Activated Carbon Filters 101

The detoxification of carbon was first discovered by ancient Egyptians. Today, carbon has become a water treatment method used in practically every home. Utilized in many different forms, activated carbon filters help food and beverages taste and smell their best. In this article, you can learn how activated carbon filters work, the types of carbon filters, and their alternatives.

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What is a carbon filter?

A carbon filter is a form of water treatment with remarkable chlorine, foul taste, and bad odor reduction capabilities. It uses a charred form of media, typically wood or coconut shells, to attract contaminants, trapping them as they flow by. Water treatment plants treat water with chlorine and chloramines that linger in drinking water, tainting it with a chemical flavor. Carbon clears water of organic compounds that make your water taste and smell bad.

How does a carbon filter work?

Carbon filters remove contaminants through a process called adsorption. Absorption soaks up particles like a sponge to water. Adsorption, on the other hand, adheres particles to a surface like a piece of Velcro. Organic compounds bond or stick to the surface of a carbon filter because water and contaminants are both polar compounds that attract one another.

Carbon filters are extremely porous and have a large surface area, making them effective at reducing bad tastes, odors, and other particles in water. A carbon filter acts as a parking lot with pores as parking spaces for contaminants as water flows through. The tiny pores are measured in microns. The smaller the micron, the finer the filtration. Low flow rate and pressure give contaminants more time to adhere to the carbon. The more contact time water has with the surface of a carbon filter, the more efficient the filtration.  

Carbon vs. charcoal filters

Carbon filters are often referred to as charcoal filters, but there are a couple differences between these filter types. First, charcoal filters are only made of wood, while carbon filters can be made from a variety of materials, such as wood, coconut husks, coal, peat, and others. Charcoal filters are generally of worse quality than carbon filters because they contain less carbon than activated carbon filters. Carbon filters are purer than charcoal filters and are overall a better option. However, charcoal filters can still effectively reduce levels of certain contaminants in water.

What is a carbon filter made of?

Activated carbon filters are most commonly made up of one of the following materials:

• Bituminous coal
• Wood-based media
• Coconut shell media

Of these three types of filter media, coconut shell carbon is the most renewable. This type of carbon is made from the shell of a coconut rather than the contents inside, so it doesn't cause allergic reactions or add flavor to water. Wood-based carbon is made from burned wood ground into a granule and resembles what the ancient Egyptians would have used. Bituminous coal is used less frequently today because traces of arsenic have been discovered in the media. 

Types of carbon filters

Granular activated carbon filters

GAC filters contain loose granules of activated carbon that allow water to flow through easily. Water flows in one direction through the cartridge and contacts carbon in the process. 

Advantage: Not as restrictive. Water flows through the carbon at a faster rate. 

Disadvantage: Channeling. Water can cut a path through the carbon and allow contaminants through. 

Carbon block filters

Carbon block is made of fine granules held together with a bonding agent, which only takes up 15% of the surface area. Impure water flows in through the side of the filter and sends filtered water out through the top. As carbon is ground to a finer granule, the surface area increases. 

Advantage: Gives 7-10 times more surface area than GAC filters, and the compact carbon prevents channeling. 

Disadvantage: More flow restrictive.

Radial flow GAC carbon filters 

Radial flow carbon filters combine the surface area of the carbon block with the flow rate of the GAC filter. Water flows to the filter through the side of the cartridge like a carbon block but contains granules for increased flow like the GAC.

Are carbon water filters safe?

Carbon water filters are safe, especially if they've been rated by a third party for material safety. All carbon filters are rated for CTO (chlorine, taste, and odor) removal, and sub-micron carbon blocks remove other contaminants like lead or cysts. Activated carbon block filters with sub-micron ratings go above and beyond to remove additional particles through mechanical filtration. Mechanical filters work like a screen door, keeping unwanted elements out and letting clean water through. Pores of a carbon block filter that measure less than one micron are too small for cysts to pass through.

What do activated carbon filters remove?

Activated carbon filters are best at removing chlorine, bad tastes, and foul odors, but some may be certified to remove other contaminants. Look for the NSF certification on a carbon filter to find out exactly what it can remove.

• Bad tastes and odors? YES
• Chlorine? YES
• Trihalomethanes (THMs)? YES
• Mercury? YES
• Pesticides and herbicides? YES
• Iron or heavy metals? If certified
• Lead? If certified
• Bacteria? If certified to remove coliform
• Hard water? NO
• Fluoride? NO
• Total dissolved solids (TDS)? NO

Dissolved minerals like calcium and magnesium flow through a carbon filter undeterred. However, a carbon filter with a pore size smaller than one micron can remove coliform, cysts, lead, arsenic, iron, and other heavy metals through mechanical filtration. Even though these contaminants do not adhere to the carbon, they cannot fit through the small pores. 

