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|Activated carbon filtration is an effective method of reducing certain organic compounds and
chlorine in drinking water. It can also reduce the quantity of lead, dissolved radon, and harmless
taste and odor causing compounds. This guide discusses the principles, processes and
requirements of activated carbon filtration systems for the domestic (household) user.
Contaminants removed from water by activated carbon filtration
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 EPA 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
During disinfection the reaction of chlorine with organic matter during drinking water chlorination
can produce compounds such as trihalomethanes (THMs) as byproducts. These disinfection
byproducts may increase the risk of certain cancers. The EPA 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 disinfection byproducts 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 has 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 have not been established.
Contaminants not removed by activated carbon filtration
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. Activated carbon filters will not
remove microbial contaminants (such as bacteria and viruses), calcium and magnesium (hard
water minerals), fluoride, nitrate and many other compounds. Refer to Extension Circular
EC03-703, Drinking Water Treatment: An Overview for a discussion of possible water quality
problems and appropriate treatments for these contaminants.
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 the water, contaminants that are 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 exceed
the 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 is not economically
feasible to test for all possible contaminants. It is essential to know what contaminants are
present, their quantities, and reasons for removal (i.e., to reduce contaminants posing health
risks, to remove tastes or odors, etc.) prior to selecting treatment methods or equipment.
There are two basic types of water filters: sediment (or mechanical) filters which filter particles by
size and adsorptive or reactive filters which contain a medium that adsorbs or reacts with a water
contaminant. Activated carbon filtration is an adsorptive process in which the contaminant is
attracted to and held (adsorbed) onto the surface of the carbon particles. The efficiency of the
adsorption process is influenced by the characteristics of the carbon (particle and pore size,
surface area, density and hardness) and the characteristics of the contaminant (concentration,
tendency of chemical to leave the water, solubility of the contaminant and contaminant attraction
to the carbon surface).
The medium for an activated carbon filter is typically petroleum coke, bituminous coal, lignite,
wood products, coconut shell or peanut shells, which are all sources of carbon. It is "activated" by
subjecting it to steam and high temperature (2300oF) 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 is 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 the carbon and the 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 that are 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 the carbon, which is determined by the rate of water flow, also affects
contaminant adsorption. Greater contact time can allow for greater adsorption of contaminant.
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 also can affect how quickly the carbon becomes full
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 is
possible that the contaminant concentration in the "treated" water may actually be higher than in
the untreated water. In order to prevent breakthrough, some AC filtration units will shut off the
water supply after a specified number of gallons have been treated; most units, however, do not
have this feature. Using two AC filters or cartridges in series can help safeguard against
Activated carbon 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
POU devices treat water at a point or points of use within the house and are useful for removal of
lead and chlorine. Point-of-use models can be either in-line, line-bypass, 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 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 the 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 the 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 are not 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 are not as effective as other devices
because contact time is limited due to the small amounts of carbon contained in the units. Figure
2 shows the different types of AC filters. 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
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 do not indicate how much water has passed
through the filter, so a consumer must estimate the number of days the filter will last before
To do this, assume that one person uses one gallon per day for drinking and cooking uses one
to three gallons per day. For example, a household with 4 people would use 4 gallons per day for
drinking and possibly one gallon for cooking, for a total of 5 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, do not take into consideration the contaminant being
removed and its concentration.
Tests done by Rodale Press Product Testing Department indicated that filter performance was
reduced significantly after 75% 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 which have been idle for a number of days or which are saturated with organic matter
provide an excellent environment for bacteria to grow. There is 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, the 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 the 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 EPA.
It is important to realize that this does not guarantee that the device is effective or has been
tested or endorsed by the EPA. It is required that water treatment equipment with an active
ingredient intended to prohibit growth of microorganisms be registered with the EPA.
A sediment or particulate filter installed ahead of any AC filter will prolong the life of the AC unit.
Sediment can easily and quickly clog the 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 do not regulate activated carbon filtration POU and POE home
systems. The industry is self-regulated. The NSF (formerly known as the National Sanitation
Foundation) and the Water Quality Association (WQA) 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 the Water Quality Association Gold Seal.
Though these certifications and validations should not be the only criteria for choosing an AC
system, they are 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 is 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. The NSF and the WQA test and certify products and this
certification can help guide selection.