or storage jar is needed. They are not suitable as whole-house filters. Because they can remove minerals, some produce a slightly aggressive water. They can be relatively expensive, take up a lot of space in a cabinet, and periodic maintenance is required. The operating cost is less than when using a distiller, but more than with a carbon filter.


(This article is from the archives of the original Healthy House Institute, and the information was believed accurate at the time of writing.)
(Note: The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)



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Water Filter Basics:  Created on August 14th, 2010.  Last Modified on April 11th, 2011


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