By Jodi Kocher, Bruce Dvorak & Sharon Skipton
Summary: Carbon filtration is an effective way to reduce certain organic compounds and chlorine in drinking water. It can also reduce lead, dissolved radon, and harmless taste and odor-causing compounds. This article discusses the principles, processes and requirements of activated carbon filtration systems for household users.
Homeowners are increasingly concerned about contaminants in their water supply that may affect health or cause taste and odor problems. Sources of these contaminants might include solvents, pesticides, industrial wastes or contaminants from leaking underground storage tanks. Contaminants such as benzene, chlorobenzenes, trichloroethylene, carbon tetrachloride, methylene chloride and vinyl chloride in drinking water may pose health risks if present in quantities above the USEPA Health Advisory Level (HAL). Activated carbon (AC) filtration can be effective in reducing some of these organic chemicals as well as certain harmless taste and odor-producing compounds.
With disinfection, the reaction of chlorine with organic matter during drinking water chlorination can produce compounds such as trihalomethanes (THMs) as by-products. These disinfection by-products (DBPs) may increase the risk of certain cancers. The USEPA mandates that public systems have less than 80 parts per billion (ppb) of THMs in their treated water. Activated carbon filtration can be effective in removing chlorine and some DBPs from drinking water.
In addition, lead from water pipes and joints may be present in water from the tap. AC filtration can reduce lead in drinking water, though another filter medium is commonly used in addition to AC for this purpose. Only very specialized AC filters effectively adsorb heavy metals. Radon, a radioactive decay product of natural uranium that’s been related to lung cancer, can be found in some groundwater. Radon gas can also be removed by AC filtration, though removal rates for different types of AC equipment haven’t been established.
No one piece of treatment equipment manages all contaminants. All treatment methods have limitations and often a combination of treatment processes is required to effectively treat the water. Different types of carbon and carbon filters remove different contaminants and no one type of carbon removes all contaminants at maximum efficiency. AC filters generally won’t remove microbial contaminants (such as bacteria and viruses), calcium and magnesium (hard water minerals), fluoride, nitrate and many other compounds.
Regardless of the water treatment system being considered, the water should first be tested to determine what substances are present. Public water systems are routinely tested for contaminants. Water utilities are required to publish Consumer Confidence Reports (CCRs), which inform consumers on the source of water, contaminants present, potential health effects of those contaminants and methods of treatment used by the utility. Depending on the population served by the utility, CCRs may be mailed, published in newspapers or posted on the Internet. A copy of the CCR can be obtained from the local water utility. Public supplies must conform to federal standards established by the Safe Drinking Water Act. If contaminants persistently exceed a Maximum Contaminant Level (MCL), the water must be treated to correct the problem and/or another source of water suitable for drinking must be provided.
In contrast, monitoring of private water systems is the responsibility of the homeowner. Therefore, contamination is more likely to go undetected in a private water supply. Knowledge of what contaminants may be present in the water should guide the testing since it’s not economically feasible to test for all possible contaminants. It’s essential to know what contaminants are present, their quantities, and reasons for removal, i.e., to reduce contaminants posing health risks, remove tastes or odors, etc., prior to selecting treatment methods or equipment.
There are two basic types of water filters: sediment (or mechanical) filters that filter particles by size and adsorptive, and reactive filters that contain a medium that adsorbs or reacts with a water contaminant. AC filtration is an adsorptive process in which the contaminant is attracted to and held (adsorbed) onto the surface of the carbon particles. Efficiency of the adsorption process is influenced by characteristics of the carbon (particle and pore size, surface area, density and hardness) and characteristics of the contaminant (concentration, tendency of chemical to leave the water, contaminant solubility, and contaminant attraction to the carbon surface).
