Carbon, granulated activated

(endorsed 2005)

Granular activated carbon is used in drinking water treatment to adsorb or biologically degrade dissolved organic matter, pesticides, algal toxins and compounds causing taste or odour problems. Use of activated carbon used before disinfection reduces the formation of disinfection by-products, by reducing the amount and reactivity of organic precursors of these by-products.

General description

Granular activated carbon (GAC) is a black, solid, extremely porous material that can adsorb impurities and contaminants from air and water. It has a complex, porous internal structure, with internal surface areas averaging about 900 $\text{m}^{2}$/g and a bulk density of 250–600 kg/$\text{m}^{3}$. Activated carbon is insoluble in water and organic solvents.

The properties of activated carbon depend on its degree of activation and the raw material from which it is produced. Coal, wood and coconut-based activated carbons each have different pore structures and different characteristics.

GAC may act as a biological carrier by housing bacteria in its internal honeycomb structure. When GAC filters are used in an enhanced biological mode, they are referred to as biological activated carbon (BAC) filters. BAC filters work through two mechanisms: biodegradation of contaminants (e.g. taste and odour compounds, and organics) and biological regeneration of the carbon’s adsorption sites.

Dry activated carbon can be stored in cast iron or steel silos. Wet activated carbon can be stored in plastic, rubber or silicon-lined containers, or in stainless steel (type 316), monel or bronze.

Chemistry

Carbon is ‘activated’ by heating carbonaceous material such as wood, coal or coconut husks to high temperatures in a controlled atmosphere of steam, or at moderate temperatures in the presence of chemicals such as acid.

The adsorptive properties of GAC vary with pore size, pore-size distribution, internal surface area of the pores and surface properties. The properties of the GAC available in the market are variable. In selecting an activated carbon product, it is important to consider factors such as the adsorptive capacity of the activated carbon, the desired application, abrasion resistance during backwashing and cost. The quality of the activated carbon can be determined by its ability to remove contaminants such as 2-methylisoborneol (MIB), geosmin, toxins and pesticides, and by a number of other factors that are listed below, together with typical ranges (actual values will depend on the raw material and the activation processes):

• iodine number: 900–1300 mg/g carbon

• apparent density: 0.2-0.6 g/cc

• moisture content: 3-8%

• abrasion resistance: 75-99%

• particle size distribution:

  5% maximum on upper sieve

  90% minimum between sieves

  5% maximum through lower sieve

• ash content: 3–15%

The adsorptive capacity of activated carbon can be inferred from the iodine number, methylene blue number or molasses number.

Effective sizes of GAC are typically 0.7–1.2 mm, with a uniformity coefficient (UC) generally specified to be less than 1.8. The GAC is installed over supporting layers of sand and gravel.

After installation in the filter bed, the GAC is carefully wetted over several hours. In some carbons, significant flotation of the carbon may occur in the wetting phase, and the floating portion of GAC is removed and disposed of. The floatable component of the GAC may vary between 0 and 30%.

Typical use in Australian drinking water treatment

In water treatment, activated carbon is used to control taste and odour-causing compounds, and to remove contaminants such as nitrates, pesticides, algal toxins, disinfection by-products, organic carbon and other trace organic chemicals.

GAC is generally used as a filter medium in beds or tanks, with the water being treated as it passes through the filter. Contaminants are removed through adsorption and biological degradation. Many taste and odour compounds (e.g. 2-methylisoborneol (MIB), geosmin and 20–50% of natural organics) can be biologically degraded, and GAC filters used in this way can operate for 10–15 years. If the contaminant is not biodegradable, the GAC medium can be used continuously until its adsorption capacity is exhausted; and can then be reactivated using a thermal process (currently not available in the Australian drinking water industry). In adsorption mode, a GAC bed is effective for about 1 month to 2 years, depending on the concentration of contaminants in the water.

GAC beds can be used either before or after conventional treatment (i.e. pre-filtration or post-filtration). GAC can also be used for a combination of filtration and adsorption, either as a full GAC bed, or as a layer of sand topped with a layer of GAC medium. The process can be preceded by ozonation, which encourages biological activity on the filter (creating a BAC filter), thus prolonging the life of the filter. Ozonation generally produces water that is more biologically stable and has a lower chlorine demand.

GAC and BAC filters are designed for a specific empty bed contact time (EBCT), which typically ranges from 5 to 25 minutes. The most economic EBCT can be determined by analysing particular contaminants of concern, at either laboratory or pilot scale.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in the ash that may be found in activated carbon:

• aluminium

• arsenic

• chromium

• iron

• lead

• manganese

• mercury

• phosphorus

• silver

• zinc

Residual and by-product formation in drinking water

When employed in drinking water treatment, activated carbon should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Degraded GAC can pass through a water treatment plant, causing black specks and deposits in the distribution system, although it is unlikely that significant quantities of carbon residues will be present in finished water.

Status

Activated carbon was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard B604-96. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Gosselin RE, Smith, RP and Hodge HC (1984). Clinical Toxicology of Commercial Products, 5th edition. Williams and Wilkins, Baltimore, II-94.

IARC (International Agency for Research on Cancer ) (1984). Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Man. World Health Organization, Geneva, Switzerland.

NIOSH (National Institute for Occupational Safety and Health) (1984). Method 5000, Carbon Black (issued 2-15-84). In: NIOSH Manual of Analytical Methods. Methods A-Z & Supplements, 4th edition.

U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health. U.S. Government Printing Office, Washington, DC.

Snoeyink VL and Summers RS (1999). Adsorption of Organic Compounds. In: Water Quality and Treatment, A Handbook of Community Water Supplies, Letterman RD (ed). American Water Works Association, 5th edition. McGraw-Hill Professional, New York, 13.1–13.76.

Urfer D, Huck PM, Booth SDJ, and Coffey BM (1997) Biological Filtration for BOM and particle removal: a critical review. AWWA Journal 89(12) 83-98.