Active carbon
Activated carbon is produced by the controlled thermalization of carbonaceous material, normally wood, coal, coconut shells or peat. This activation produces a porous material with a large surface area (500–1500 m2/g) and a high affinity for organic compounds. It is normally used in either powdered (PAC) or granular (GAC) (Granular filtration) form. When the adsorption capacity of the carbon is exhausted, it can be reactivated by burning off the organics in a controlled manner. However, PAC (and some GAC) is normally used only once before disposal.
Different types of activated carbon have different affinities for types of contaminants. The choice between PAC and GAC will depend upon the relative cost-effectiveness, frequency and dose required. PAC would generally be preferred in the case of seasonal or intermittent contamination or where low dosage rates are required. PAC is dosed as a slurry into the water and removed by subsequent treatment processes, together with the waterworks sludge. Its use is therefore restricted to surface water treatment works with existing filters. GAC in fixed-bed adsorbers is used much more efficiently than PAC dosed into the water, and the effective carbon use per water volume treated would be much lower than the dose of PAC required to achieve the same removal.
Suitable conditions
GAC is used for taste and odour control. It is normally used in fixed beds, either in purpose-built adsorbers for chemicals or in existing filter shells by replacement of sand with GAC of a similar particle size. Although at most treatment works it would be cheaper to convert existing filters rather than build separate adsorbers, use of existing filters usually allows only short contact times, and they are not capable of facile reactivation.
Activated carbon is used for the removal of pesticides and other organic chemicals, taste and odour compounds, cyanobacterial toxins and total organic carbon.
| Advantages | Disadvantages/limitations |
|---|---|
| - The activated carbon filter makes water taste good and eliminates chlorine and various organic compounds. |
- The filter does not filter out nearly enough contaminants, and cannot be considered as a primary filter. AC (activated carbon) filtration should only be used on water that has been tested and found to be bacteria free or effectively treated for pathenogenic bacteria. |
Construction, operations and maintenance
It is common practice to install additional GAC adsorbers (in some cases preceded by ozonation) between the rapid gravity filters and final disinfection. Most groundwater sources do not have existing filters, and separate adsorbers would need to be installed.
The service life of a GAC bed is dependent on the capacity of the carbon used and the contact time between the water and the carbon, the empty bed contact time, controlled by the flow rate of the water. Empty bed contact times are usually in the range 5–30 minutes. GACs vary considerably in their capacity for specific organic compounds, which can have a significant effect upon their service life. A guide to capacity can be obtained from published isotherm data. Carbon capacity is strongly dependent on the water source and is greatly reduced by the presence of background organic compounds. The properties of a chemical that influence its adsorption onto activated carbon include the water solubility and octanol–water partition coefficient.
As a general rule, chemicals with low solubilities and high log octanol–water partition coefficients are well adsorbed.
Acknowledgements
- Guidelines for Drinking-water Quality. Fourth edition. WHO, 2011.
- McAllister, Skye. Analysis and Comparison of Sustainable Water Filters. May, 2005.