2.3 Applications of Activated Carbon
2.3.2 Liquid-phase applications
The following list indicates the wide-ranging scenarios of liquid-phase applications allocated to for AC.
i. Drinking water availability, to improve taste, smell and colour including removal of chlorinated compounds and other VOCs.
ii. Improvements to ground water purity, contaminants coming from disused sites of heavy industries.
iii. Treatments of both industrial and municipal wastewater.
iv. Mining operations require feed water treatment, metallic ion adsorption (gold and other metals), adsorption of excess flotation reagents and adsorption of Natural Organic Materials (NOM).
v. Pharmaceutical processes, including purification of process water, use with fermentation broths and purification of many products.
vi. The food, beverage and oil industries for removal of small, colour and unacceptable tastes.
vii. The dry-cleaning industries require purification of solvents.
viii. The electroplating industries require purification of wastewaters containing Pb, Cr, etc.
ix. Household water purification, cleaning of aquaria and use in oven-extract hoods.
x. The sugar and sweetener industries need decolourization agents for the production of white sugar, etc.
Both GAC and powdered AC are used in liquid-phase applications, the former being used in continuous processes because they are capable of regeneration, whereas the latter are generally used in batch processes (after completion, the carbon is separated from the liquid and discarded or eluted). Liquid-phase applications require AC with a larger pore size than gas-phase ones, because of the need for rapid diffusion of the liquid to the interior of the carbon particles and because of the large size of many dissolved molecules to be retained. It is usual for AC for Liquid-phase applications to be prepared by chemical activation of wood, peat, lignite, etc. There are two types of liquid-phase applications for AC: odour, colour, or taste removal from a solution, and concentration or recovery of a solute from solution including water purification and sugar and sweetener decolourization as principal applications.
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322.3.2.1 Water treatment
The application of AC in water treatment is mainly centered in the removal of pollutant
organic compounds. These compounds can be classified in three different categories:NOM, synthetic organic compounds and by- products of chemical water treatment. NOM is mainly composed of residues of the metabolism of living things. These compounds produce bad tastes and odours, and also may constitute a source of infection. Among the synthetic organic compounds that can be present in water, one can find oil, benzene and toluene, phenols and chlorophenols, trichloromethane and carbon tetrachloride, detergents, pesticides, dyes and so on.
Finally, trihalomethanes are the most important group of compounds that are to be found in water as a product of chemical treatments to disinfect water with chlorine: CHCI3, CHBrCl2, CHBr2Cl and CHBr4. They are very strongly adsorbed on AC, and this is the reason for the increasing number of potable-water plants using AC as the polishing step.
The removal of water contaminants by active carbon is the major market for liquid-phase applications. AC is used both as a primary treatment, to render the effluent more amenable to other purification processes, and as the final tertiary stage in the purification of the effluent. Of the total water treatment market, about 50% is in drinking water, 40% in wastewater and the rest in ground water markets. Both powdered and GAC are used in water treatment. The tendency being toward use of the granular type because of its regeneration capability (Rodriguez-Reinoso, 2002).
When the powdered form of AC is used, it is added to the water as slurry with automatic feeders.
After a suitable contact time, it is removed by clarification or filtration. Dosage rates of AC in taste and odour control depend on the type of carbon and the level of impurities in the water, but in general terms the dosage is low, and the carbon can last for up to one year. As a result, it is not usually economic to regenerate the carbon, and spent carbon is generally discarded. GAC is preferred when there is a persistent problem with taste and odour control, and it is also used in special filters and disposable cartridges in industrial, commercial and residential installations.
GAC is used in gravity columns, through which water flows continuously for a set contact time.
Contacting systems can be of the up-flow or down-flow type, the former adsorbing organic compounds, whereas the latter filters suspended solids in addition. In an up-flow system, replacement of the spent carbon is carried out from the bottom of the column, with addition of new carbon at the top, while the unit remains in operation. In a down-flow system that does not
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33have pre-filtration, suspended solids may accumulate at the top of the bed, requiring periodic back-washing of the bed to relieve the pressure drop caused by the accumulated solids. This type of bed is operated in series or in parallel. As the carbon will be exhausted first at the top of the bed, it is necessary to remove the entire bed in order to replace the carbon.
2.3.2.2 Waste water treatment Wastewater is treated in four stages.
1. The first stage, or pre-treatment, is carried out when the wastewater contains toxic or non-biodegradable compounds that can affect the subsequent biological treatments. Redox reactions, followed by precipitation and filtration are used to separate the metals contained in the wastewater. Ozone treatment followed by adsorption into GAC is used to eliminate high molecular weight organics, whereas light organics and ammonia are eliminated by air-stripping.
2. The secondary treatment involves the removal of suspended solids, non-soluble oil and floating material by treatment with lime or other chemicals, followed by nitration, as well as neutralization by addition of acids or bases.
