Home Water Treatment Using Activated Carbon
Michigan State University Extension
MSU Extension Water Quality Bulletins - WQ239201 07/14/97

Introduction

Activated carbon (AC) filters have been employed in home water purification systems primarily for removal of taste and odor. Taste and odor, although undesirable, are generally not considered unhealthy. In recent years, however, AC filters have been used for the removal of some of the contaminants that have been discovered in water supplies.

This bulletin discusses the use of AC filters for home water treatment. 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 all treatment systems, it cannot be used for removal of all possible drinking water contaminants.

Because AC systems are limited in the types of compounds they can effectively remove, it is essential that the water contaminants be determined prior to the purchase of such a system. Anyone with a suspected water quality problem should first obtain a water analysis from 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 aid in the interpretation of the water analysis results.

Activated Carbon

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 accumulate on the surface of the AC. This is called adsorption. The two main reasons that chemicals adsorb onto AC is 1) a "dislike" of the water and 2) attraction to the AC.

Adsorption of most contaminants results from a combination of 1 and 2. Many organic compounds, such as chlorinated and nonchlorinated solvents, gasoline, pesticides and trihalomethanes can be adsorbed by AC. AC is also effective for removal of chlorine and moderately effective for removal of some heavy metals. Metals that are bound to organic molecules will also be removed by AC. Fluoride, chloride, nitrate, hardness (calcium and magnesium) and most metal ions are not removed by AC to any significant degree.

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.

AC Filters

Home AC treatment systems are quite simple. The AC is normally packaged in filter cartridges which 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). This means that they are installed near the point of use, 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 by-pass 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. A point-of-entry (POE) system is more appropriate, if a contaminant is present which may 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 when water is used for showering and washing. In this case, it is more economical to have a large POE system which treats water as it enters the home than POU systems 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 that the 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 characteristics affecting the amount and rate of pollutant removal. More carbon in a cartridge means more capacity for chemical removal and, therefore, leads to 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. Clogging by rust, scale, sand or other sediments can be a problem with any AC filter. A solution to this problem is the placement of 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 can cause a drop in water pressure.

Design of any AC filter must ensure that the filter is 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 is dependent 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.

Operation, Maintenance and Cost

AC filters have a limited lifetime. The surface of the AC will eventually 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. The cartridge, at this time, needs to be replaced. Knowing when breakthrough will occur and thus 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, breakthrough 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 this happens. 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 of the treatment device can make better estimates of the filter's useful lifetime based on water usage (flow rate) and pollutant concentrations shown in the chemical analysis. These calculations should be done before purchasing the system. However, it should be remembered that this will not be successful if pollutant concentrations increase over time and testing is not performed to reveal this change.

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 that can pass through the filter before breakthrough occurs and the homeowner can replace the filter when this number is reached. Remember, any prediction for filter replacement must be based on the actual pollutant concentrations present in the water. This again emphasizes the necessity of an initial water analysis and periodic routine analyses.

Some systems are sold with claims that the device will alert the user when replacement of the cartridge is needed. This is based on a pressure drop across the filter, which 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. Thus, these types of devices may not protect you. The uncertainty in predicting breakthrough suggests that there may be benefit in replacing the cartridge more often than the manufacturer recommends. It has been suggested that in some cases, cartridge replacement be performed twice as often as recommended by the manufacturer. In addition, reduction in water pressure, change in taste, or sediment in the water are indicators of filter malfunction. When these occur, the cartridge should be replaced.

Another problem with AC treatment is that AC can be an excellent place 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 may pose a health threat, or whether they are harmless.

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 primarily 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 filters vary in cost as they do 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. Tests show that under-the-sink models generally have more carbon, superior performance and greater convenience than faucet or countertop models.

Certification and Validation

POU and POE home systems are not regulated by federal, state or local laws. The one exception, is that the US EPA does require AC filters with silver or other chemicals that may kill bacteria, be registered. Registration does not imply that EPA has tested or endorses use of these filters. The industry is self regulated. Performance, construction, advertising, and operation manual information
are evaluated by the National Sanitation Foundation (NSF) and the Water Quality Association (WQA). The NSF program is generally considered a certification program and establishes many performance standards which must be met for endorsement. The WQA program has less stringent standards and may be considered a validation program. Certification and validation are helpful in ensuring effectiveness but should not be regarded as the major criterion for a choosing a particular AC treatment system.

Summary

Home water purification using AC is one option to be considered 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 that can also be common contaminants of drinking water.

The selection of an AC filter should be based upon water analysis and a thorough assessment of the individual homeowners situation. A well informed decision will be the best insurance for protecting health.

AC Filter Guidelines

Make sure the filter contains AC.
Know the quantity of AC in the filter since this will determine the amount and rate of pollutant removal.
Use prefilters to add life to AC filters.
Replace both prefilters and AC filters regularly.
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.

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Home Water Treatment Using Activated Carbon Michigan State University Extension MSU Extension Water Quality Bulletins - WQ239201 07/14/97