GAC Activated Carbon Filter Media – Coconut AF (25KG)
Granular Activated Carbon GAC Filter Media – Coconut (25KG)
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Granular Activated Carbon Water Filter Media
Granular Activated Carbon (GAC) is commonly used for removing organic compounds and residual disinfectants such as chlorine from water supplies. GAC improves taste, smell and minimises health risks, it also protects other water treatment systems such as reverse osmosis membranes and ion-exchange resins from possible damage due to oxidation or organic fouling. Granular Activated carbon is a widely used water treatment technique because of its multifunctional nature and the fact that it adds nothing to the purified water.
Most granular activated carbons (GAC) are made from raw materials such as coconut shells, wood and coal. The typical surface area for granular activated carbon is approximately 1,000 square meters per gram (m2/gm).
However, different raw materials produce different types of activated carbon varying in hardness, density, pore and particle sizes, surface areas, extractable, ash and pH. These differences in properties make certain granular activated carbons preferable over others in different water purification applications.
The two principal mechanisms by which GAC removes contaminants from water are adsorption and catalytic reduction. Organics are removed by adsorption and residual disinfectants are removed by catalytic reduction.
8 Factors that affect the performance of activated carbon are:
GAC Performance based on Molecular weight
As the molecular weight increases, the activated carbon adsorbs more effectively because the molecules are lea soluble in water. However, the pore structure of the carbon must be large enough to allow the molecules to migrate within. A mixture of high and low molecular weight molecules should be designed for the removal of the more difficult species.
Water feed pH for GAC
Most organics are less soluble and more readily adsorbed at a lower pH. As the pH increases, removal decreases. A rule of thumb is to increase the size of the granular activated carbon filter media bed by twenty percent for every pH unit above neutral (7.0).
GAC Contaminant concentration
The higher the contaminant concentration, the greater the removal capacity of granular activated carbon. The contaminant molecule is more likely to diffuse into a pore and become adsorbed. As concentrations increase, however, so do effluent leakages. The upper limit for contaminants is a few hundred parts per million. Higher contaminant concentration may require more contact time with the GAC filter media. Also, the removal of organics is enhanced by the presence of hardness in the water, so whenever possible, place the granular activated carbon filtration units upstream of the ion removal units. This is usually the case anyway since GAC filter water media is often used prior to ion exchange or membranes to remove chlorine.
GAC Grain size
Granular Activated Carbon is commonly available in 8 by 30 mesh (largest), 12 by 40 mesh (most common), and 20 by 50 mesh (finest). The finer mesh gives the better contact and the best removal, but at the expense of higher pressure drop. A rule of thumb here is that the 8 by 30 mesh gives two to three times better removal than the 12 by 40, and 10 to 20 times better kinetic removal than the 8 by 30 mesh.
GAC Flow rate
The lower the feed water flow rate, the more time the contaminants will have to diffuse into a pore and be adsorbed by the granular activated carbon. Adsorption by activated carbon is almost always improved by a longer contact time with the filter media. Again, in general terms, a GAC bed of 20 by 50 mesh can be run at twice the flow rate of a bed of 12 by 40 mesh, and a carbon bed of 12 by 40 mesh can be run at twice the flow rate of a bed of 8 by 30 mesh. Whenever considering higher flow rates with finer mesh carbons, watch for an increased pressure drop!
Higher water temperatures decrease the solution viscosity and can increase die diffusion rate of GAC, thereby increasing adsorption. Higher temperatures can also disrupt the adsorptive bond and slightly decrease adsorption. It depends on the organic compound being removed, but generally, lower temperatures seem to favour granular activated carbons adsorption.
Organic Removal with GAC
Organic material in public water supplies comes from decaying plant life, which becomes more soluble in water over time and exists as large, high-molecular weight organic acids (non-polar weak acids). Eventually, smaller molecular weight acids of varying sizes form. Typical organic acid molecules range in molecular weight from a few hundred to tens of thousands.
The size, number and chemical structure of organic acid molecules depend on a large number of factors, including water pH and temperature. Accordingly, there exists an almost infinite number of organic acids. As a result, removing organics can be difficult and is always site-specific.
Granular activated carbon’s adsorption properties are used to remove organics. Generally, adsorption takes place because all molecules exert forces to adhere to each other. GAC filter media adsorbs organic material because the attractive forces between the carbon surface (non-polar) and the contaminant (non-polar) are stronger than the forces keeping the contaminant dissolved in water (polar). The adsorptive forces arc weak and cannot occur unless the organic molecules are close to the carbon’s surface. The large surface am of the activated carbon, due to its particle size and pore configuration, allows for the adsorption to take place.
GAC’s Ability to Remove Residual Disinfectants
Granular Activated Carbon can remove and destroy residual disinfectants (chlorine and chloramine) through a catalytic reduction reaction. This is a chemical reaction that involves a transfer of electrons from the activated carbon surface to the residual disinfectant. In other words, GAC acts as a reducing agent.
Activated carbon’s removal of chlorine reduces the chlorine to a non-oxidative chloride ion. The reaction is very fast and takes place in the first few inches of a new activated carbon bed. (Where removal of organics by activated carbon takes minutes, removal of chlorine literally takes seconds). The chlorine capacity of new activated carbon is 500 gram of chlorine per 500 gram of granular activated carbon at a flow rate of 11 to 18 l / min and a bed depth of 900mm.
Chloramine removal by activated carbon is a much slower process. The predominant species of chloramine in city water supplies (pH about 7 to 8) is mono-chloramine. The reaction with granular activated carbon and mono-chloramine also renders a non-oxidative chloride ion. Since the rate of reaction is considerably slower, the flow rate should be 2 l / min, and the bed depth greater than 900mm.