Often materials are added to carbon to improve its reduction capabilities. Adding silver to carbon creates a media that kills bacteria. Iron-reduction carbon includes KDF (Kinetic Degradation Fluxion) media made of ground copper and zinc to create a chemical reaction. The KDF converts heavy metals from a ferrous (dissolved) to a ferric (solid) state to capture the solids in the matrix of the carbon. Although certain carbon blends can reduce coliform, a UV system disinfects bacteria-infested water most effectively.

Activated carbon and chlorine

Removing chlorine is the most common reason to use a carbon filter. Chlorine makes your food, beverages, and drinking water nasty and emits a gas that you can inhale in the shower. Chlorine does not adhere to carbon. Instead, a carbon filter removes chlorine through a chemical reaction. Activated catalytic (more reactive) carbon chemically alters the chlorine molecules, converting them into chloride.

Learn more: How to remove chlorine from drinking water

Do carbon filters remove chloramines?

Many water treatment plants use chloramine to disinfect water because it is a stable compound that does not dissipate like chlorine or create by-products like trihalomethane. However, chloramine gives water a foul taste and smell. Chloramines are more difficult to remove than chlorine, so catalytic carbon is used. When chloramine hits the carbon filter, the carbon breaks the ammonia from the chlorine and turns it into chloride. 

Learn more: How does city water treatment work?

Where are carbon filters used?

Carbon filters are used in many applications. They can be used as point-of-entry (POE) systems to filter the whole house or point-of-use (POU) filters to clean water before you drink or cook with it. Refrigerator water filters often contain a blend of sediment and carbon filtration. Some shower heads include carbon filtration to prevent negative chlorine effects on skin and hair. Carbon filters are also part of a reverse osmosis system or an ultrafiltration (UF) system. A carbon filter added to a UF system provides organic and chemical particulate reduction along with lead reduction.

Learn more: What is ultrafiltration?

If you use a water softener to soften water treated by a municipal plant, then you should install a carbon filter before the water softener. If chlorine is removed prior to softening, then the softener resin lasts longer. Chloramines can cause O-rings, gaskets, and rubber seals to deteriorate, so removing chemical disinfectants with catalytic carbon helps water-using appliances, like your tankless water heater, last longer.

Learn more: What is a water softener work and how does it work?

How often should you change your carbon filter?

Change your carbon filter every six months to one year. Never use a carbon filter for longer than one year. Waiting to change the filter could make your water worse than it was before treating it. If the pores of a carbon filter are full, collected particles will begin to break from the carbon and flow into your water supply. 

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Alternatives to carbon water filters

Sediment filters

Carbon and sediment filters serve two different functions. Sediment filters catch dirt and debris, while carbon captures organic compounds that cause bad tastes and odors. A sediment filter is a mechanical filter. It has a certain porosity that water can flow through but traps anything larger than the micron rating of the filter. Because a carbon filter removes particles by adsorption, it does not make a good sediment filter. You often need one of each.

Learn more: What is a sediment filter and how does it work?

Ceramic filters

A ceramic filter is another type of mechanical filter. Unlike sediment filters, ceramic filters contain a pore size small enough to block some living organisms. Often, carbon is either blended with ceramic or added to the inner core of a ceramic filter to enhance the scope of reduction or target specific types of contaminants. Adding carbon inside of a ceramic filter gives both carbon and sediment filtration to reduce some chemicals and debris. 

Learn more: What is a ceramic filter and how does it work?

Reverse osmosis systems

Carbon filtration is a part of the reverse osmosis (RO) treatment process. It is present in at least one stage of reverse osmosis systems, but it is also common for an RO system to contain two or three carbon filters. Unlike a simple carbon filter, a reverse osmosis system uses a membrane to separate dissolved inorganic compounds and radionuclides. Contact with chlorine can damage the RO membrane, so carbon filters are used within the system to protect the membrane.

Learn more: What is a reverse osmosis system and how does it work?

If you have any additional questions, please do not hesitate to contact us.

