Carbon Filtration Explained: Raw Materials, Pore Structures, Blends, and Certifications

Many people assume that a "carbon filter" describes a specific type of water filter. In reality, the phrase only identifies one material used inside the system and says very little about what the filter is actually designed to remove.   

Products sold under the carbon filter category include inexpensive pitcher filters that are designed to reduce chlorine, whole house water filters that are engineered to remove chlorine from large volumes of water at a high flow rate, and no-compromise point-of-use carbon filters that carry third-party certifications for things like PFAS, lead, volatile organic compounds (VOCs), and agricultural pesticides. 

Quick Summary

• The term "carbon filter" describes a category of filter, not a performance specification for a product.
• Raw material selection influences pore structure and adsorption behavior.
• Mechanical processing and surface modification can extend the capability of carbon filtration media.
• Additional active filtration media broaden the performance of carbon filters to perform against contaminants like lead and arsenic. 
• Third-party certification validates the complete system under defined laboratory conditions. 

Granular Activated Carbon Filters vs. Carbon Blocks 

Activated carbon is primarily used in two physical forms: granular activated carbon (GAC) and as powder that gets manufactured into solid carbon blocks.  The differences between these two forms can influence filtration and flow rate. 

Granular Activated Carbon (GAC)

GAC consists of loose carbon grains that get loaded into a cartridge or tank. The grain sizes for GAC used in water treatment systems range from less than 1mm up to several mm in diameter.  In GAC systems, water flows through the void spaces that are created between the particles. 

Products that use GAC produce comparatively low pressure drops and are well-suited for high-flow, high-volume applications, such as whole house and municipal systems.  Because the pore structure of GAC is large enough for water to flow through without added pressure, it is commonly used in small gravity-fed products like water pitcher filters.  

In GAC-based systems that are connected to a home’s water pressure, the flow rate must be controlled to ensure the water is in contact with the activated carbon for a long enough time. Engineers know this parameter as empty bed contact time (EBCT).  

One practical consideration to be aware of in granular systems is that if flow through the GAC becomes uneven due to a void or heterogeneous media packing, water can flow preferentially through localized pathways within the bed, a process called channeling.  Channeling limits the efficacy and capacity of a GAC-based system because the water is moving too fast through the media, and most of the system’s media is excluded (partially or fully) from the flow. 

Carbon Block Water Filters

In a carbon block filter, powdered carbon is mixed with a binder and formed (either by molding or extrusion) into a rigid material. Because the carbon is fixed in a block’s three-dimensional pore structure, the flow pathways are generally more uniform, and the contact is comparably predictable and homogeneous under higher pressure conditions.   

This geometry reduces the likelihood of preferential flow patterns associated with loose beds. Properly engineered carbon blocks operate at comparably higher pressure drop than GAC systems of the same size, but they provide predictable hydraulic behavior at pressures commonly found in the home.  These characteristics make carbon blocks a common choice for point-of-use filters that are driven by the home’s water pressure or appliances that have a standalone pump. 

How Raw Materials Influence Activated Carbon Filtration Performance

Carbon is activated by heating carbon-rich materials (like coconut shells, coal, or wood) to high temperatures in a low-oxygen environment, followed by oxidation with steam or chemicals. This process creates a porous internal structure with a surface that can adsorb a wide range of contaminants from water. 

The raw material used to produce activated carbon impacts its pore size distribution and adsorption behavior and therefore impacts which contaminants the material is best suited to address when built into a filter.  

Surface Area and Pore Structure of Activated Carbon Filters

Activated carbon is highly porous, and engineers use a metric called Iodine Number to describe how much iodine is adsorbed into the carbon. A higher Iodine Number means a higher degree of activation, adsorption capacity, and surface area. For activated carbon, ranges between 500–1600 mg/g are common for carbon used in water filtration. 

Another metric that describes the pore structure of activated carbon is pore size characterization. Different raw materials produce different pore structures. 

The nomenclature used by the International Union of Pure and Applied Chemistry (IUPAC) to classify pores is: 

• Micropores: less than 2 nanometers 
• Mesopores: 2 to 50 nanometers 
• Macropores: greater than 50 nanometers 
  

Coconut shell typically produces carbons rich in micropores with high abrasion resistance. This structure performs well for smaller organic compounds such as VOCs and disinfection byproducts. 

Bituminous coal produces a broader distribution of micropores and mesopores. This structure improves diffusion of larger organic molecules and therefore provides performance against a broad range of chemicals. 

Wood precursors typically produce carbons with a higher proportion of macropores. This structure favors adsorption of larger organic molecules and is commonly used in color removal and specialty industrial applications. Its pore architecture differs substantially from shell- and coal-based carbons used in potable water treatment. 

Mechanical and Chemical Processing of Carbon

Raw material should be thought of as the starting point for carbon, because activated carbon can be further processed mechanically and chemically to expand its performance. 

Mechanical Reagglomeration 

When bituminous coal undergoes reagglomeration, it is pulverized into a powder, mixed with a binder, pressed into briquettes, and re-crushed.   

This process produces more uniform particle size and improved mechanical durability. Reagglomerated coal-based carbons are commonly used in residential systems designed for longer contact times, including some that carry PFAS reduction claims. 

Catalytic Carbon 

While standard activated carbon is effective at reducing free chlorine, it is less effective at reducing chloramine, which is used in a growing number of municipalities due to its comparative stability and persistence. 

