WHAT ARE ACTIVATED CARBON FILTERS?
Activated carbon filter is manufactured from a wide variety of carbonaceous material such as coal, bituminous coal (lignite), bone, wood and coconut shell. The basic carbonaceous material is subjected to a manufacturing process which creates a huge internal area within the carbon particles. For example one gram of ground coal has an internal surface area of only 10 m²; during the process of activation this internal surface areas up to 1000 m². Hence, the process of activation transforms the carbon particle into a useful adsorbent. The most common process of activation consists of thermal treatment which is carried out in two stages. First, the material is carbonized at about 700◦C in the absence of oxygen to reduce the volatile content and convert the raw material to a char. This is followed by further heating up to 1000◦C in a regulated oxygen and steam environment. The steam penetrates the carbon particle and selectively oxidizes the internal carbon atoms to create the large internal surface area.
Within the furnace the main gaseous reactions can be represented as follows:
H2O(g) + C(s) ⇒ CO(g) + H2(g) endothermic
CO2(g) + C(s) ⇒ 2CO(g) endothermic
then
CO(g) + H2O(g) ⇒ CO2(g) + H2(g)
When using oxygen the reaction is exothermic and it is more difficult to control the attack on the carbon structure.
3/2O2(g) + C(s) ⇒ CO2(g) + CO(g)
There are two principle forms of activated carbon used in water treatment:
- granular activated carbon (GAC), and
- powdered activated carbon (PAC).
Although both types can be produced from the same raw material their typical average particle diameters vary from greater than 1 mm for the GAC to less than 0.1 mm for PAC. This variation in size can affect the rate of adsorption but not the amount adsorbed, since the greater proportion of the particle’s surface area lies within its internal structure (pores) and not on its external surface.
Carbons are generally categorized by pore size according to the classification below: macro pore >50 nm, mezo pore 2–50 nm, micro pore 1–2 nm, minimicro pore <1 nm.
APPLICATIONS
Activated carbon is widely used in the water industry to remove organic and inorganic compounds. In particular it is used to remove (i) natural organic matter and color, (ii) pesticides, (iii) taste and odor (iv) algal toxins. There are some general rules that can be used to predict if GAC is likely to remove a molecule: larger molecules adsorb better than smaller molecules, non-polar molecules adsorb better than polar molecules, slightly soluble molecules adsorb better than highly soluble molecules, pH may have an influence on the extent of adsorption as it can control both polarity and solubility of a molecule.
The best available technology for removing pesticides is activated carbon. The adsorption capacity of activated carbon to remove pesticides is affected by concentration and physical / chemical properties of the contaminant as well as loading rate and empty bed time. Generally, activated carbon has an affinity for contaminants that are hydrophobic (low solubilities), so any pesticides exhibiting a high organic adsorption coefficient (KOC) and low solubility are expected to exhibit high binding affinities for activated carbon.
Two compounds that are commonly identified as causing taste and odor problems. These are earthy, musty odor compounds produced as secondary metabolites by some cyanobacteria and actinomycetes. They can cause significant problems for drinking water as they are perceived by most consumers as an unpleasant odor and taste in water at very low concentrations. The compounds, aliphatic, tertiary alcohols, are not easily oxidized by chlorine or ozone, consequently activated carbon is currently considered the best technology for the treatment of affected water.
Advantage: Effective at removing a wide range of organic contaminants such as VOCs and pesticides.
Carbon block and precoat designs have been validated as effective for lead reduction.
Often effective for reducing taste and odor problems.
Does not use electricity or generate wastewater.
Disadvantages: Not effective at removing inorganic materials such as hardness, iron, nitrate or fluoride.
Bacteria growth may occur in the carbon filter if not maintained properly.
May require post-disinfection facilities.