All About Activated Carbon Filters
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Activated carbon filters are used during the process of removing organic compounds and/or extracting free chlorine from water, therefore making the water suitable for discharge or use manufacturing processes.
High-Tech Filtration There are various types of high-tech activated carbon filters available for industrial filtration systems. The methods used to create the varying AC materials lead to distinct differences across the range of media available. Coconut shells and coal (anthracite or bituminous) are both organic sources of activated carbon. Carbon forms when an organic source is burned in an environment without oxygen. This process leaves only about 30% of the organic mass intact, driving off heavy organic molecules. Prior to being used for process water treatment, the organic mass must then be “activated.” The process of activation opens up the carbon’s massive number of pores and further filters out unwanted molecules. The open pores are what allow the carbon to capture contaminants, known as “adsorption.”
Here are the two main methods for AC activation: Steam Activation – Steam activation is carried out using steam at temperatures of between 800°C and 1000°C. At these temperatures an instant Water-Gas reaction occurs, gasifying the carbonized material. Air is then introduced to burn out the gasses, without burning the carbon. This process produces a graded, screened and de-dusted form of activated carbon. Carbon activated by steam generally has a fine pore structure, ideal for adsorbing both liquid phase and vapor phase compounds. Chemical Activation – During chemical activation, the carbon is first filled with a powerful dehydrating agent, typically a paste form of phosphoric acid (P2O5) or zinc chloride (ZnCl2). The paste is heated to temperatures between 500°C and 800°C to activate the carbon. Chemical activation produces activated carbon with a very open pore structure, making it more suitable for adsorbing large molecules.
How it Works The activated carbon water treatment method is used for two water treatment methods and each works in completely 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. One advantage to using AC is its low operating cost and virtual “fail safe” operation once installed. Spent carbon can be re-activated, however, re-activated filters should only be used in wastewater treatment applications. 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 five 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:
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All About Activated Carbon Filters
WaterProfessionals
®
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-- 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. 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. -- 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. -- Contaminant Properties: Activated Carbon is best for use in filtering out large organic molecules. AC and organic molecules are similar materials, which means they 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 concentrations of contaminants. -- 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 absorbable as temperatures and pH levels decrease. -- Length of Exposure: The length of time in which the contaminants are in contact with the AC filter also influences the adsorption process. The longer the length of contact, the greater the number of contaminants that are 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.
Filtration Equipment Activated carbon filters are similar to those used in multi-media filtration, except without the air scour step in the backwash process. Since certain organics require an extended exposure time to the filter to be removed, higher filter vessel shell sides may be used to provide deeper carbon beds for extended reaction times. WaterProfessionals® can specify high-tech filtration methods for the identified contaminants and the level of purity required. This is why it is critical to match up the correct activated carbon bed with the particular need. This will achieve the most efficient filtering and the longest use interval for the equipment.
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1-800-999-4195