How To Choose The Right Activated Carbon Filter: CTO Or GAC?

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Activated carbon filters play a pivotal role in water treatment, effectively removing contaminants and significantly enhancing water quality. Through a direct chemical reaction with carbon, these filters can eliminate chlorine and other impurities, ensuring cleaner, safer water.

Among the various types of carbon filters, Carbon Block (CTO) and Granular Activated Carbon (GAC) filters are particularly notable. They excel at removing impurities like chlorine, volatile organic compounds (VOCs), and certain heavy metals.

This article will explore the functions, manufacturing processes, and applications of CTO and GAC filters, providing you with the insights needed to make an informed choice for your water filtration needs.

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What is a GAC Filter?

Definition and Materials

Granular Activated Carbon (GAC) filters are produced from raw organic materials like coconut shells or coal, which contain high levels of carbon. These materials go through an activation process to form granules of activated carbon that are used in the filters. During the activation, the carbon is treated with heat and an oxidizing agent, like steam, to produce a porous structure that increases its adsorption capability.

Function and Limitations

GAC filters operate by capturing specific chemicals in the granules as water flows through them. They are highly efficient at eliminating chlorine, volatile organic compounds (VOCs), and undesirable tastes and odors. However, GAC filters are not as effective at removing inorganic impurities like iron and nitrate. The loose structure of the granules enables faster flow rates but may result in less effective filtration due to potential channeling and reduced contact time between water and the carbon surface.

Manufacturing Methods

GAC filters are manufactured by placing activated carbon granules into a plastic housing. End caps are attached to both ends by welding equipment, and non-woven filter pieces are inserted to prevent carbon powder and black water during use. This design allows for customization based on customer requirements. The granules are typically sized between 0.2mm and 5mm, which provides a coarse mesh and sets GAC apart from carbon block filters.


GAC filters provide a much higher flow rate, allowing more water purification in less time. However, their filtration efficiency is lower compared to other filter types. GAC filters are prone to channeling, where water creates paths of least resistance, reducing the effective contact area with the carbon. Additionally, they can harbor bacterial growth in areas where water becomes stagnant.


GAC filters are suitable for large-scale applications such as municipal, commercial, and industrial water treatment systems. They are also used in residential systems where high flow rates are required. Common applications include whole-house filters, point-of-use filters like under-sink units, and pour-through units such as water filter pitchers.

What is a CTO Filter?

Definition and Materials

CTO (Chlorine, Taste, and Odor) filters are carbon block filters made from compressing loose carbon blocks together. Common sources of carbon for these filters include bituminous coal, wood, and nutshell. The carbon is ground into a fine powder and then bonded together using a food-safe adhesive to form a solid block.


CTO filters remove contaminants through adsorption, where impurities are attracted to the surface of the activated carbon and held there. This process is similar to how a magnet attracts and holds iron filings. The high surface area of the compressed carbon block provides numerous sites for adsorption, making CTO filters highly effective at removing a wide range of contaminants.

Manufacturing Methods

CTO filters use coal-activated carbon and coconut shell-activated carbon with high adsorption capabilities. These materials undergo sintering and compression using food-grade adhesive to form a durable cartridge. A layer of non-woven fabric is wrapped around the filter to prevent carbon powder release. This ensures the carbon core remains intact and maintains its filtration efficiency.


CTO filters have high adsorption capabilities and a compact structure, making them more effective at filtration. They are capable of removing fine particulates, VOCs, and other dissolved chemicals. The dense structure of CTO filters provides a longer contact time between water and carbon, enhancing filtration efficiency and extending the filter’s lifespan.


CTO filters are ideal for point-of-use systems, such as under-sink water filters. They effectively remove fine particulates and dissolved chemicals, making them suitable for high filtration efficiency applications. CTO filters are also used in refrigerator water filters and reverse osmosis systems to provide clean, safe drinking water.

the ultimate guide to activated carbon filters comparing cto and gac likefilter

How Activated Carbon Filters Work

Activated carbon filters eliminate impurities from water or air through the adsorption process.

Here is how it works:

1. Adsorption Process

Activated carbon features an extensive surface area with numerous pores that can trap and retain contaminants. As water or air flows through the filter, these impurities adhere to the surface of the carbon. The adsorption efficiency relies on factors such as pore size, pore distribution, and the duration of contact between the water and the carbon.

2. Chemical Reactions

Certain impurities are eliminated through chemical interactions with activated carbon. Chlorine reacts with carbon to form chloride, thus removing it from water. Catalytic carbon filters are specially processed to enhance these chemical reactions, making them particularly effective at eliminating contaminants like chloramines.

3. Pore Structure

The porous nature of activated carbon significantly increases its surface area, which allows for more contaminants to be captured and removed. The efficiency and capacity of the filter are influenced by the size and distribution of these pores, which can vary based on the type of activated carbon used.

