WQ-13
Activated carbon (AC) filters primarily remove taste and odors in home water treatment systems. Taste and odor, although undesirable, are generally not considered health risks. In recent years, AC filters have been used to remove some of the contaminants discovered in water supplies.
AC effectively removes organic compounds such as volatile organic compounds, pesticides and benzene. It can also remove some metals and radon. As with any treatment system, it cannot eliminate all possible drinking water contaminants.
AC systems are limited in the types of compounds they can effectively remove. The homeowner must determine which water contaminants are present before purchasing such a system. If you suspect a water quality problem, first, have your water analyzed by the local health department or a reputable laboratory. These analyses may be costly, but worth the expense. They are necessary to determine the appropriate home treatment system and how best to operate the system. A state or local health official can interpret water analysis results. Some laboratories may also provide this service.
Home water treatment should be considered only a temporary solution. The best solutions to a contaminated drinking water problem are to stop the practices causing the contamination or to change water sources.
AC is a black, solid substance resembling granular or powdered charcoal. It is extremely porous with a very large surface area. One ounce of AC has an estimated 30,000 square yards of surface area. Certain contaminants collect on the surface of the AC in a process called adsorption.
The two main reasons chemicals adsorb onto AC are a ''dislike'' of the water, and an attraction to AC. Adsorption of most contaminants results from a combination of these reasons. Many organic compounds, such as chlorinated and non-chlorinated solvents, gasoline, pesticides and trihalomethanes can be adsorbed by AC. AC effectively removes chlorine and is moderately effective in removing some heavy metals. AC will also remove metals bound to organic molecules. Fluoride, chloride, nitrate, hardness (calcium and magnesium) and most metal ions are not removed by AC to any significant degree.
Carbon is not necessarily the same as AC. AC removes far more contaminants from water than does ordinary carbon. Figure 1 shows how contaminants adsorb onto an activated carbon filter.
Home AC treatment systems are quite simple. The AC is normally packaged in filter cartridges inserted into the treatment device. Water needing treatment passes through the cartridge, contacting the AC on its way to the faucet. AC filters eventually become fouled with contaminants and lose their ability to adsorb pollutants. The filters then need replacement.
AC treatment systems are typically point-of-use (POU) installed where they treat water used only for drinking and cooking. AC filters can be placed on the end of the faucet, on the countertop or under the sink. Tests show under-the-sink models generally have more carbon and provide superior performance and greater convenience than faucet or countertop models.
POU systems often have a bypass so water for purposes other than drinking and cooking can be dispensed at the tap without being treated. The bypass increases the life of the AC, reducing the time between filter replacements.
A point-of-entry (POE) system is more appropriate if large quantities of contaminants exist. Or if contaminants pose a health threat from general use as well as from consumption. Volatile organic compounds and radon are examples of this type of contaminant. These contaminants may get into the indoor air from water used for showering and washing. In such cases, a large POE system, which treats water as it enters the home, is more economical than a POU system at each tap.
AC filters used for home water treatment contain either granular activated carbon (GAC) or powdered block carbon. Although both are effective, one study comparing GAC with block AC filters showed block AC filters were more effective in removing chlorine, taste and halogenated organic compounds.
The amount of AC in a filter is one of the most important features. It affects the amount and rate of pollutant removal. More carbon in a cartridge means more capacity for chemical removal, resulting in a longer cartridge lifetime. This means fewer cartridge changes and less chance of drinking contaminated water. Particle size will also affect the rate of removal; smaller AC particles generally show higher adsorption rates.
Rust, scale, sand or other sediments can clog an AC filter. Foam or cotton filters (often called sediment or fiber filters) placed between the cartridge and incoming water solve the problem. When sediment filters become clogged, they need to be replaced or they will cause water pressure to drop.
An AC filter must be deep enough so pollutants will adsorb to the AC in the time it takes the water to move through the filter. The filter depth required depends on the flow rate of water through the filter. The slower the flow rate, the better the removal. The poor performance of some end-of-faucet devices is probably due to improper filter depth.
Physical and chemical characteristics of the water will also affect performance. Acidity and temperature can be important. Greater acidity and lower water temperatures tend to improve the performance of AC filters.
AC filters have a limited lifetime. Eventually, the surface of the AC becomes filled with adsorbed pollutants, and no further treatment occurs. 'Break-through' takes place when pollutants break through the filter and emerge in the treated water. Contaminant concentrations in the treated water can possibly be even higher than those in the untreated water. The cartridge then needs to be replaced. Knowing when breakthrough will occur and when to replace the cartridge is a major problem with AC treatment.