Effective Against: 

• unwanted tastes
• odors
• chorline
• detergents
• radon

• some man-made, organic chemicals such as many pesticides

• volatile organic chemicals, such as paint thinners

Not Effective Against: 

• microbial contamination (eg. coliform bacteria)
• heavy metals (eg. sodium, nitrate, flouride, and lead)

How Activated Carbon Treatment Works

Activated carbon filters are used to remove unwanted tastes, odors, radon, and some man-made volatile organic contaminants from drinking water. The efficiency of the unit depends on the type of activated carbon installed, the filter bed depth, the type of contaminants in the water and their concentration, and the contact time between the water and the carbon filter. Activated carbon filters do not adsorb every type of contaminant equally well. Carbon filters are easy to install and maintain, and operating costs are usually limited to filter replacement. Depending on the type and concentration of the contaminant being removed, some carbon filters may require special hazardous waste handling and disposal, which can be costly. Other filter types are available including charcoal and ceramic materials that treat the water similarly, through adsorption of the contaminant onto the filter.

The solid material used in an activated carbon filter is a specialized carbon manufactured for this purpose. Contaminants adhere to the surface of these carbon granules or become trapped in the small pores of the activated carbon. Generally, an activated carbon filter is used with a pre-treatment filter to remove sediment or iron particles that may be present and can clog the carbon filter.

Granular activated carbon (GAC) treatment is most common for private water systems. Granular activated carbon is an effective filter for removing organic chemicals that may be in drinking water. Many other small devices use carbon that has been pressed into a solid block. Use of block carbon may result in a significant drop in home water pressure. These devices also clog easily if the water is cloudy or turbid.

A synthetic resin may be a substitute for carbon. Although activated carbon is efficient in removing a variety of organic chemicals, a specially formulated synthetic resin may be a better absorber for a specific contaminant.

Types of Units

Activated carbon filters come in both pointof- entry (POE) units and point-of-use (POU) units. POE and POU devices refer to the location where the device is installed. POE units treat all the water coming into the house. POU units treat water at specific areas, i.e. below the kitchen sink to treat only tap water in the kitchen. POU units include pour through, faucet mount, in-line, and line bypass.

All activated carbon units differ in level of treatment, installation, storage requirements, and cost. Also, some devices are better at treating certain contaminants than others. Discuss the options with the product distributor. In addition, confirm that the treatment unit has been tested to meet manufacturer&#;s claims. See the Product Certification section at the end of this factsheet.

Pour-through units

similar in design to a drip coffee maker, are the simplest type of activated carbon filter. A quantity of untreated water is poured through the carbon, and the treated water is collected in a receptacle. These units are not connected to the water supply and usually sit on the counter. They are portable, require no installation and are convenient for camping or picnicking. Pour-through devices will treat small quantities of water at a given time and are not generally as effective as larger, automatic units.

Faucet mount units

are attached to the faucet (usually in the kitchen) or placed on the counter with a connection to the faucet. !ere are two basic designs: the bypass option, which has a valve to filter water used for cooking and drinking (this prolongs the life of the carbon cartridge), and the non-bypass option, which filters all water passing through the faucet. Because the quantity of carbon contained in a faucet-mount unit is not large enough to provide extensive contact time with the water, these devices are not recommended for removal of organic chemicals.

The in-line device

is installed beneath the kitchen sink in the cold water supply line to treat water for drinking or cooking. If both hot and cold water come from a single faucet, the treated (cold) water can mix with the untreated (hot) water. Treated water is assured only when using cold water for drinking and cooking.

The line-bypass unit

is also attached to the cold water pipe, but a separate faucet installed at the sink provides treated water for cooking and drinking. !e regular tap delivers untreated water. This design increases the life of the carbon by allowing a choice of treated or untreated water, depending on the intended use.

Whole house treatment or point-of-entry

is recommended for treatment of volatile organic compounds (VOCs). Since VOCs easily vaporize from water into the air, POE treatment prevents inhalation of hazardous vapors from the shower, dishwasher, washing machine, or other times when large amounts of water are used, or exposure through skin contact. !is device should meet certain guidelines concerning the application rate of water to the carbon, contact time between the water and the carbon, the type of carbon used, and the concentration of contaminants to be removed.

Unit Effectiveness

The effectiveness of an activated carbon unit depends on the extent of contact between the carbon and the untreated water. During regular operation of any device, channels form within the carbon filter, allowing some water to bypass the filtering material, short-circuiting treatment. Since treatment depends on the carbon granules adsorbing the chemical contaminants, these channels decrease the contact time and the effectiveness of the carbon filter unit. A disadvantage of block carbon is that if it is unevenly compressed when manufactured, irregular flow patterns may affect the effectiveness of contaminant removal.

