Activated Carbon Filters
Activated carbon (AC) is generally used in water treatmentfor removing free chlorine and / or organic compounds. Removal of organics from potable water could be to prevent common organic acids such as humic or fulvic from reacting with chlorine to form trihalomethanes (a class of known carcinogens) or, to treat waste water to remove any number of organic compounds to make the water suitable for discharge. Similar to other types of water treatment, however, AC filtration is effective for some contaminants and not effective for others. AC filtration does not remove microbes, sodium, nitrates, fluoride, and hardness. Lead and other heavy metals are removed only by a very specific type of AC and this would typically only be feasible for point-of-use household filters.
Sources and Preparation: AC derives from an organic source. Common ones used in water and waste water treatment include coal (bituminous or anthracite) and coconut shells. Carbon is formed when the basic source is burned in virtual absence of oxygen which drives off the heavy, organic molecules leaving about 30% of the original mass remaining. This carbonaceous material must be further treated or “activated” before it is useful in water treatment. Activation fulfills several functions, including further driving off unwanted molecules, but its major function is to open up huge numbers of pores in the media; it is these pores that result in such high adsorptive properties. The pore structure is so important because pores offer the extremely high surface area for contaminant adsorption.
How it Works
AC is basically used for two water treatment purposes and each work in totally different ways.
Chlorine Removal:AC catalyzes removal of free chlorine with little consumption or degradation of the carbon during the process. This ability, however, requires tremendous surface area and organics in the water will gradually adsorb onto the carbon particle, blocking or occupying pores. This leads to gradual loss of dechlorination ability and the need to replace the carbon. Such carbon can be re-activated and this is frequently done; however, reprocessed carbon should only be used in waste water applications. Dechlorination occurs very rapidly and filter flow rates are typically high. One advantage of carbon for dechlorination is its low operating cost once installed and virtual “fail safe” operation. A disadvantage, however, is that once the chlorine is removed in the top one inch or so of the media, the damp environment and entrapped organics to serve as food source provide an ideal growth environment for bacteria which will proliferate throughout the bed. This can cause problems in medical applications or when using carbon as pretreatment for reverse osmosis.
Organics Removal:AC removes organic compounds by attracting and holding certain chemicals (a process known as "adsorption") as water passes through it. The adsorption process depends on the following factors: 1) physical properties of the AC, such as pore size distribution and surface area; 2) the chemical nature of the carbon source, or the amount of oxygen and hydrogen associated with it; 3) chemical composition and concentration of the contaminant; 4) the temperature and pH of the water; and 5) the flow rate or time exposure of water to AC. Some additional considerations are described below:
Physical Properties:Forces of physical attraction or adsorption of contaminants to the pore walls is the most important AC filtration process. The amount and distribution of pores play key roles in determining how well contaminants are filtered. The best filtration occurs when pores are barely large enough to admit the contaminant molecule (Figure 1). Because contaminants come in all different sizes, they are attracted differently depending on pore size of the filter. In general AC filters are most effective in removing contaminants that have relatively large molecules (most organic chemicals). Type of raw carbon material and its method of activation will affect types of contaminants that are adsorbed.
Chemical Properties:Processes other than physical attraction also affect AC filtration. The filter surface may actually interact chemically with organic molecules. Also electrical forces between the AC surface and some contaminants may result in adsorption or ion exchange. Adsorption, then, is also affected by the chemical nature of the adsorbing surface. The chemical properties of the adsorbing surface are determined to a large extent by the activation process. AC materials formed from different activation processes will have chemical properties that make them more or less attractive to various contaminants. For example chloroform is adsorbed best by AC that has the least amount of oxygen associated with the pore surfaces.
Contaminant Properties: Large organic molecules are most effectively adsorbed by AC. A general rule of thumb is that similar materials tend to associate. Organic molecules and activated carbon are similar materials; therefore there is a stronger tendency for most organic chemicals to associate with the activated carbon in the filter rather than staying dissolved in a dissimilar material like water. Generally, the least soluble organic molecules are most strongly adsorbed. Often the smaller organic molecules are held the tightest, because they fit into the smaller pores.
Concentration:Concentration of organic contaminants can affect the adsorption process. A given AC filter may be more effective than another type of AC filter at low contaminant concentrations, but may be less effective than the other filter at high concentrations. This type of behavior has been observed with chloroform removal. The filter manufacturer should be consulted to determine how the filter will perform for specific chemicals at different levels of contamination.
Water Temperature and pH:Adsorption usually increases as pH and temperature decrease. Chemical reactions and forms of chemicals are closely related to pH and temperature. When pH and temperature are lowered many organic chemicals are in a more absorbable form.
Exposure Time: The process of adsorption is also influenced by the length of time that the AC is in contact with the contaminant in the water. Increasing contact time allows greater amounts of contaminant to be removed from the water. Contact is improved by increasing the amount of AC in the filter and reducing the flow rate of water through the filter. Bed depth and flowrate are critical design parameters.