What is granular activated carbon?
Granular activated carbon (GAC) is activated carbon in particle sizes typically ranging from 0.4 mm to 4 mm — large enough to pack into a bed and small enough to provide the surface area needed for effective adsorption. Unlike powdered carbon, which is dosed and then removed, GAC stays in place and the fluid flows through it.
This seemingly simple difference — the fluid moves through the carbon, rather than the carbon moving through the fluid — has profound engineering consequences. Fixed-bed GAC systems can treat enormous volumes continuously, with low operator intervention and predictable performance.
How a GAC bed works
In a GAC contactor, contaminated water or air enters at one end and exits at the other. As the fluid passes through the bed, contaminants are adsorbed onto the carbon surface. Over time, the carbon nearest the inlet becomes saturated and stops adsorbing — the contaminants pass deeper into the bed. This advancing front is called the mass transfer zone.
When the mass transfer zone reaches the outlet, the contaminant begins to appear in the treated effluent — this is called breakthrough. At breakthrough, the carbon must be replaced or thermally reactivated to restore its capacity.
A well-designed GAC system can treat millions of litres of water before the carbon needs replacement. The key is matching the carbon grade, bed depth, and flow rate to the specific contamination load.
Key applications for GAC
- Municipal drinking water treatment — chlorine removal, taste and odour control, micropollutant removal
- Industrial effluent polishing — removing residual organics before discharge
- Process water in food, beverage, and pharmaceutical manufacturing
- Air purification systems — VOC and odour control in industrial facilities
- Groundwater remediation — removing BTEX compounds and chlorinated solvents
Grade selection for GAC
GAC grades differ in particle size (mesh), activity (iodine number), hardness, and base material (wood or coconut shell). For water treatment, coconut shell GAC at 8×30 or 12×40 mesh is standard. For air applications, finer-pore coconut grades with higher CTC are preferred. For large organic molecules — such as in industrial decolorisation — wood-based GAC with macroporous structure performs better.
Rajindra’s RC830 and RC1240 series are purpose-designed for water treatment, with consistent iodine number, controlled mesh specification, and hardness above 98% to withstand backwash cycles without generating fines.