Granular Activated Carbon

Millimeter-sized granular activated carbon can remove contaminants to concentrations below analytical detection limits, and it requires only about a quarter of the amount of carbon between the influent and effluent water compared to powder.

However, the plant needs the proper infrastructure to install fresh carbon and remove used granular activated carbon for furnace reactivation. The cost of reactivated activated carbon is about half that of fresh or unused granular activated carbon. The use of granular activated carbon is a continuous process and it is a multi-purpose product based on thermal reactivation. Thermal regeneration allows carbon to be classified as "green chemistry".

Where the potential for industrial pollution is high, more activated carbon must be available for possible emergencies. It can be kept in a fixed container between the inlet and outlet water, which also requires more powdered carbon.

Finally, granular or oversized carbon particles are used to control hydrogen sulfide and other odors in gas-phase municipal wastewater. These relatively large forms of activated carbon allow the gas stream to flow uninhibited through the carbon bed. This reduces the use of fans and energy required to blow the airflow through the compact bed. Conventional and catalytic carbons are used for hydrogen sulfide odor control.

With conventional carbon, the flowing hydrogen sulfide is oxidized to a fixed amount of sulfur that accumulates on the carbon surface. The use of elemental sulfur accumulation on working carbon has determined when carbon needs to be replaced with fresh carbon in the laboratory. Catalytic carbon converts hydrogen sulfide to sulfuric acid by oxidation. The sulfuric acid on this catalytic carbon can be washed off the used carbon with water and can be reused many times in the field.

Mass Transfer Zone

As more contaminated water or gas passes through the bed, a moving contaminant mass transfer zone (MTZ) is formed by the application of the aqueous and gas phases. Carbon beds are typically 3 to 10 feet deep and consist of layered activated carbon, with the smaller size particles at the top of the working carbon bed and the largest size particles at the bottom.

Maximizing Performance

For improved performance and economy, the typical configuration for multi-bed activated carbon operation is a continuous series. Multiple beds in series allow for complete carbon bed use through breakthrough, where the influent and effluent contaminant concentrations are equal. This is because any remaining backup beds in the series will start another MTZ as needed during operation.

This overrun and lag bed configuration is capable of treating the maximum number of gallons of water per pound of activated carbon before waste carbon must be replaced with fresh carbon.

The working goal is to obtain high quality drinking water at the lowest cost. The final activated carbon bed in the continuous series finishes polished to remove trace contaminants and provide safe, high quality drinking water. By replacing the earlier fully depleted carbon bed with fresh carbon (when the bed influent and effluent concentrations are equal), the later bed functions longer as the final polisher and provides a margin of safety.

When sampling a carbon bed for profiling, samples should be taken from the top, middle and bottom. This type of sampling allows for a more accurate estimate of the MTZ and remaining service time of the carbon bed.

Waste Activated Carbon

Activated carbon does not last forever. It needs to be replaced periodically with fresh virgin carbon or regenerated carbon. The nanoscale volume of pores or physical adsorption spaces between the graphite chips will eventually fill up and no longer be able to remove the adsorbate. The carbon pores are heterogeneous and the adsorption energy varies from strong to weak. Note the graphite flake spacing in Figure 1. Carbon graphite flakes close together provide high adsorption potential energy, while wide flake spacing has relatively low adsorption energy.

Drinking water plants have two main replacement options: purchase virgin or unused carbon or use recycled carbon. After several reactivation cycles, the reactivated carbon becomes less effective and must be replaced with fresh virgin carbon.

Sometimes it is beneficial to expand the pore size distribution by reactivation, especially for larger molecules and higher molecular weight adsorbents. However, trace concentrations of water-soluble low molecular weight compounds such as trihalomethanes may not be readily adsorbed and may yield longer MTZs when used with reactivated carbon having a wider pore size distribution.

Further discussion of this topic will cover test methods to help water plant personnel select the best activated carbon for a given application and to monitor the efficacy and life cycle of the carbon through final disposal.