Activated carbon (AC) filters have been used in home water purification systems primarily to remove taste and odor. Taste and odor, although undesirable, are generally not considered unhealthy. In recent years, however, AC filters have been used to remove some of the contaminants that have been discovered in water supplies.
AC is most effective at removing organic compounds such as volatile organic compounds, pesticides and benzene. It can also remove some metals, chlorine and radon. As with any treatment system, it cannot remove all possible drinking water contaminants. Because AC systems are limited in the types of compounds they can effectively remove, it is essential that the homeowner determine which water contaminants are present before purchasing such a system. Anyone who suspects they have a water quality problem should first have their water analyzed by their local health department or a reputable laboratory These analyses are costly, but worth the expense since they are necessary to determine the appropriate home treatment system and how best to operate such a system. A state or local health official can interpret water analysis results. Some laboratories may also provide this service. Note that home water treatment is considered only a temporary solution. The best solutions to a contaminated drinking water problem are to either end the practices causing the contamination or change water sources. AC is a black solid substance resembling granular or powdered charcoal. It is extremely porous with a very large surface area. Certain contaminants accumulate on the surface of the AC in a process called adsorption. The two main reasons that chemicals adsorb onto AC are a "dislike" of the water, and attraction to the AC. Many organic compounds, such as chlorinated and non-chlorinated solvents, gasoline, pesticides and trihalomethanes can be adsorbed by AC. AC is effective in removing chlorine and moderately effective in removing some heavy metals. AC will also remove metals that are bound to organic molecules. It is important to note that carbon is not necessarily the same as AC. AC removes vastly more contaminants from water than does ordinary carbon. Home AC treatment systems are quite simple. The AC is normally packaged in filter cartridges that are inserted into the purification 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. At this time, they need to be replaced. AC treatment systems are typically point-of-use (POU) --- installed where they typically treat water used for drinking and cooking only. AC filters can be placed on the end of the faucet, on the countertop, or under the sink. POU systems often have a bypass so that water for purposes other than drinking and cooking can also be dispensed at the tap without being treated. This increases the life of the AC, reducing the time between filter replacements. (Vis. 1) A Point-of-Entry (POE) system is more appropriate if a contaminant is present that poses 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 when water is used for showering and washing. In this case, it is more economical to have a large POE system that treats water as it enters the home than to have POU systems at each tap. AC filters used for home water treatment contain either granular activated carbon (GAC) or powdered block carbon. The amount of AC in a filter is one of the most important characteristics affecting the amount and rate of pollutant removal. More carbon in a cartridge means more capacity for chemical removal, resulting in 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 any AC filter. A solution to this problem is to place foam or cotton filters (often called sediment or fiber filters) between the cartridge and incoming water. 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 that the pollutants will adsorb to the AC in the time it takes the water to move through the filter. The appropriate filter depth 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. The 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 will be saturated with adsorbed pollutants and no further purification will occur. This is called breakthrough --- the pollutants have broken through the filter to emerge in the treated water. When this occurs, it is possible that the contaminant concentrations in the treated water will be even higher than those in the untreated water. At this time, the cartridge needs to be replaced. Knowing when breakthrough will occur and thus when to replace the cartridge is a major problem with AC treatment. Some cartridges are sold with predictions about their longevity. These are generally only crude estimates since they do not take into consideration factors that are characteristic to a specific water source, such as pollutant concentration. The retailer you purchase the treatment device from can make better estimates of the filter's useful lifetime based on water usage (flow rate) and pollutant concentrations shown in the chemical analysis. Hence, to get the most accurate estimates, you should learn what these amounts are before purchasing the system. Note that if pollutant concentrations increase over time and testing is not performed to reveal this change, such estimates may turn out to be not very practical or useful. Unfortunately, AC filters can be excellent places for bacteria to grow. Conditions for bacterial growth are best when the filter is saturated with organic contaminants, which supply the food source for the bacteria, and when the filter has not been used for a long period of time. It is still unclear whether the bacteria growing on the carbon poses a health threat. Some manufacturers have placed silver in the AC in order to prevent the growth of bacteria. The effectiveness of this procedure has not been independently verified. In addition, silver may contaminate the drinking water. The above considerations have led public health officials to consider AC home treatment a temporary solution to 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 Filter Guidelines 1. Make sure the filter contains AC. 2. Know the quantity of AC in the filter since this will determine the amount and rate of pollutant removal. 3. Use prefilter to add life to AC filters. 4. Replace prefilters and AC filters regularly. 5. Determine appropriate intervals for replacement of AC filters based on contaminant concentration, water characteristics, water flow rate, depth of filter, type and amount of AC and prefilter. Retailers can help in this analysis. This information comes from Michigan State University