The medium for an AC filter is typically petroleum coke, bituminous coal, lignite, wood products, coconut shell or other shells, which are all sources of carbon. It’s “activated” by subjecting it to steam and high temperature (2,300°F) without oxygen. In some cases, the carbon may also be processed by an acid wash or coated with a compound to enhance the removal of specific contaminants. This activation produces carbon with many small pores and, therefore, a very high surface area. It’s then crushed to produce a granular or pulverized carbon product. This creates small particles with more outside surface area available for compounds to enter the AC, which results in greater contaminant removal. The source of carbon and activation method determine the effectiveness of removal for specific contaminants. For instance, the carbon that most effectively removes lead is obtained from a different source and activation method than the carbon that most effectively removes chlorine.
If more than one contaminant is present in the water, those contaminants easily and strongly adsorbed to the carbon will adsorb in greater quantity than less well-adsorbed contaminants. This is called competitive adsorption and can significantly affect the dynamics of carbon adsorption. Competitive adsorption can result in a less well-adsorbed compound leaving the filter while a better-adsorbed compound is still being removed. The length of contact time between the water and carbon, which is determined by water flow rate, also affects contaminant adsorption. Greater contact time allows for greater adsorption of contaminants. Also, the amount of carbon in the filter affects contaminant removal. For instance, less carbon is generally required to remove taste and odor-producing compounds than for trihalomethane (THM) removal.
The amount of carbon can also affect how quickly the carbon becomes full (saturated). When all the adsorption sites on the activated carbon become full of contaminants, the filter is saturated and has reached its capacity. At this point, some contaminants may not be adsorbed, or some may move from the carbon back into the water. This is called breakthrough since the contaminant “breaks through” the filter and is in the “treated” water. When this occurs, it’s possible the contaminant concentration in the “treated” water may actually be higher than in the untreated water. To prevent breakthrough, some AC filtration units will shut off the water supply after a specified number of gallons are treated; most units, however, don’t have this feature. Using two AC filters or cartridges in series can help safeguard against breakthrough.
AC filtration units can be either point-of-use (POU) or point-of-entry (POE) treatment. A POE device treats all water coming into the house. This type of setup is recommended for treatment of some nuisance compounds, radon and volatile organic compounds (VOCs). VOCs can easily vaporize from water in showers, washing machines and dishwashers, and come in contact with skin. A POE device that reduces the contaminant at the point of entry is appropriate for such a situation. Devices for this purpose should meet guidelines for contact time, the type and amount of carbon used, and the wastewater discharge.
POU devices treat water at a point or points of use within the house and are useful for removal of lead and chlorine. POU models can be either in-line, line-bypass, countertop, faucet-mounted or pour-through. An in-line device is installed beneath the kitchen sink in the cold water supply line. In this situation, if hot and cold come through one spigot, the treated cold water mixes with the untreated hot water; only the cold water can be considered treated. In the line-bypass system, a separate faucet is installed at the sink. The unit is attached to the cold water pipe and provides drinking and cooking water with the regular tap providing untreated water for non-consumptive use. This arrangement increases life of the carbon by allowing a choice of treated or untreated water, depending on use.
Faucet-mounted devices are attached to the faucet or sit on the counter with connections to the faucet. These can have a bypass valve that allows selective filtering, if water is going to be used for cooking or drinking, which prolongs life of the carbon. Pour-through models (such as pitchers with a filter) are the simplest type of AC filter. Water is simply poured through the carbon and collected in a container. These units aren’t connected to the water supply. Both pour-through and faucet-mounted units are inexpensive and simple, but will treat only limited quantities of water at a time and aren’t as effective as other devices because contact time is limited due to the small amounts of carbon contained in
One measure of the AC’s capacity to remove organic compounds is the iodine number. This is the amount of iodine (in milligrams) adsorbed by one gram of AC under set conditions. A higher iodine number generally indicates greater adsorptive capacity.
Carbon cartridges must be replaced regularly. Replacement intervals should be determined based on daily water flow through the filter and the contaminant being removed. Some manufacturers state a recommended water treatment capacity in gallons, beyond which the AC should be replaced.
