Lesson - Activated Carbon
Please watch the Carbon Adsorption video by Dr. Mohamad Darwish to begin this week's module. Feel free to take notes and turn on closed caption on the video.
Activated carbon is commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment. Adsorption is the physical and chemical process of accumulating a substance at the interface between liquid and solids phases.
Activated carbon filters are used to remove unwanted tastes, odors, radon, and some man-made volatile organic contaminants from drinking water. The efficiency of the unit depends on the type of activated carbon installed, the filter bed depth, the type of contaminants in the water and their concentration, and the contact time between the water and the carbon filter. Activated carbon filters do not adsorb every type of contaminant equally well.
The solid material used in an activated carbon filter is a specialized carbon manufactured for the purpose. Contaminants adhere to the surface of these carbon granules or become trapped in the small pores of the activated carbon. Generally, an activated carbon filter is used with a pre-treatment filter to remove sediment or iron particles that may be present and can clog the carbon filter.
Granular activated carbon is an effective filter for removing organic chemicals that may be in drinking water. Although activated carbon is efficient in removing a variety of organic chemicals, a specially formulated synthetic resin may be a better absorber for a specific contaminant.
As surface waters continue to be threatened by contaminants, groundwater is increasingly the source of potable and non-potable applications, and it is important to remove these trace organic contaminants. Treatment of such waters for the removal of organic contaminants varies based on the intended uses, however, in most cases, even trace amounts of the contaminants require some level of removal to meet drinking water requirements.
Organic substances are composed of two basic elements, carbon and hydrogen, and are often responsible for taste, odor, and color problems in ground waters. As such, treatment is used generally to improve aesthetically objectionable water.
Sources of organic compounds (VOC's, taste & odor compounds, disinfection by-products, and free chlorine) found in ground waters may include leaking underground gasoline/storage tanks, agricultural runoff containing herbicides or pesticides, solid waste or hazardous waste landfills, and improperly disposed industrial/chemical waste.
The ability of organic compounds to cause health effects varies greatly from those that are highly toxic, to those with no known health effect. The extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. Eye and respiratory tract irritation, headaches, dizziness, visual disorders, and memory impairment are among the immediate symptoms that some people have experienced soon after exposure to some organics.
The technologies most suitable for organic contaminant removal in process waters are granular activated carbon (GAC) and aeration. GAC has been designated by the U.S. Environmental Protection Agency (EPA) as the best available technology (BAT) for organic chemical removal. Activated carbon adsorption provides an effective and reliable treatment for removing organic contaminants and is suitable for treating a wide range of organics over a broad range of concentrations. GAC filtration will also remove chlorine.
GAC filtration is recognized by the Water Quality Association as an acceptable method to maintain certain drinking water contaminants within the limits of the EPA National Drinking Water Standards. Organics that are readily adsorbed by activated carbon include:
- Aromatic solvents (benzene, toluene, nitrobenzenes)
- Chlorinated aromatics (PCBs, chlorobenzenes, chloroaphthalene)
- Phenol and chlorophenols
- Polynuclear aromatics (acenaphthene, benzopyrenes)
- Pesticides and herbicides (DDT, aldrin, chlordane, heptachlor)
- Chlorinated aliphatics (carbon tetrachloride, chloroalkyl ethers)
- High molecular weight hydrocarbons (dyes, gasoline, amines, humics)
Various types of GAC are available for removing organics from water, however, the most frequently used carbon in the US. is coal-based carbon due to its hardness, adsorption capacity, and ease of availability.