With catalytic carbon, the surface is modified to introduce nitrogen-containing functional groups that accelerate chloramine reduction kinetics. Unlike traditional activated carbon, where Iodine Number reflects adsorption capacity, catalytic carbon uses a metric called Peroxide Number to specify how quickly it can break down a solution of hydrogen peroxide. A lower number references a faster breakdown. 

Silver Impregnation 

Water filters are a wet environment that can support microbial growth under some conditions. To mitigate this, manufacturers sometimes impregnate the carbon matrix with silver salts. 

When carbon is impregnated with silver, the silver functions strictly as a bacteriostat, meaning that it inhibits bacterial growth and biofilm formation on the media surface, which protects the filter from premature biofouling. It does not disinfect the incoming water stream, nor does it increase the adsorption capacity of the underlying carbon. 

Iron-Coated Carbon  

Often referred to as iron-oxide-coated activated carbon (IOCAC) or granular ferric carbon (GFC), iron-coated carbon is a specialized filtration medium used primarily to filter arsenic, phosphorus, and heavy metals from water. By bonding iron-oxide nanoparticles to the high surface area of activated carbon, the filter can simultaneously adsorb organic contaminants and chemically bond with arsenic. 

Expanding Performance with Additional Active Media 

Activated carbon is effective for many organic compounds and chlorine-based disinfectants. However, it is not the primary solution for other contaminants that water filter systems are configured to reduce. 

GAC water filters are often paired with other types of active media.  For example, it’s common for whole house systems on chlorinated water supplies to pair a GAC-filled tank with a softening resin tank, because chlorine can degrade the resin.  On wells, GAC systems are often paired with specialized media to reduce contaminants like arsenic or manganese. 

In carbon blocks, it’s common to blend in other types of active media to achieve heavy metal reduction. For example, blocks that have a lead reduction claim will have a lead sorbent blended into the carbon before the block is fabricated.  Some specialty carbon blocks also blend in active media to reduce arsenic, but this is less common.   

In short, products that fall under the carbon filter umbrella can have different contaminant reduction claims depending on the active media blend materials included, even if they have the same outward appearance. Without knowing the media formulation and the certification claims, you cannot predict how a “carbon filter” will perform. 

Carbon Filter Micron Ratings 

In addition to reducing dissolved contaminants, carbon filters, especially when engineered into a block, can protect against particulates found in water. Water filter micron ratings only apply to performance against particulates, and don’t give meaningful information about how a product performs against dissolved contaminants. Two of the most common particulate contaminants addressed by carbon blocks are cysts and microplastics. 

Nominal and Absolute Ratings 

Two terms often found on spec sheets for carbon filters are nominal and/or absolute pore size ratings. A nominal micron rating is a relatively loose term that indicates partial reduction at a stated particle size under defined testing. An absolute rating is a more rigorous standard that indicates nearly all particles of that size or larger are retained. 

NSF/ANSI Certification of Carbon Filters

Because most design decisions described above are not made public by carbon filter companies, reputable water filter companies usually obtain third-party certifications. These certifications demonstrate how effectively a filter targets certain contaminants, thereby protecting consumers from aggressive marketing claims and evasive language. They also demonstrate a filter’s material safety and structural integrity. 

The drinking water treatment industry relies on standards developed by NSF International and the American National Standards Institute (ANSI). The three primary standards common in carbon-based water filtration systems are: 

• NSF/ANSI Standard 42: Evaluates reduction of contaminants that affect water’s taste, odor, or appearance 
• NSF/ANSI Standard 53: Establishes performance criteria for contaminants associated with health effects, including lead, volatile organic compounds, arsenic, and PFAS 
• NSF/ANSI Standard 401: Covers emerging contaminants not currently regulated by the EPA but associated with health concerns, including pesticides, microplastics, and pharmaceuticals 

There are a few important things to know about NSF/ANSI certifications: 

• “Certified to” and “tested to” NSF/ANSI standards are completely different.  Products that carry certifications by NSF, WQA, or IAPMO have demonstrated through rigorous testing that the product doesn’t leach chemicals, is structurally sound, and performs against the claimed contaminants.  “Tested to” (and other similar phrases) could mean that the manufacturer chose a subset of testing.  For example, the claim may be accurate for the first gallon or so of water processed by the filter but not the entire filter lifetime (which is required by certified systems). 
• Certification to NSF/ANSI standards is conducted on a contaminant-by-contaminant basis.  Carbon filters that are certified to NSF/ANSI Standard 42 for chlorine reduction do not carry an NSF/ANSI Standard 53 certification for lead reduction unless they explicitly make the claim.   
• There are three certification bodies that can issue NSF/ANSI Certifications:  NSF International, Water Quality Association (WQA), or International Association of Plumbing and Mechanical Officials.  All three of these accredited bodies maintain listings of certified products on their websites.   

How to Choose the Right Carbon Filter for You

Activated carbon is one of the most widely used filtration media in drinking water treatment, but the term “carbon filter” alone does not describe a product’s performance. The raw materials used to make activated carbon, resulting pore structure, processing methods applied to the media, and the additional materials blended into the system, all influence what contaminants a filter can reduce.  

In other words, two filters with the same  “carbon filtration” label may have completely different specifications and produce very different results. For that reason, the most reliable way to evaluate a carbon filter is by reviewing third-party certifications verifying performance.