Activated carbon has more than 2,500 commercial product applications. Most wastewater plants use the carbons to purify the water and air leaving a facility.
Activated carbon is an inert solid adsorbent material commonly used to remove diverse, dissolved contaminants from water and process gas-phase streams. It is made from almost any feedstock that contains carbon, including coconut shells and coal family members, as many readers will already know.
Adsorption is the accumulation of a gas or liquid on the surface of a liquid or solid substrate, as opposed to absorption, in which the encroaching substance enters the substrate’s bulk or volume.
Activated carbon is porous, inexpensive and readily available for use as adsorbents, furnishing a large surface area to remove contaminants. It has more useful surface area per gram than any other material available for physical adsorption. In fact, a teaspoon of activated carbon has more surface area than a football field.
Because of its rare characteristics, activated carbon possesses an exceptional ability to capture water-dissolved contaminants that include taste-, odor-, color- and toxic-promoting species. Removal takes place through adsorption phenomena based on surface interactions between contaminants and carbon graphitic platelet surfaces.
These contaminant-carbon surface interactions occur through Van der Waal forces and induced dipole interactions. Activated carbon graphitic platelets induce neutral organic molecules into intra-molecular dipoles. The induced dipoles cause the molecules to be attracted to each other and stick together, so they precipite out of solution in the carbon’s nano-sized pores or adsorption spaces. This is referred to as premature condensation, facilitated by the activated carbon.
Activated carbon manufacturers use different feedstocks and process parameters to make a variety of pore size distributions available. Proper pore structure selection is essential to solve aqueous- and gaseous-phase problems with activated carbon.
Above figure shows realistic representations of activated carbons manufactured from wood, coconut shells and bituminous coal. These carbon types are sold and used in different forms: powder, granular, pellets, blocks and composites. The difference is seen in the size of the graphitic platelets represented by the heavy black lines and how close they are together, as indicated in the figure.
Powdered, micron-sized activated carbon particles are milled from millimeter granular activated carbon and exhibit faster kinetics and a greater capacity for contaminant removal, when compared to carbons with larger particle sizes.
Powdered activated carbon can be used for sporadic contaminant episodes, such as algae blooms and industrial spills, that contaminate municipal influent waters. Powder can be added to the clarification process settling unit to remove these contaminants with activated carbon. It can also protect fixed activated granular carbon beds against sudden influent contamination.
Plants can use powder instead if they lack the infrastructure to use granular activated carbon or do not have enough granular carbon between the influent and the effluent to economically use for removal in sporadic contaminant episodes. The single-use powdered activated carbon is used as a batch process to remove contaminants to acceptable regulated maximum contamination levels (MCLs) but not necessarily to zero or non-detected contamination.
Millimeter-sized granular activated carbon can remove contaminants to concentrations below analytical detection limits, and compared to powder, it requires only about one-fourth the amount of carbon between influent and effluent.
However, a plant needs proper infrastructure to install the fresh carbon and remove the spent granular activated carbon for furnace reactivation. Reactivated activated carbon costs about half as much as fresh or unused granular activated carbon. Granular activated carbon use is a continuous process, and it is a multiple-use product based on thermal reactivation. Thermal reactivation enables the carbon to be classified as “green chemistry.”
Where the possibility of industrial pollution is relatively high, more activated carbon must be readily available for possible emergencies. It can be kept in fixed vessels between the influent and effluent, and more powdered carbon is needed as well.
Finally, pellets, or extra-large carbon granules, are used to control vapor phase municipal wastewater hydrogen sulfide and other odors. These relatively large forms of activated carbon enable gas streams to flow through carbon beds uninhibited. This decreases the use of fans and energy necessary to blow gas streams through tight beds. Regular and catalytic carbons are used for hydrogen sulfide odor control.
With regular carbon, mobile hydrogen sulfide is oxidized to immobilized sulfur, which accumulates on the carbon surface. Using elemental sulfur build-up on working carbon has determined when the carbon needs to be replaced with fresh carbon at laboratories. Catalytic carbons transform hydrogen sulfide to sulfuric acid by oxidation. Sulfuric acid on this catalytic carbon can be washed from used carbon with water and be reused on site many times.
During water filtration through activated carbon, contaminants adhere to the surface of these carbon granules or become trapped in the small pores of the activated carbon . This process is called adsorption. Activated carbon filters are efficient to remove certain organics (such as unwanted taste and odours, micro-pollutants), chlorine, fluorine or radon, from drinking water or wastewater. However, it is not effective for microbial contaminants, metals, nitrates and other inorganic contaminants.Activated carbon filtration is commonly used in centralized treatment plants and at household level, to produce drinking water and in industries to treat effluents. It is also an upcoming treatment applied for the removal of micro-pollutants both in drinking water production and for the purification of treated wastewater before disposal
There are two basic types of water filters: particulate filters and adsorptive/reactive filters. Particulate filters exclude particles by size, and adsorptive/reactive filters contain a material (medium) that either adsorbs or reacts with a contaminant in water. The principles of adsorptive activated carbon filtration are the same as those of any other adsorption material. The contaminant is attracted to and held (adsorbed) on the surface of the carbon particles. The characteristics of the carbon material (particle and pore size, surface area, surface chemistry, etc.) influence the efficiency of adsorption.
The characteristics of the chemical contaminant are also important. Compounds that are less water-soluble are more likely to be adsorbed to a solid. A second characteristic is the affinity that a given contaminant has with the carbon surface. This affinity depends on the charge and is higher for molecules possessing less charge. If several compounds are present in the water, strong adsorbers will attach to the carbon in greater quantity than those with weak adsorbing ability.
At centralised level, they are generally part of one of the last steps, before the water is fed into the water distribution network. At decentralised level, activated carbon filtration units can either be point-of-use (POU) or point-of-entry (POE) treatment. A POE device is recommended for the treatment of radon and volatile organic compounds because these contaminants can easily vaporise from water in showers or washing machines and expose users to health hazards. POU devices are useful for the removal of lead and chlorine. The structure of POU devices can either be in-line, line-bypass faucet mounted (see also advanced filters) or pour-through (similar to the design of ceramic candles, colloidal silver or biosand filters).
Activated carbon filters can also be used as a tertiary treatment in wastewater treatment plants to remove micropollutants from municipal effluents or recalcitrant contaminants from industrial effluents.
In air filters, the activated charcoal adsorbs many types of allergens and pollutants, leaving the resulting air fresh and clean. For those who suffer from allergies, asthma, or other breathing problems, activated charcoal air filters could be the best investment you ever make. They'll create a marked improvement in your home, enhancing your quality of life and helping you breath easier.
Carbon air filters are the filters most commonly used to remove gases. They are designed to filter gases through a bed of activated carbon (also called activated charcoal) and are usually used to combat volatile organic compounds (VOCs) released from common household products. They are also often used to remove odors from the air, such as the smell of tobacco smoke. They cannot remove fine particles like mold, dust, or pollen from the air.