Activated Carbon: also called activated charcoal, is a black, solid material resembling charcoal that has been treated with oxygen to open millions of small pores between the carbon atoms, it is an extremely porous material with a large internal surface area which accounts for its power to hold impurities, contaminants or colour bodies by a process known as adsorption. Numerous types of activated carbon are available, including pelletized, granular, powdered, impregnated, virgin and reactivated, made out of several different materials such as coal, coconut shell and wood. carbon is the most adsorbent material known to man and has been used to purify air and water for thousands of years.
Activated Carbon FAQ
How is Carbon Activated?
There are two methods to make carbon active:
Chemical Activation of Carbon: By adding a chemical substance, eg: (phosphoric acid, and zinc chloride) the pores of the raw material are opened and further activated at temperatures of 400-700 C. This method renders a product with predominantly large pores, mainly suitable for adsorbing large molecules, such as colour compounds from liquids.
Thermal Activation of Carbon: The raw material is heated to 900-1000 C and under the addition of steam (to prevent carbonisation) a product with predominantly medium and small pores is obtained. By varying the process conditions temperatures, steam dosing, kiln retention time, the size, ratio and number of pores can be influenced.
How does Activated Carbon Work?
Volume ratio: One gram of a typical commercial activated carbon will have a surface area equivalent to 1,000 square meters.
How does Activated Carbon Work? The positively charged active carbon attracts negatively charged impurities. This process is called ‘adsorption’. The gas molecules of chemicals and other impurities get stuck in the numerous tiny pores on the surface area of the activated carbon, present in the filter. The amount of impurities attracted by the filter would be in proportion to the amount of carbon and its thickness.
Activated Carbon is the leading technology for the removal of Volatile Organic Compounds (VOCs) from air and other gases. In most cases, the compound can be removed to below the detection limit, which means that the most stringent air quality regulations can be exceeded.
Additionally, activated carbon is used in the hazardous waste remediation area to treat off-gases from air strippers and from soil vapour extraction and remediation projects.
What are three main forms of Activated Carbon?
- Granular Activated Carbon: (GAC) – irregularly shaped particles with sizes ranging from 0.2 to 5mm. This type is used in both liquid and gas-phase applications.
- Powder Activated Carbon: (PAC) – pulverized carbon with a size predominantly less than 0.18mm. These are mainly used in liquid phase applications and for flue gas treatment.
- Extruded Activated Carbon: – extruded and cylindrical shaped with diameters from 0.8 to 5mm. These are mainly used for gas phase applications because of their low-pressure drop, high mechanical strength and low dust content.
Coconut shell carbon’s superior level of hardness makes it cleaner than most other carbons and gives it a longer life expectancy. This, combined with their high activity levels, makes them well-suited for use in any kind of carbon filter or system.
What is Activated Carbon Used For?
Activated carbon has been utilised for many years to reduce fumes and gases from the air. Currently, a wide range of carbons are available for this purpose. The ever-increasing awareness of this problem by the health authorities and environmentalists has resulted in a increase in the use of activated carbon odour filters. Carbon filters are desirable in many installations where airborne odours and chemical or toxic fumes are present and need to be reduced or eliminated. Carbon will adsorb the chemical molecules in the airstream in varying degrees according to the particular contaminants involved.
How Effective is Activated Carbon?
Several factors influence the effectiveness of activated carbon. The pore size and distribution vary depending on the source of the carbon and the manufacturing process. Large organic molecules are adsorbed better than smaller ones. Adsorption tends to increase as pH and temperature decrease. Contaminants are also removed more effectively if they are in contact with the activated carbon for a longer time, so the flow rate through the carbon affects filtration. The dwell time (i.e. the period of time the air is in contact with the carbon) is recommended to be approximately 0.1 seconds or a face velocity of 38 fpm.
Can Activated Carbon be Reused?
Carbon can be reused if the adsorbed substances are removed. This process is known as “regeneration,’ simply heating the spent carbon at a given temperature for an adequate length of time can regenerate activated carbon to the point where it can be reused for wastewater treatment, regeneration inevitably results in the loss of carbon.
Can Activated Carbon Target Specific Elements?
