GAC stands for granular activated carbon, a filtering material used in water treatment to remove organic chemicals, chlorine, and unpleasant tastes and odors. It works by trapping contaminants onto its highly porous surface as water flows through a bed of carbon granules. GAC is one of the most widely used technologies in both municipal water plants and home filtration systems.
How GAC Is Made
Granular activated carbon starts as a carbon-rich raw material. Common sources include coconut shells, coal, wood, and various biomass wastes chosen for their availability and natural carbon content. The raw material goes through two key steps: carbonization (heating it in the absence of oxygen to burn off everything except the carbon skeleton) and activation (exposing it to steam or chemicals at high temperature to carve out millions of tiny pores). The result is a granular material with an enormous internal surface area, typically between 1,800 and 2,450 square meters per gram. To put that in perspective, a single teaspoon of activated carbon can have the internal surface area of a football field.
The size of the granules is what distinguishes GAC from its cousin, powdered activated carbon (PAC). GAC particles are large enough to pack into a fixed bed or column that water flows through continuously. PAC, by contrast, is a fine powder mixed directly into the water and later filtered out. GAC’s larger particle size makes it better suited for permanent installations where water passes through a dedicated filter vessel.
How Adsorption Works
GAC removes contaminants primarily through adsorption, a process where molecules in the water stick to the carbon’s surface. This is different from absorption (think of a sponge soaking up liquid). In adsorption, pollutant molecules are attracted to and held on the solid surface by weak chemical forces. The more porous and interconnected the carbon’s internal structure, the more surface area is available for contaminants to latch onto.
Porosity matters enormously. Carbon with a porosity around 0.55 (meaning roughly 55% of its volume is open pore space) can adsorb about 1.2 kilograms of water per kilogram of carbon. Drop the porosity to 0.2, and adsorption falls to just 0.3 kilograms per kilogram of carbon. Pore geometry plays a role too. Irregular, disorganized pore structures can reduce adsorption by 20% compared to well-connected pore networks, even at similar porosity levels. This is why the choice of raw material and activation method directly affects how well a GAC filter performs.
What GAC Removes
GAC excels at pulling organic chemicals out of water. In column tests evaluating 83 different micropollutants, including pharmaceuticals, pesticides, PFAS compounds, artificial sweeteners, and personal care products, GAC achieved average removal rates above 97%. That far outperformed other filter materials like lignite (68% average) and sand (47% average). For PFAS specifically, removal depends partly on chain length: longer-chain PFAS molecules, which are more hydrophobic, tend to adsorb more readily onto carbon surfaces.
Beyond lab-measured pollutants, GAC is commonly used to strip chlorine, chloramines, and the taste and odor problems they cause. Interestingly, the way GAC handles chloramines is not simple adsorption. Research using isotopically labeled trichloramine showed that activated carbon actually breaks trichloramine down through a chemical reaction on its surface, converting it into harmless nitrogen gas. Sulfur-containing groups on the carbon surface appear to drive this reductive reaction. The practical result is the same: the chlorine taste and smell disappear.
What GAC Does Not Remove
GAC has clear blind spots. It is not effective against dissolved minerals like iron and nitrate, which aren’t attracted to carbon surfaces. It also won’t remove dissolved salts, hardness minerals (calcium and magnesium), or most heavy metals at meaningful levels. For those contaminants, technologies like reverse osmosis or specialized ion exchange resins are needed. GAC is also not designed to kill bacteria, viruses, or other pathogens, so it should not be relied on as a standalone disinfection method.
How GAC Filters Operate
In a treatment plant, water flows through a column or vessel packed with GAC granules. A key design parameter is empty bed contact time (EBCT), which measures how long water spends in contact with the carbon bed. Typical EBCT values range from 10 to 45 minutes. Longer contact times generally improve removal, but they also require larger vessels or slower flow rates, which increases cost.
As the filter operates, the carbon bed gradually fills up with adsorbed contaminants. The zone where active adsorption is happening slowly moves from the top of the bed (where water enters) toward the bottom. Eventually, contaminants begin appearing in the treated water on the other side, a point known as “breakthrough.” Tracking when breakthrough occurs for specific pollutants is how operators know the carbon is exhausted and needs replacement or regeneration. The speed of exhaustion depends on the concentration of organic matter in the incoming water, the specific contaminants present, and the EBCT.
Regeneration and Lifespan
Spent GAC doesn’t have to be thrown away. The standard industrial approach is thermal regeneration, which heats the carbon to 800 to 1,000°C to burn off the adsorbed contaminants and restore the pore structure. This is energy-intensive, and each cycle causes some carbon loss.
Newer methods aim to reduce that energy cost. One approach uses a mild alkaline solution at just 150°C to strip contaminants from the carbon. This hydrothermal method achieved 91.6% regeneration efficiency with only 4.2% carbon loss in a single cycle. Even after four rounds of use and regeneration, total carbon loss was 13.3%, significantly less than conventional high-temperature methods. The ability to reuse GAC multiple times is important for keeping long-term operating costs manageable, especially in large municipal systems that process millions of gallons daily.
GAC vs. PAC
Both forms of activated carbon remove the same types of contaminants, but they fit into treatment systems differently. GAC is packed into fixed beds or columns where water flows through continuously. This makes it well suited for permanent installations and allows for regeneration and reuse. PAC is dosed as a powder directly into the water stream, typically at an earlier stage of treatment, and then removed along with other solids during sedimentation or filtration.
GAC tends to increase particle sizes in the water it contacts, which can help with downstream filtration. PAC does the opposite, creating finer particles. In membrane-based systems, both have been shown to reduce membrane fouling, but through different mechanisms: GAC increases the scouring action that keeps membranes clean, while PAC adsorbs the organic compounds that would otherwise clog membrane pores. The choice between them often comes down to whether a facility wants a dedicated, reusable filter bed (GAC) or a flexible, dose-as-needed additive (PAC).