4. Regeneration Methods

The activated carbon can become filled with contaminants after being used for some time. To restore the filter’s effectiveness, it can be regenerated either by replacing the activated carbon media or through processes such as washing and heating, which release the trapped contaminants. This regeneration process prolongs the filter’s useful life and maintains its performance.

Comparison of the GAC vs. CTO Filters

1. Structure

  • GAC: Made of loosely held carbon granules. The loose distribution allows for higher flow rates but less effective filtration.
  • CTO: Made of fine carbon powder compressed into a solid block. The compact structure provides higher filtration efficiency and longer contact time with water.

2. Filtration Accuracy

  • CTO: Finer carbon particles offer higher filtration precision, effectively removing smaller particles and dissolved chemicals.
  • GAC: Larger granules provide less effective filtration, with the potential for channeling and bypassing some carbon particles.

3. Dirty-holding Capacity

  • CTO: More contaminants filtered due to the compact structure, leading to a higher dirty-holding capacity and longer filter life.
  • GAC: Lower capacity due to loose granules, requiring more frequent replacements.

4. Flow Rate

  • GAC: Higher flow rate, less effective filtration. Suitable for applications requiring rapid water filtration.
  • CTO: Lower flow rate, more effective filtration. Ideal for applications requiring thorough filtration.

5. Filter Costs

  • CTO: More expensive due to the manufacturing process. Higher initial cost but longer lifespan and better filtration efficiency.
  • GAC: Lower cost, more affordable option. Requires more frequent replacements, leading to higher long-term costs.

6. Applications

  • CTO: Better for reducing particulates and VOCs. Suitable for point-of-use systems and applications requiring high filtration efficiency.
  • GAC: Better for taste, odor, and color removal. Suitable for large-scale and high-flow applications.
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Contaminants Removed by Activated Carbon Filters

Activated carbon filters are efficient in removing and reducing various contaminants from water which include but not limited to below:

1. Chlorine and Chemicals

Activated carbon filters are highly effective at removing chlorine and a variety of chemical compounds, thereby enhancing the taste and odor of water. The interaction between chlorine and carbon results in chlorine being converted into chloride, which is safe for consumption.

2. Volatile Organic Compounds (VOCs)

Activated carbon filters efficiently absorb and reduce VOCs, including substances like solvents, herbicides, and pesticides. VOCs often contaminate water sources due to industrial discharges and agricultural activities.

3. Sediments and Particles

Activated carbon filters excel at trapping and removing sediments such as rust, dirt, and sand. This process enhances water clarity and protects downstream equipment and plumbing from potential damage.

4. Organic Compounds

Activated carbon filters also lower the concentration of specific organic compounds, including benzene, toluene, and xylene. These compounds, commonly introduced by industrial activities, can pose health risks if not adequately removed from water.

5. Heavy Metals and Bacteria (if certified)

Certain activated carbon filters are certified to eliminate heavy metals like lead and bacteria, such as coliform and cysts. This certification ensures that the filters adhere to specific standards for contaminant removal and safety.

Considerations of Choosing the Right Filter

When selecting between a carbon block filter (CTO) and a granular activated carbon filter (GAC), consider the following factors:

1. Filtration Efficiency

CTO filters offer higher filtration efficiency due to their compact structure and finer carbon particles. They effectively remove a wider range of contaminants, including fine particulates and dissolved chemicals.

2. Water Flow Rate

GAC filters provide a higher water flow rate, making them suitable for applications requiring rapid filtration. They are ideal for whole-house systems and high-flow applications.

3. Contaminant Removal

CTO filters are more effective at removing a wider range of impurities, including heavy metals and microbial contaminants. GAC filters excel at removing chlorine, VOCs, and improving taste and odor.

4. Filter Lifespan

CTO filters typically have a longer lifespan due to their higher capacity for holding contaminants. GAC filters require more frequent replacements, especially in high-contaminant environments.

5. Cost

GAC filters are generally more affordable but less effective at comprehensive filtration. CTO filters have a higher initial cost but provide better long-term value due to their efficiency and durability.

6. Specific Needs and Priorities

Assess your specific needs and priorities to make an informed decision. Consider the types of contaminants in your water, the desired flow rate, and the cost implications. Sometimes, a combination of CTO and GAC filters can achieve the best results by leveraging the strengths of both types.

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Both CTO and GAC filters are highly effective in improving water quality by removing various contaminants with unique strengths. Understanding the differences between these filters and their respective applications helps in choosing the right filter for specific needs.

LikeFilter offers a range of high-quality CTO and GAC filter cartridges to meet diverse water filtration requirements. For further assistance, please contact our team of experts.

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