Activated Carbon Filters

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What Is Activated Carbon?

Also called “activated charcoal,” activated carbon is a form of carbon that has been processed to have an incredibly large surface area. Tiny, low-volume pores provide a surface for either adsorption or chemical reactions.

One gram of activated carbon has over 32,000 square feet (3,000 square meters) of surface area — and one teaspoon has roughly the area of a football field.his enormous area alone makes activated carbon extremely useful for a wide range of applications, but activated carbon can undergo further chemical treatment to increase its adsorption properties.

Activated carbon is derived from a carbonaceous source material. Examples include wood, bamboo, sawdust, willow peat, coconut shell, peach pits, coir, petroleum pitch, and a variety of coal. But regular carbon isn’t the same as activated carbon. To form millions of tiny pores across its surface, carbon must first be “activated.”

This is produced by one of two methods: thermal or chemical activation.

1. Thermal Activation

In thermal activation, the source material is formed into activated carbon utilizing hot gases. After being exposed to heat, steam is usually introduced to help open the pores. Thermal activation is typically done through a process involving reduction of moisture, reduction of volatiles, carbonization, and a steam treatment. This process is carried out in an inert atmosphere using gases such as carbon dioxide, nitrogen or argon.

2. Chemical Activation

In chemical activation, chemicals are added to the raw source material before the carbonization process. Typically, the chemical used is a strong base, acid, or salt. The material is then exposed to heat, and it activates quickly.

How it Works

Activated carbon water treatment is basically used for two water treatment purposes and each work in totally different ways.

1. Chlorine Removal: Activated carbon may be used to remove chlorine with little degradation or damage to the carbon. Dechlorination occurs rapidly and flow rates are typically high. However, this process requires an extensive amount of surface area, and organics in the water will eventually fill up and block the pores of the carbon. Ultimately, the activated carbon filter will need to be replaced as its ability to dechlorinate the water will slowly decline. Spent carbon can be re-activated; however, re-activated filters should only be used in waste-water treatment applications. One advantage to using AC is its low operating cost and virtual “fail safe” operation once installed. One disadvantage is that as the chlorine is removed from the topmost layer of the media, the AC provides a damp environment ideal for the growth and proliferation of bacteria. Bacteria can cause problems in medical applications, or when using carbon as a pretreatment to reverse osmosis.

2. Removal of Organic Matter: As water passes through an activated carbon filter, organic particles and chemicals are trapped inside through a process known “adsorption”. The adsorption process depends upon 5 key factors:

1) physical properties of the activated carbon (surface area and pore size distribution);

2) the chemical makeup of the carbon source (amount of hydrogen and oxygen);

3) the chemical makeup and concentration of the contaminant;

4) water pH and temperature; and

5) the length of time the water is exposed to the activated carbon filter (called empty bed contact time or EBCT).

Additional considerations for organics removal are discussed below:

Physical Properties: Pore size and distribution have the greatest impact on the effectiveness of AC filtration. The best filtration occurs when carbon pores are barely large enough to allow for the adsorption of contaminants (Figure 1). The type of contaminants an AC filter attracts will depend on the pore size of the filter, which varies based on the type of carbon used and the activation method. AC filters tend to work best for removing organic chemicals with larger molecules.

Figure 1. Molecular screening in the micropores of an activated carbon filter. (after G. L. Culp and R. L. Culp)

Chemical Properties: The surface of an activated carbon filter may also interact chemically with organic molecules. Electrical forces between the AC surface and the chemical nature of some contaminants may result in ion exchange or adsorption. The activation process determines, to a large extent, the chemical properties of the AC filter, making the filter attractive to various contaminants. Different activation processes will yield activated carbon with different chemical properties. For example, AC that has the least amount of oxygen in pore surfaces will absorb chloroform the best.
Contaminant Properties: Activated Carbon is best for use in filtering out large organic molecules. AC and organic molecules are similar materials, which means they will tend to associate with each other. This means organic chemicals will have a stronger tendency to associate with the AC filter rather than remaining dissolved in water. The less soluble organic molecules are, the more likely they are to be adsorbed. Smaller organic molecules fit the smallest pores and are held the tightest.
Concentration: The adsorption process can be affected by the concentration of organic contaminants. For example, with chloroform removal one AC filter may be more effective than another at filtering high concentrations of contaminants, and less effective at filtering low concentration of contaminants. Consult with the manufacturer to determine how an activated carbon filter will perform at different concentration levels for a specific chemical.
Water Temperature and pH: The rate of adsorption will usually be higher at lower temperatures and pH levels. Chemical reactions and chemical forms are closely related to water temperature and pH. In most cases, organic chemicals are more adsorbable as temperatures and pH levels decrease.
Length of Exposure: The length of time in which the contaminant is in contact with the AC filter also influences the adsorption process – the longer the length of contact, the greater the number of contaminants that will be removed. A greater amount of active carbon and a slower flow rate will improve the effectiveness of the filtration process. Bed depth and flow rate are critical design parameters. Carbon filtration is often engineered to provide a specified residence time of water in contact with the carbon bed, referred to as empty bed contact time or EBCT.

HYOX have years of experience to help in supplying these systems. Contact for additional information

Hyox Water Scientists is a firm of repute, which has been in the field for a decade providing effective solutions to drinking water purity problems of not only homes but industrial and business organization as well.