GRANULAR ACTIVATED CARBON AS AN ADSORPTION AND FILTRATION MEDIUM
About granular activated carbon
Granular activated carbon (GAC) is a hybrid mixture of a wide variety of graphite platelets that are interconnected by nongraphitic carbon bonding. The adsorptive capacity of GAC makes it ideal for removing a variety of contaminants from water, air, liquids and gases. GAC is also an environmentally responsible product that can be reactivated through thermal oxidation and used multiple times for the same application.
The U.S. Environmental Protection Agency (EPA) and most state-based departments of health consider adsorption by GAC to be the best available technology for the removal of many organic materials in surface water. On its own or paired with an ultraviolet (UV) disinfection system, GAC can facilitate the removal of:
Disinfection byproducts (DBPs) associated with chlorine and alternative disinfectants
Algal toxins, such as microcystin-LR, cylindrospermopsin and anatoxin-A
Pharmaceuticals and personal care products
Taste and odor-causing compounds
Organic materials from decaying plants and other naturally occurring matter which serve as the precursors for DBPs
The following is a review of the fundamental aspects of activated carbon and specifically the use of GAC as a filter and adsorbent in potable water treatment.
All images courtesy of Calgon Carbon
Activated carbon production
Activated carbons are produced from a wide variety of carbon-rich precursor materials such as bituminous coal, anthracite, subbituminous coal, lignite, wood, coconut shells and peat.
These materials are converted into activated carbon by either thermal or chemical activation processes. Thermal treatment typically includes steam gasification (activation) and chemical activation uses reactive, inorganic additives at relatively lower temperatures.
Activated carbons produced from bituminous coal can be classified as direct activated or reagglomerated. Direct activation involves sizing the coal to approximately the final particle size required and thermally activating the sized coal. Direct activation can produce a less costly product compared to reagglomeration.
Reagglomeration involves first pulverizing and briquetting the coal with organic binders. The briquettes are stage crushed to achieve the desired particle size. Once the reagglomerated material has been activated, the binder is also converted to a graphitic structure that interconnects the activated coal particles. The hardness and abrasion characteristics of reagglomerated and direct activated GAC are often comparable. However, reagglomerated carbons tend to have a more homogenous pore structure that can be better for certain organic contaminant removal.
During thermal treatment, all the carbonaceous raw material reacts by means of condensation reactions to form increasingly larger aromatic plate structures. The density of the structure is influenced by the characteristics of the raw materials. The denser the raw material, such as bituminous coal, the more extensive the structures will be. These extended flat graphite platelets are arranged randomly within the GAC particle to provide the extensive internal structure needed for adsorption to occur.
Adsorption results from the interaction of the electronic structure of an adsorbent, such as activated carbon, with an adsorbate, such as a taste- and odor-causing compound like geosmin. Molecular compounds are kinetically attracted to the porous surface area of the carbon.
- Granular activated carbon filtration
GAC has also shown to be an effective physical filtration medium in water treatment plants, and has the added benefit of providing water quality protection by adsorption of taste and odor compounds or chemical contaminants.
Prior to retrofitting an existing multimedia filter with GAC, or designing a new GAC filter, several practical considerations are necessary, including hydraulic requirements, filter on-stream time, and backwash water availability. The properties of the GAC, such as adsorption performance, abrasion resistance and density must be considered as well. Additionally, the effective cost of converting the filter to GAC must be evaluated.
Backwashing requirements are another factor affecting how deep the GAC bed should be, particularly when retrofitting a multimedia filter. The filter design should allow for the optimal expansion while allowing several inches of clearance between the top of the expanded bed and the bottom of the backwash trough. Since the water velocity increases between the troughs, any GAC that is expanded to the level between the troughs will more than likely be washed out of the filter.
The mechanics of installing GAC in a filter retrofit or in a new filter are much the same as for installing sand and anthracite. A notable exception is that the filter and initial gravel/sand or underdrain/sand support must be disinfected prior to installing the GAC. This is because suitable disinfectants, such as chlorine, will rapidly react over GAC, leaving no disinfectant residual. A second variance is the GAC should be submerged for 24 hours prior to the final backwash before bringing the new filter into service. This soak time allows the air entrapped within the GAC pore structure to be removed and it allows the water to thoroughly wet the GAC internal surface.
- GAC filter operation recommendations
Conventional GAC filters are sensitive to the same mechanical operating upsets as typical multimedia filters. As a result, the filters should not be subjected to sudden changes in water velocity during production operation, during backwash or startup online. Depending on the filter design, it is recommended that a minimum of 30 seconds is used to bring backwash water to full flow in the filter. When the unit is brought back online, a minimum of 10 minutes is recommended for bringing the filter back to full flow. Operators should account for seasonal water density changes during backwashing and periodically observe a backwash event.
To better maintain the filter, it is recommended that core samples be taken once per year. The purpose of the procedure is to collect an accurate sample from the core of the filter. The GAC can then be tested for residual activity, which is determined using an iodine number test. Historical data suggests that once the iodine number is between 450 and 550, the GAC should be reactivated or exchanged in the near future.
Article Resources: http://www.watertechonline.com/granular-activated-carbon-as-an-adsorption-and-filtration-medium/