Activated Carbon

 

 

Introduction and Background

The main objectives of drinking water treatment are to produce high quality water that is safe for human consumption, has aesthetic appeal, conforms to state and federal standards, and is economical in production. One of the tools that helps to achieve these goals is activated carbon.

Close-up of Sample Activated Carbon Particles

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Activated carbon is useful in drinking water treatment because it acts as an adsorbent, and can effectively remove particles and organics from water. These organics are of great concern in water treatment because they react with many disinfectants, especially chlorine, and cause the formation of disinfection-by-products, DBP's.

These DBP's are often carcinogenic and therefore highly undesirable. This problem has been addressed most recently by the new standards that were created by the 14th Title of the Public Health Service Act. This Act, most commonly known as the Safe Drinking Water Act set maximum contaminant levels (MCL's) in treated water for such DBP's as volatile organic chemicals and trihalomethanes.

Activated carbon is one of the best tools which can be used to reduce risks to human health and provide an aesthetically pleasing product at reasonable cost.

 

 

Adsorption and how it works

 

Adsorption is the process by which Activated Carbon removes substances from water. Defined, adsorption is "the collection of a substance onto the surface of adsorbent solids." It is a removal process where certain particles are bound to an adsorbent particle surface by either chemical or physical attraction. Adsorption is often confused with Absorption, where the substance being collected or removed actually penetrates into the other solid.

 

Close-ups of Carbon Surface and Pores
magnification increases left to right

 

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The reason that activated carbon is such an effective adsorbent material is due to its large number of cavernous pores. These provide a large surface area relative to the size of the actual carbon particle and its visible exterior surface. An approximate ratio is 1 gram = 100 m2 of surface area.

 

 

Scanning Electron Microscope Images of Pores

 

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Activated Carbon uses the physical adsorption process whereby attractive van der Waals forces pull the solute out of solution and onto its surface. Once the solute is bound to the carbon is it considered "removed" from the water. The animation below illustrates this process where the organics are drawn toward the activated carbon by these forces.


Animation of Adsorption in Stages

Activated carbon adsorption proceeds through 3 basic steps

  1. Substances adsorb to the exterior of the carbon granules

  2. Substances move into the carbon pores

  3. Substances adsorb to the interior walls of the carbon

 



Adsorption efficiency decreases over time and eventually activated carbon will need to be replaced or reactivated. Isotherms are empirical relations which are used to predict how much solute can be adsorbed by activated carbon. The three most well-known isotherms are the Freundlich, Langmuir and Linear. In environmental engineering and specifically drinking water treatment application the most commonly used isotherm is the Freundlich. Shown to the right is the Freundlich isotherm equation in general form.

 

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The two graphs below illustrate a general Freundlich isotherm equation and a sample breakthrough curve. Each individual type of GAC has its own isotherm curve and breakpoint characteristics. These help to predict the adsorptive capacity of particular activated carbons and give a design estimate for adsorptive life. Reactivation becomes necessary once the breakpoint has been reached.

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Properties of Activated Carbon

The process of activated carbon generation begins with the selection of a raw carbon source. These sources are selected based on design specifications since different raw sources will produce activated carbon with different properties. Some of the more common raw sources include wood, sawdust, lignite, peat, coal, coconut shells, and petroleum residues.

Characteristics of importance in choosing carbon types include pore structure, particle size, total surface area and void space between particles. After selection of a source, preparations for use are made. These preparations most often include dehydration, carbonization, and activation. Dehydration and carbonization involve slow heating of the source in anaerobic conditions. Chemicals such as zinc chloride or phosphoric acid can be used to enhance these processes. The stage of activation requires exposure to additional chemicals or other oxidizing agents such as a mixture of gases. Depending upon the specifics of the process and the source carbon, the newly activated carbon can be classified according to density, hardness, and other characteristics.

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Carbonized Coconut

Activated Coconut

 

As mentioned previously, another important characteristic of activated carbon is the isotherm or breakpoint characteristic for each particular type. This is most often determined by modeling, testing, cost analysis, and pilot studies. The pilot studies ensure that the chosen carbon type effectively removes the desired substances for the particular raw water source and allows the plant to reach desired levels of quality before treatment continues. Once a breakpoint of a particular carbon has been determined the plant operators know approximately how long the carbon will effectively function. As this time approaches the carbon must be changed to ensure adequate removal.

The "spent" carbon, as it is called, is removed and sent for re-activation treatment. This is done primarily with granular activated carbon because PAC particles are too small to be effectively re-activated. This process allows for recovery of approximately 70% of the original carbon. This number also allows for any physically lost in the shipment process. The re-activated carbon is then mixed with a portion of new carbon for higher effectiveness and is then returned to its place in the plant process.

Reactivation Process Specifics

#

Stage

Temperature (degrees C)

Action

1

Drying

< 100

GAC dewatered to 50% of original weight

2

Desorption

100 - 649

volatile materials driven off

3

Pyrolysis

100 - 649

heavy organics burnt leaving residue

4

Gasification

>>649 and >>1038

vapors and residues from previous stages driven out of pores

 


 

Applications in the water industry

 

Carbon Columns

The water industry uses activated carbon in several forms, typically powdered and granular, to deal with a variety of undesirable aspects in raw water. To the right is an image of pilot scale carbon columns at a water treatment plant.

Seasonal application of powdered activated carbon (PAC) at the raw water intake or rapid mix unit is used by some plants to correct short term raw water quality problems such as algal blooms. PAC is basically used to correct taste and odor problems which are primarily an aesthetic quality of the water. Other uses such as residual ozone destruction and chemical contamination prevention exist, but are not as well documented. Contact time is needed to allow adsorption to occur. The PAC is removed from the water by the processes of coagulation, flocculation, and sedimentation. Once the PAC has been separated from the water it is disposed of along with sedimentation sludge. Some non-traditional systems such as sludge-blanket clarifiers also use PAC (Hoehn, 1996).

Comparison of Powdered and Granular Activated Carbon

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PAC

 

PAC

 

GAC

 

GAC

 

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Granular activated carbon is typically found in beds or filter columns as a Granular Activated Carbon (GAC) Cap and will treat water continuously when raw water quality problems exist year round. The GAC Cap is typically found above the filter media as a distinct layer. In some applications the sand layer can be replaced by GAC. When GAC is used for long term applications it can be more economical since the carbon can be reactivated following decreased adsorption efficiency.

If a GAC cap follows
ozonation in the treatment process, a biological layer can be cultivated in the granular activated carbon cap. Since ozone disinfection leaves no residual disinfectant this biological layer is able to grow freely and metabolize some of the organics in the water, enhancing the overall removal by the GAC column.

Type

Size(typical)

Usage

Economics

Granular

12x30, 12x40, 8x30 mesh

6 month to 2 years

more $

Powdered

less than 150 mesh

short term, as needed

less $