EFFECTIVE AGAINST: Unwanted tastes, odors, chlorine and iodine residuals, detergents, radon, and some man-made, organic chemicals such as many pesticides, and volatile organic chemicals, such as paint thinners. Lead and other heavy metals are removed only by a very specific type of activated carbon filter.
NOT EFFECTIVE AGAINST: Activated carbon filtration will not remove microbes, sodium, nitrates, fluoride, and hardness. Unless the manufacturer states that its product will remove heavy metals, the consumer should assume that the filter is not effective in removing them.
These devices are commonly known as carbon filters. They actually absorb contaminants to an activated carbon medium. Activated carbon filters are used to remove unwanted tastes, odors, radon, and some man-made volatile organic contaminants from drinking water. The efficiency of the unit depends on the type of activated carbon installed, the filter bed depth, the type and concentration of contaminants in the water, and the contact time between the water and the carbon filter. Activated carbon filters are recommended for water without bacteria present.
Carbon filters are easy to install and maintain, and operating costs are usually limited to filter replacement. Depending on the type and concentration of the contaminant being removed, some carbon filters may require special hazardous waste handling and disposal, which can be costly. Other filter types are also available including charcoal and ceramic materials.
Activated carbon filters do not adsorb every type of contaminant equally. The solid material used in an activated carbon filter is a specialized carbon manufactured for these purposes. Contaminants adhere to the surface of these carbon granules or become trapped in the small pores of the filter. Generally, an activated carbon filter is used with a pre-treatment filter to remove sediment or iron particles that may be present and can clog the carbon filter.
Granular activated carbon treatment is most common for private water systems. Granular activated carbon is an effective filter for removing organic chemicals that may be in drinking water. Many other small devices use carbon that has been pressed into a solid block. Use of block carbon may result in a significant drop in home water pressure. These devices also clog easily if the water is cloudy or contains particles. A synthetic resin may be a substitute for carbon. Although activated carbon is efficient in removing a variety of organic chemicals, a specially formulated synthetic resin may be a better absorber for a specific contaminant.
Activated carbon filters come in both point-of-entry units and point-of-use units. All activated carbon units differ in level of treatment, installation, storage requirements, and cost. Also, some devices are better at treating certain contaminants than others. Discuss the options with a treatment specialist. In addition, confirm that the treatment unit has been tested to meet manufacturer’s claims. See questions to ask for more details on product certification.
There are five basic types of activated carbon systems. The particular activated carbon device appropriate for a given situation is determined by the type and concentration of the contaminant and the unit's design, including how much carbon it contains.
Similar in design to a drip coffee maker, pour-through units are the simplest type of an activated carbon filter. A quantity of untreated water is poured through the carbon, and the treated water is collected in a storage container. These units are not connected to the water supply and usually sit on the counter. They are portable, require no installation and are convenient for camping or picnicking. Pour-through devices will treat only small amounts of water at a given time and are not generally as effective as larger, automatic units. They are inexpensive but do not contain enough activated carbon or provide sufficient contact time between the carbon and the water for reliable contaminant removal. The filter material needs to be replaced. Significant bacterial growth can occur on the carbon in these units, so they should be flushed thoroughly if unused for more than a few days.
Faucet mount units are attached to the faucet (usually in the kitchen) or placed on the counter with connections to the faucet. There are two basic designs: the bypass option, which has a valve to filter water used for cooking and drinking (this prolongs the life of the carbon cartridge), and the non-bypass option, which filters all water passing through the faucet. Because the quantity of carbon contained in a faucet-mount unit is not large enough to provide extensive contact time with the water, these devices are not recommended for removal of organic chemicals. As with the pour-through units, the filter material is usually replaced. Significant bacterial growth can occur on the carbon in these units, so they should be flushed thoroughly if unused for more than a few days.
The in-line device is installed beneath the kitchen sink in the cold water supply line to treat water for drinking or cooking. If both hot and cold water come from a single faucet, the treated (cold) water can mix with the untreated (hot) water. Treated water is assured only when using cold water for drinking and cooking.
