The activated carbon is a material that, with its highly porous structure and high specific surface area, is able to retain many molecules of other substances, adsorbing them in its extended surface area.
Thanks to these features, it can be used in various fields, such as filtration, removal of the organic substances, deodorization, decoloration and dechlorination of liquids.
However, this filter is not able to separate the salinity or dissolved heavy metals (Pb, Cu, Zn, Ni, Cd, Fe, etc.) or the hardness (Ca, CO3) and, once saturated, it is necessary to remove the filling material and replace it with new activated carbon.
The activated carbon is available in two forms:
- Granular (or GAC): it is formed by particles of about 0,8 mm (similar to those of the sand) and is used in the cases where it is necessary a material with small pores and high specific surface;
- In powder (or PAC): is formed by smaller particles and is used in case it is necessary an activated carbon with larger pores and it is sufficient a lower specific surface area.
In the market, there are different types of activated carbon, which can be identified after the following main parameters:
- Superficie specifica (mq/g): Specific surface area (m²/g): the absorption is a phenomenon linked to the contact surface, for which an higher specific surfaces correspond to greater absorption;
- Bulk density (kg/m): this value has great importance to evaluate the success of possible regeneration process;
- Volume of pore (cc/g): this value indicates the ability of the carbon to absorb substances having different molecular weights;
- Uniformity Coefficient: is the index of uniformity of the particle size of the carbon and is important for the evaluation of the load losses through the filling material (and therefore for the assessment of energy consumption);
- Iodine value (mg/g): is related to the capacity of activated carbon to absorb substances with low molecular weight and it is defined as the milligrams of absorbed iodine by a gram of carbon at a concentration of 0.02N of iodine. It is also useful to assess the success of the regeneration process.
- Index of molasses: indicates the capacity of carbon to absorb substances with high molecular weight.
In the water treatment plants, this solution can be used as a finishing stage, for example, after chemical-physical treatments; however, it is preferable to use other technologies such as biological oxidation for the removal of COD or as the decolouring resing filters (which are regenerable) for the removal of color, because of its high operating costs.
Following, there are some data concerning the removal capacity of the activated carbon in water treatment:
- - 1 Kg of activated carbon (good quality) retains approximately 450-500 ppm of COD;
- - 1 Kg of activated carbon (good quality) retains approximately 30-40 ppm of detergent anionic or cationic or non-ionic;
- - 1 Kg of activated carbon (good quality) retains approximately 500-800 ppm of units of Pt/Co.
As a dechlorination treatment, it removes from water the traces of chlorine that, if they reach the osmotic membranes or ion exchange resins, will adversely affect their life cycle. During this process, residual chlorine, all chlorine-compounds and also various toxic compounds (such as pesticides, disinfectants and other organic micro-pollutants) are retained.
The dechlorination is a catalytic reaction that occurs on the surface of the active carbon and is not in connection with the available or total surface area of the activated carbon itself. The decomposition of chlorine does not occur in a short time, as the molecule must be hydrolyzed, and it depends on the concentration of chlorine and from of its origin (gas chlorine, dioxide chlorine and hypochlorite).
For this reason, the filters planned for the water dechlorination are often filled with granular activated carbon of very thin size to have more available contact surface, but also to reduce the flow velocity and extend the contact time.
For this type of application, it is useful to know the "length of semi-dechlorination", i.e. the layer of activated carbon which will halve the concentration of chlorine in the water to be treated.
The active carbon filter is structurally very similar to the Quartz filter, but uses filtration speed lower than it (about 7-8 m/h), so as to allow a contact time between carbon and liquid of about 15-20 minutes.
This technology is used as an integral part of the treatment process in a Primary Water Plant or Drinking water plant.