Activated carbon can be impregnated with chemicals to enhance its ability to control problem contaminates. Control of acid gases such as Hydrogen Sulfide and Mercaptans is greatly enhanced by impregnation with a caustic compound. Carbon is treated with Potassium Hydroxide for this purpose. This gives carbon the ability to chemically neutralize acidic gases and increase the volume of air able to be treated by the carbon.
How Effective is Activated Carbon?
Carbon adsorption is an extremely versatile technology. For many water treatment applications, it has proved to be the least expensive treatment option. Adsorption is particularly effective in treating low-concentration waste streams and in meeting stringent treatment levels. Due to the large internal surface area of activated carbons, it can actually adsorb up to 60% of its weight
The ability of activated carbon to remove contaminants is determined not by its weight or volume, but by its adsorption capacity, i.e., the amount of impurity removed by a given amount of activated carbon. The higher this capacity, the more contaminants are removed per cubic foot, and the less carbon is needed to perform a particular job.
The Blame Game – Water Filtration
Most problems associated with activated carbon water filtration systems are not caused by inferior-quality carbon, but by users not following proper operating procedures.
Many users, when they experience such problems as:
- Pressure drop
- Premature organic breakthrough
- Decreased flow rate
- Significant pH rise
Assume that something is wrong with the carbon in their system. However, carbon quality rarely is the cause of performance problems, such problems typically are caused by customers who fail to take the proper steps when putting a unit containing activated carbon online. By familiarizing themselves with potential problems and how to prevent them, users can ensure that their activated carbon systems operate effectively and require minimal maintenance.
Problems that sometimes arise shortly after start-up and carbon change out include high-pressure drops, premature organic breakthroughs, and significant pH increases. Operators often assume that high-pressure drop is due to excessive carbon fines in the bed, premature breakthrough is due to poor carbon quality that results in channeling and a rise in pH is due to something leaching from the activated carbon. In most cases, none of these assumptions are valid.
What problems do Water Filtration Systems experience?
Improper Activated Carbon Preparation: Improper wetting of granular activated carbon is the root cause of many operating system problems and poor performance, if activated carbon is placed online without being pre-wetted properly, air pockets develop in the carbon bed, causing high-pressure drop throughout the bed, premature organics breakthrough. As the water flows downward through the bed, the air forms pockets or bubbles that do not exit the bed but cause channelling, high-pressure drop and premature organic breakthroughs.
Wetting granular activated carbon properly is time-consuming. In a typical system, carbon is only 90% wetted after 24 hours. Therefore, if an adsorber is put online immediately after being filled with granular activated carbon and water, pressure will build and premature organics breakthrough will occur. The adsorber should be filled with water and allowed to stand for at least 24 hours. For systems not designed to be back-washed, water should be drained and the vessel refilled with water.
PH Rise Phenomenon: When most carbons initially are put online, effluent pH will rise to a value between 9 and 12, with the final value depending on the water source. This rise can hinder the carbons’ ability to adsorb sufficient amounts of certain organics and also can cause iron or calcium to precipitate. It is due to the carbon adsorption of chlorides, sulfates, nitrates and other anions from the water. Water pH remains elevated until the carbon pores become filled with these anions. Typically, 100 to 200 bed volumes of water must pass through the carbon before pH drops to 8 or 8.5. The purchase of pre-oxidized carbons is one way to deal with and stop the pH rise.
High-pressure Drop: High-pressure drop, premature organic breakthrough, carbon losses and carbon fines elution with intermittent operation are the most common problems that surface after a unit has been online for several weeks or months.
Several typical causes of high-pressure drops exist:
- Suspended solids are present in the influent. A pre-filter must be installed upstream of the carbon. If the bed can be back-washed, back-washing should be performed on a regular basis.
- Iron or calcium deposits have formed on the carbon. When iron or calcium is the cause of pressure problems, the element should be removed from water before it reaches the carbon
- Air has entered the bed via the influent piping system, as a result of siphoning, air can enter the bed if the influent piping to the pump is not designed correctly. Care must be taken to ensure that the pump’s suction piping is designed to prevent vortex formation in the supply tank and therefore introduction of air.