The line-bypass unit is also attached to the cold water pipe, but a separate faucet installed at the sink provides treated water for cooking and drinking. The regular tap delivers untreated water. This design increases the life of the carbon by allowing a choice of treated or untreated water, depending on the intended use. The separate faucet system slows the flow rate, which increases contact time and adsorption effectiveness. Neither in-line nor line-bypass units are recommended for removing radon or volatile organic chemicals.
Whole-house treatment or point-of-entry is recommended for treatment of volatile organic compounds (VOCs), radon and other contaminants that should be treated before entering the house. Since VOCs easily vaporize from water into the air, point-of-entry treatment prevents inhalation and skin contact of hazardous vapors from the shower, dishwasher, washing machine, or other times when large amounts of water are used. This device should meet certain guidelines concerning the application rate of water to the carbon, contact time between the water and the carbon, the type of carbon used, and the concentration of contaminant(s) to be removed. Point-of-entry devices are installed where the water enters the house, so they treat all the water used in the home. Systems used should be vented to the outside atmosphere so as not to cause a build up of harmful vapors inside the home. There is no bypass option, although water can be diverted for outdoor use prior to treatment.
The effectiveness of an activated carbon unit depends on the extent of contact between the carbon and the untreated water. During operation of carbon filters it is possible for channels to form within the filter which will allow some of the water to pass through untreated. Since treatment depends on the carbon granules adsorbing the chemical contaminants, these channels decrease the contact time and the effectiveness of the carbon filter unit. A disadvantage of block carbon is that if it is unevenly compressed when manufactured, irregular flow patterns may affect contaminant removal.
There are two main procedures to determine how much a carbon filter can adsorb: the iodine test and the phenol test. The iodine number is defined as the amount of iodine (in milligrams) adsorbed by one gram of carbon under a certain set of conditions. The larger the number, the more the carbon filter can adsorb. Phenol is another measure of effectiveness. The lower the phenol number, the better the carbon is at removing organics. When comparing home carbon devices, pay attention to these values.
Please keep in mind that manufacturers tend to report the best removal rate for their product. The values are frequently the amount the unit will remove at the beginning of use for a single contaminant at moderate concentrations. Units having a larger bed volume of carbon usually remove a greater quantity of contaminants. Some types of activated carbon will remove specific contaminants better than others. Buyers need to examine as many products as possible and know exactly what contaminant they want the unit to remove. You will also need to know how much water is used in the home on an average day to adequately size the treatment system. Additionally, have both the raw water (prior to treatment) and the treated water retested after a unit is installed to ensure it is functioning properly.
Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless maintained in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a log book to record water test results, equipment maintenance and repairs.
Most activated carbon filter units need to have the carbon changed periodically. For small specialty units, the entire unit is normally replaced. Cartridge filters are the easiest to change.
Before purchasing a unit, consider the ease of opening the filter housing and the amount of space required to change the filter. Service periods vary greatly. Small units with heavy loads of contaminants may need replacement monthly or more often, while a six-month service interval is frequently advised for the cartridge filters. Since some filters treat up to a particular amount of water, a water meter installed on the filter can help judge when carbon replacement is necessary.
Depending on the type and concentration of the contaminant being removed, some carbon filters may require special hazardous waste handling and disposal, which can be costly. An example would be carbon filters used to remove radon in drinking water. It is important to establish beforehand how handling and disposal will work and whether alternative treatment methods or locating alternative water sources (such as installing a new well in a new location, tying into a public water supply if available, or using bottled water) would be more effective in the long run.
Activated carbon is an excellent medium for growing bacteria. Bacterial growths will plug and coat the activated carbon and reduce the effectiveness of the filter and can introduce bacteria into your drinking water. Allowing the water to run through the filter for at least 30 seconds is a good practice to flush out bacteria that may have accumulated and grown in the filter. Backwashing and regular filter replacement also help reduce bacteria build-up.
Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if an activated-carbon filter is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.
Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.