There are two main procedures to determine how much a carbon filter can adsorb: the iodine test and the phenol test. The iodine number is defined as the amount of iodine (in milligrams) adsorbed by one gram of carbon under a certain set of conditions. The larger the number, the more the carbon filter can adsorb. Phenol is another measure of effectiveness. The lower the phenol number, the better the carbon is at removing organics. When comparing home carbon devices, pay attention to these values.

Please keep in mind that manufacturers tend to report the best removal rate for their product. The values are frequently the amount the unit will remove at the beginning of use for a single contaminant at moderate concentrations. Units having a larger bed volume of carbon usually remove a greater quantity of contaminants. Some types of activated carbon will remove specific contaminants better than others. Buyers need to examine as many products as possible, and know exactly what contaminant they want the unit to remove. You will also need to know how much water is used in the home on an average day to adequately size the treatment system. Additionally, have both the raw water (prior to treatment) and the treated water retested after a unit is installed to ensure it is functioning properly.

Maintenance

Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless maintained in accordance with the manufacturer&#;s recommendations for maintenance, cleaning, and part replacement. Keep a logbook to record equipment maintenance and repairs.

Most activated carbon filter units need to have the carbon changed periodically. For small specialty units, the entire unit is normally replaced. Cartridge filters are the easiest to change. Before purchasing a unit, consider the ease of opening the filter housing and the amount of space required to change the filter.

Service periods vary greatly. Small units with heavy loads of contaminants may need replacement monthly or more often, while a six-month service interval is frequently advised for the cartridge filters. Since some filters treat up to a particular amount of water, a water meter installed on the filter can help judge when carbon replacement is necessary.

Depending on the type and concentration of the contaminant being removed, some carbon filters may require special hazardous waste handling and disposal, which can be costly. An example would be carbon filters used to remove radon in drinking water. It is important to establish beforehand how handling and disposal will work and whether alternative treatment methods or locating alternative water sources (such as installing a new well in a new location, tying into a public water supply if available, or using bottled water) would be more effective in the long run.

Other Considerations

Ensure the system you choose is installed and operated according to the manufacturer&#;s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the raw and treated water annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water. Frequent testing will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

Activated carbon is an excellent medium for growing bacteria. Bacterial growths will plug and coat the activated carbon and reduce the effectiveness of the filter and can introduce bacteria into your drinking water.

Silver is often added to the carbon media of some carbon filters with a claim that it controls bacteria growth. However, the U.S. Environmental Protection Agency (EPA)- sponsored tests have shown that silver is not effective in treating bacterial growth in activated carbon filters. Allowing the water to run through the filter for at least 30 seconds is a good practice to flush out bacteria that may have accumulated and grown in the filter. Backwashing and regular filter replacement also help reduce bacteria build-up.

Questions to Ask Before You Buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present and their concentration. This will help you determine if activated carbon is an effective treatment method for your situation. See the factsheet Questions to Ask When Purchasing Water Treatment Equipment for more information.

Consumers should inquire about the following before purchasing an activated carbon filter:

• What type of filter best meets your water supply needs?
• What contaiminants will the filter remove?
• Can the homeowner replace the filter or is a service technician required? At what cost? Is special handling and disposal of hazardous waste required due to the type of contaminant being treated? What should the frequency of monitoring and filter replacement be?
• How do you determine when replacement is necessary?
• Where can you purchase filters and what is the cost?
• Are there any costs for disposing of used filters?
• Has the treatment system been tested and certified by a third party to ensure that it meets manufacturer's claims?
• Are there any special installation requirements that may add to the equipment costs, for instance changes to your household plumbing?

Product Certification

NSF International is a non-profit organization that sets performance standards for water treatment devices. Because companies can make unsubstantiated statements regarding product effectiveness, the consumer must evaluate test results of the device to determine if claims are realistic. Products that have been tested or evaluated by NSF and meet their minimum requirements are entitled to display the NSF listing mark on the products and in advertising literature for products. Manufacturers and models that meet NSF&#;s standard are included in a listing published twice a year. For more information contact NSF at: 800-NSF-MARK (800-673-) or http://www.nsf.org/consumer-resources

Resources

This publication is adapted from a URI fact sheet by the same name produced by the Rhode Island Department of Health and the University of Rhode Island Cooperative Extension Water Quality Program.

This project was funded, in part, by a grant from US EPA.

This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. --.

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