Unfortunately, no alarms accompany breakthrough. Unless the pollutants are smelled or tasted, they can be unknowingly consumed. In most cases, break-through can be positively verified only by chemical testing. Frequent chemical testing is impractical and expensive. However, occasional sampling may be useful in helping to predict when breakthrough will occur and alert the user to replace the filter before it happens.
Some cartridges are sold with predictions about their longevity. These are generally only crude estimates since they do not consider the characteristics of a specific water source, such as pollutant concentration. The retailer from whom you purchase the treatment device can better estimate a filter's useful lifetime based on water usage (flow rate) and pollutant concentrations, shown in the chemical analysis. To make the most accurate estimates, you should learn what these amounts are before purchasing the system. If pollutant concentrations increase over time, and without testing done to reveal the change, such estimates may not be very practical or useful.
The water flow rate through the filter can either be estimated or measured with a flow meter installed near the AC filter. The retailer can calculate the maximum allowable number of gallons which can pass through the filter before breakthrough occurs. The homeowner can then replace the filter when the number is reached. Remember, any prediction for filter replacement must be based on the actual pollutant concentrations present in the water. The necessity of an initial water analysis and periodic routine analyses becomes all the more important.
Some systems claim the device will alert the user when the cartridge should be changed. The claims state a pressure drop across the filter occurs when the cartridge needs replacement. A drop in water pressure may or may not result from saturation of the filter. Saturation and breakthrough may occur long before a filter becomes sufficiently clogged to cause excess pressure drop. These types of devices may not protect you.
Predicting when breakthrough will occur is not always easy or accurate. It may be beneficial to replace the cartridge more often than the manufacturer recommends. In some cases cartridge replacement should be performed twice as often as recommended by the manufacturer. Drop in water pressure, change in taste or sediment in the water indicate filter malfunction. When these occur, the cartridge should be replaced.
AC filters can be excellent places for bacteria to grow. A filter saturated with organic contaminants, or one that has not been used for a long time, provides ideal conditions for bacterial growth. A saturated filter supplies the food source for the bacteria. It is still unclear whether bacteria growing on the carbon pose a health threat. Some manufacturers place silver in the AC to prevent bacterial growth. The effectiveness of the silver has not been independently verified. In addition, the silver may contaminate the drinking water.
The above considerations have led public health officials to consider AC home treatment a temporary solution. It should be used only until the source of contamination can be eliminated and the water supply is safe. Even with proper installation, maintenance and operation, malfunction of home water treatment systems can occur.
AC filters vary in cost as much as in effectiveness. Good under-the-sink models cost between $200 and $700, while POE devices can cost as much as $3,000. End-of-faucet devices can be purchased for as low as $10. (NOTE: Dollar values are provided as a rough guide to compare costs of different systems. Current prices are likely to be higher than those quoted.)
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Certification of treatment products is available from independent testing laboratories, such as the National Sanitation Foundation (NSF). Results from NSF tests provide good measures of the effectiveness of devices designed to treat water for both esthetic and health reasons. The Water Quality Association (WQA) is a self-governing body of manufacturers and distributors. WQA offers voluntary validation program to its members. Validation is less stringent than certification. Certification or validation does not ensure effective treatment; all systems must be designed for each particular situation and maintained properly.
Home water treatment using AC is one option often used by people with a drinking water quality problem. AC is considered the best home method for treating certain organic compounds. However, it is not recommended for metals and other ions commonly found in drinking water.
The selection of an AC filter should be based upon water analysis and a thorough assessment of the individual homeowner's situation. A well-informed decision is a your best insurance for protecting health.
For further information on water quality contact your county Cooperative Extension office or local health department. The following bulletins in the WQ series may also be helpful:
- WQ 1 "Water Testing Laboratories"
- WQ 2 "What Is Ground Water?"
- WQ 3 "How to Take a Water Sample"
- WQ 4 "Why Test Your Water?"
- WQ 5 "Interpreting Water Test Results Part One: Inorganic Materials"
- WQ 6 "Buying Home Water Equipment"
- WQ 9 "Water Quality for Animals"
- WQ 10 "Wetlands and Water Quality"
- WQ 11 "Sulphur Water Control"
- WQ 12 "Distillation For Home Water Treatment"
- WQ 14 "Reverse Osmosis for Home Treatment of Drinking Water"
- WQ 16 "Bacterial Contamination of Household Water"
Kamrin, Michael, Nancy Hayden, Barry Christian, Dan Bennack and Frank D'Itri, WQ 23 ``Home Water Treatment Using Activated Carbon,'' Cooperative Extension Service, Michigan State University, 1990.
*Reviewed and revised by Adel Pfeil, Extension Specialist, Department of Consumer Sciences and Retailing.
Editor: Cheri L. Janssen, Department of Agronomy
Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.