3. Dissolved and colloidal organic compounds like proteins, sugars, starches and phenols are removed in the secondary treatment by biological oxidation. These two processes remove about 85% of the suspended solids and of the Biological Oxygen Demand (BOD) of wastewater. For many purposes, however, further purification is needed to satisfy stringent effluent regulations.
4. This treatment involves the removal of inorganic and organic compounds by adsorption. It results in an extremely high-purity effluent, where the BOD can be reduced by over 99%, to 1 mgL-1 (Marsh et al., 1997).
There are three possible locations in a wastewater treatment plant for treatments with AC. This can be used as an adsorbent after primary and secondary biological processes, it can be used as an independent physico-chemical treatment, or it can be added to biological aeration tanks and used as part of the secondary biological treatment. This last choice has been used effectively to obtain a high-quality effluent. The selection of an appropriate treatment depends on the nature and contaminant loading of the wastewater, the scale of operation, specific requirements for effluent purity and the cost of carbon regeneration compared with alternative available treatments. The use of tertiary AC processes on wastewater that has already undergone
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34conventional secondary biological treatment results in a very high-purity effluent. These processes consist of packed beds of granular material arranged either for down-flow in series, for down flow in parallel, for up-flow in series, or in moving beds. The efficiency of a tertiary treatment depends on the consistent and efficient operation of the previous treatment. Changes in wastewater composition, large variations in flow and the presence of toxic material can all disrupt the biological oxidation process. One method of improving efficiency is to treat the influent waste streams with ozone prior to treatment with AC, as is widely done in Western Europe (Marsh et al., 1997).
AC is also used in the treatment of industrial wastewater to upgrade the water for reuse or to pre-treat effluents prior to discharge into municipal pre-treatment plants, rivers and streams. Adsorption by AC may be used as the only treatment before biological treatment or as a tertiary process after biological treatment. AC is used to purify industrial wastewater, as it removes not only biodegradable organic compounds, but also chemicals that are not responsive to, or are toxic to, conventional biological treatments. These include pesticides, phenols, organic dyes and detergents.
AC is used to treat effluent wastes from chemical factories, rubber tread factories, fabric dyeing, fertilizer plants, pulp and paper mills, etc. AC systems are more flexible than biological ones as they can handle sudden fluctuations in the concentration of impurities, and the water purity can be controlled to meet specific requirements (Marsh et al., 1997).
2.3.2.3 Removal of oil from effluent water in petroleum refining.
Another important use for AC is the removal of oil from effluent water in petroleum refining, petrochemicals, metal extraction, detergent, margarine and soft fat manufacture and mineral extraction. The presence of oil in the effluent inhibits the biological treatment in sewage works.
AC is also used to remove oil and organic material from recycled condensate in boilers, because water contaminated with oil would cause foaming or priming even in low-pressure boilers. In high-pressure boilers, where the feed water is de-ionized, the oil causes serious fouling in the ion-exchange resins.
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352.3.2.4. Food and beverage processing
AC is used in food and beverage industries to remove colour or odour from products.
Applications include the processing of fruit juices, honey, sugar, sweeteners, vegetable oils and fats, alcoholic beverages, soft drinks, yeast, maple, syrup etc (Marsh and Rodriguez-Reino, 2006).
2.3.2.5 Chemicals and pharmaceutical
By removing impurities in chemical processes, AC helps to control product quality; it is also used for the removal of toxic chemicals. AC is used to extract pharmaceuticals in processes involving fermentation. Antibiotics, vitamins and steroids are adsorbed into the AC and recovered by solvent extraction followed by distillation. Other uses include filters for the dialysis of poisons and drugs, felts for wounds, etc.
2.3.2.6 Adsorption of dyes
It is estimated that about 40,000 tonnes of dyes are not used but are discharged (lost) into wastewaters. This is out of a total production of about 450,000 tonnes. A large variety of dyestuffs is available with such names as acid, basic, reactive, direct, disperse, sulphur and metallic dyes. Losses are at a minimum (5%) in the dyeing of acrylic fibers but can be as high as 50% for the use of reactive dyes with cottons. Pereira et al. (2003) review much of the earlier literature of adsorption of dyes by AC. This appears to have been confined to studies of adsorption behaviour in terms of surface areas and pore sizes. As a result, little progress was made in understanding of adsorption behaviour by dyes. Pereira et al. (2003), being aware of the importance of surface functionality of carbons in adsorptions from solution (e.g. phenols from aqueous systems), as well as in catalysis on carbon surfaces, explored this aspect using dyes and AC with surfaces modified by oxidation and reduction procedures.
2.3.2.7 Other applications
AC has many smaller-scale applications such as industrial dry cleaning, as well as in coin operated dry-cleaning machines, cleaning of electroplating solutions, public and private aquariums, decaffeination, etc. Medical applications include oral ingestion into the stomach to remove poisons or toxic materials.