Water utilities must protect the public health by producing a final product that meets all regulatory requirements. In addition, the water must be pleasing to the customer, with no taste or odor issues. And finally, utilities must stay abreast of emerging contaminants, health advisories, and new regulations. It’s a constant challenge to shoulder these responsibilities while staying within tight budgets. Utilities need a technology that helps them achieve multiple goals cost-effectively.
Calgon Carbon has been a global provider of water treatment solutions for over 75 years. Water Online spoke with Calgon Carbon about purification methods that can remove many contaminants simultaneously.
How does granular activated carbon (GAC) work to remove contaminants from water?
GAC removes contaminants through adsorption, the binding of the contaminants to the surface of the carbon. GAC is a hybrid mixture of graphite platelets, of various shapes and sizes, that are interconnected by non-graphitic carbon bonding. This structure creates an enormous internal surface area, which makes GAC ideal for removing a variety of contaminants from water.
GAC is effective for removing many organic contaminants that are found in drinking water via adsorption and has the added benefit of acting as a physical filtration medium.
The U.S. Environmental Protection Agency (EPA) and most state-based health departments consider adsorption by GAC to be the best available technology for the removal of many organic materials in surface and groundwater. GAC can facilitate the removal of:
- Disinfection byproducts (DBPs) associated with the use of chlorine and alternative disinfectants
- Organic materials from decaying plants and other naturally occurring matter that serve as the precursors for DBPs
- Per- and polyfluoroalkyl substances (PFAS)
- 1,2,3-trichloropropane (1,2,3-TCP)
- Volatile organic carbons (VOCs)
- Algal toxins such as microcystin-LR and anatoxin-A
- Endocrine-disrupting compounds (EDCs)
- Pharmaceuticals and personal care products (PPCPs)
- Taste- and odor-causing compounds (T&O)
Are there different types of GAC, and are some better at removing certain contaminants than others?
Yes, there are many different kinds of GAC, made from a variety of different raw materials using different manufacturing processes.
The primary raw materials used to make GAC are high in carbon content, such as coal, coconut shells, wood, or peat. Reagglomerated bituminous coal-based material is commonly used for adsorption of organic contaminants in drinking water applications. This is because the resulting GAC has a wide range of pore sizes to adsorb a broad variety of organic chemicals.
Reagglomerated GAC is produced by grinding the raw material to a powder, adding a suitable binder for hardness, re-compacting, and crushing to the specified size. The carbon-based material is then thermally activated in a furnace using a controlled atmosphere and high temperature. The final steps in production include screening to remove unwanted oversized and undersized material (to provide predictable and favorable hydraulic characteristics during operation), followed by packaging.
Reagglomerated carbon is generally preferred because it is a robust material with fully developed porosity. It also has the necessary strength to withstand use and reuse through reactivation.
Water treatment plants have different water sources, including surface waters and groundwater. And sometimes source waters are blended. How capable is GAC of treating water from various sources?
GAC is highly capable of treating water from various sources. GAC is used to treat drinking water from surface water, groundwater - and combinations of the two - all over the United States.
Can different types of GAC be combined for simultaneous removal of multiple contaminants?
GAC of any base material can remove multiple types of organic contaminants simultaneously. A variety of GAC products should be evaluated through performance testing to determine what product will provide the most cost-effective solution.
In addition to regulated contaminants, has GAC been successful with removing emerging contaminants like 1,2,3-TCP, per- and polyfluoroalkyl substances (PFAS), GenX, pharmaceutical and personal care products (PPCPs), and other contaminants of concern?
Yes. As just one example, Calgon Carbon’s FILTRASORB(R) GAC has been used to remove PFAS compounds from water for more than 15 years. These successful projects include more than 45 large-scale and more than 1,000 point-of-entry GAC installations across the United States. Additionally, GAC is used widely across California as the established treatment solution for 1,2,3-TCP, and pilot studies have shown GAC as highly effective at removing PPCPs from drinking water.
Which contaminants are most commonly removed by GAC at water utilities?
Historically, some of the most common contaminants removed by GAC at drinking water utilities are: natural organic matter (NOM), total organic carbon (TOC), methylisoborneol (MIB), geosmin, trihalomethanes (THMs), haloacetic acids (HAA), algal toxins, and volatile organic compounds (VOCs).
GAC also commonly removes many emerging contaminants at the same time it removes the currently regulated compounds.
How can a utility determine the best treatment solution for its specific water treatment facilities?
The best way to evaluate the most cost-effective solution is through performance testing with the technologies and source waters of interest. The evaluation should mimic the full-scale treatment parameters as closely as possible to generate the most representative and meaningful data.
What can utilities do with GAC that no longer has adsorption capacity?
GAC can be reactivated and subsequently reused. The reactivation of activated carbon is a well-established, high-temperature process for the thermal destruction of adsorbed chemicals. Spent carbon is reactivated in a multi-hearth furnace or rotary kiln by volatilizing and destroying the adsorbed contaminants. This process restores the activated carbon to a virgin-like state. High-temperature thermal reactivation with off-gas abatement ensures the total destruction of all adsorbates. This eliminates future contaminant disposal liabilities.
The destruction of adsorbates on spent activated carbon is a two-step process. First, the adsorbates are volatilized or desorbed from the carbon surface. Some of the desorbed contaminants are destroyed in the reactivation furnace. Adsorbates that are removed and not destroyed in the furnace are drawn through an abatement system, consisting of a thermal oxidizer/afterburner, a scrubber, and a baghouse. The abatement system is designed to destroy organics to at least 99.99 percent efficiency. The system neutralizes acid gases formed during the process and captures particulates. Efficiency and functionality of the abatement system is verified by agency approved and verified stack testing.
Are there other technologies that work well in tandem with GAC to provide effective and cost-saving drinking water treatment?
a. Ultraviolet light disinfection/advanced oxidation process (UV/AOP) is commonly used in conjunction with GAC to treat taste and odor compounds and VOCs; it is also used to remove and treat 1,4-dioxane.
b. IX (ion exchange) is commonly used to remove ionic species that are not well removed by GAC, such as perchlorate and nitrates.
What treatment processes could be replaced or retrofitted with GAC systems to provide the same or better treatment?
One of the most common processes that can be supplemented by GAC is physical filtration. In treatment plants that have gravel/sand/anthracite filtration, some or all of the media could be potentially replaced by GAC. This provides the benefit of adsorption of organic compounds while maintaining adequate filtration of suspended solids.