Most devices on the market don’t indicate how much water has passed through the filter, so a consumer must estimate the number of days the filter will last before needing replacement. To do this, assume one person uses one gallon per day for drinking and uses one to three gallons per day for cooking. For example, a household with four people would use four gallons per day for drinking and possibly one gallon for cooking for a total of five gallons of water use per day. A 200-gallon capacity filter would last 40 days under these conditions (200 gallons/5 gallons a day = 40 days). These estimates, however, don’t take into consideration the contaminant being removed and its concentration. Tests done by the Rodale Press Product Testing Department indicated that filter performance was reduced significantly after 75 percent of the manufacturer’s recommended lifetime. Therefore, it may be safer to replace the filter more often than recommended by the manufacturer. Retailers can assist in determining replacement intervals as well; however, the only way to be certain if a filter is successfully removing contaminants is by repeated testing of the filtered water for the contaminants that are to be removed.
Some systems claim to alert the user when the cartridge should be changed. This may be determined by a pressure drop across the filter; however, a pressure drop may or may not result from reaching the filter’s adsorption capacity, as saturation and breakthrough can occur long before a pressure change occurs. When a change in water pressure occurs, or a change in taste, odor or sediment is noticed, malfunction is indicated and the filter should be replaced.
AC filters that have been idle for a number of days, or which are saturated with organic matter, provide an excellent environment for bacteria to grow. There’s little risk to healthy people consuming harmless (non-pathogenic) bacteria found on most AC filters; however, there may be some concern for the very young, very old, and those with weakened immune systems. Flushing the filter for about 30 seconds after it has sat idle for several hours (such as in the morning) may help limit the amount of bacteria on the filter. Some filters are impregnated with silver to try to prevent bacterial growth. Studies have indicated that this practice makes little difference in reducing bacteria. Any advantage seen from silver is only apparent in the first month of use. The best practice is to replace the filter as often or more often than the manufacturer recommends.
AC filtration products with silver in them are registered with the USEPA. It’s important to realize this doesn’t guarantee the device is effective or has been tested or endorsed by the USEPA. It’s required that water treatment equipment with an active ingredient intended to prohibit growth of microorganisms be registered with the USEPA. A sediment or particulate filter installed ahead of any AC unit will prolong the life of the filter. Sediment can easily and quickly clog pores of an AC filter and a good sediment filter can be obtained for a fraction of the price of most high-volume AC filters. Cost of AC equipment varies with the type of filter installed. Pour-through and faucet- mounted filters for taste and odor removal are generally less costly than high-volume units used for reducing health risks from specific trace contaminants.
Federal, state or local laws don’t regulate activated carbon filtration POU/POE home systems. The industry is self-regulated. NSF International and the Water Quality Association evaluate performance, construction, advertising and operation manual information. The NSF program establishes performance standards that must be met for endorsement and certification. The WQA program uses the same NSF standards and provides equivalent American National Standards Institute (ANSI)-accredited product certifications. WQA-certified products carry its Gold Seal. Though these certifications and validations shouldn’t be the only criteria for choosing an AC system, they’re helpful to ensure effectiveness of the system.
Drinking water treatment using activated carbon filtration is one option for a homeowner to treat drinking water problems. AC is an effective method for treating certain organic compounds, unpleasant tastes and odors and chlorine, though it’s not effective for metals, nitrates, microbial contaminants and other inorganic contaminants. Selection of an AC system should be based on water analysis and assessment of the individual homeowner’s needs and situation. Regular replacement of the filter/cartridge is a critical factor in maintaining effectiveness and reducing bacterial contamination of the filter. NSF and the WQA test and certify products and this certification can help guide selection.
About the authors
Jodi Kocher is an extension engineer, Bruce Dvorak is an extension environmental engineering specialist, and Sharon Skipton is an extension educator at the University of Nebraska-Lincoln Cooperative Extension, Institute of Agriculture and Natural Resources. This article first appeared in the institute’s online library in its October 2003 issue. It’s re-printed here with slight wording changes. The authors can be reached at (402) 472-3305, email: [email protected] or website: http://ianrpubs.unl.edu