Q: What is activated carbon (AC) and how is it used in water treatment?
A: AC is carbonaceous organic material that has been thermally treated to produce active sites capable of binding many chemicals when water (or other liquid or gas) is passed through the medium.
Some forms of AC were used by the ancient Egyptians to purify oils and for medicinal applications. About 90 million pounds are expected to be used annually in U.S. drinking water plants, and up to 800 million pounds are needed for removal of mercury from flue gas in coal-fired power plants.
AC production and application technologies have a very large and complex scope with many technical aspects and applications. By necessity this discussion will just touch upon several aspects that are pertinent to the water treatment industry.
What is it and how is it made?
There are numerous types of AC commercially available, and they are intended to apply to different water treatment purposes. They are primarily carbon, but also contain some ash. ASTM D2652 defines the properties of AC. Carbon used in drinking water must also meet applicable ANSI/NSF standards. AC as a water treatment medium is manufactured in extruded block, granular or powdered forms. It is produced by heating carbonaceous materials, e.g., lignite and bituminous coal or cellulose-based substances like wood or coconut shells to 600 to 900o C in the absence of air to form a carbonized char, then activating (oxidizing) its surface usually at 800 to 1,200o C in the presence of steam, carbon dioxide or air, creating a highly porous carbon structure. Chemical activation can also be accomplished by impregnating the raw material with acids like phosphoric acid, bases like sodium hydroxide or salts like zinc chloride and then heating to lower temperatures. They produce ACs with different surface properties and affinities to different contaminants. Thus, activated charcoal, although a common term, is not really appropriate because charcoal is not activated in that way, and it has much lower capacity to bind chemicals.
The activation process produces a very large surface area on the order of 500 to 1,200 to 2,000 square meters per gram. The surface area derives from millions of molecular-sized pores and depressions, and in itself it could provide some chemical adsorption (not absorption) at the surface. In addition, the surface chemistry of the carbon causes adsorption by chemical bindings and Van der Waals forces involving weak interactions between the electrons and nuclei of atoms that are in close proximity.
ACs have numerous applications in addition to water and wastewater treatment, including for removal of contamination from gases, purification and decolorizing juices, oils and alcoholic and nonalcoholic beverages, including wines and liquors.
What types of AC have drinking water applications?
There are many different varieties of commercial ACs that can have affinities to different substances. The forms of the ACs are tailored to the particular applications. ACs are evaluated using several techniques, a few of them are iodine number, molasses number and dechlorination half-length. The iodine number is a measure of the micropore content. It is the measured iodine uptake to about 1,200 mg/g and it is a general indication of the capability to adsorb small molecules. The molasses number is a measure of the mesopores of the carbon and its ability to adsorb larger molecules (molecular weights of about 100 to 600 daltons).
Dechlorination capacity is evaluated as the dechlorination half-length. It is the depth of a carbon column required to halve the free chlorine content in a test solution. This catalytic reduction occurs on the particle surface and in pores. Chloramine reduction is much slower than free chlorine reduction, so this must be understood when granular activated carbon (GAC) is being used in fish tanks or when the intent is to remove chloramines from drinking water.
Powdered activated carbon (PAC) is provided in fine powders or granules and sieved to desired size ranges. In water treatment PAC is used intermittently, such as during a chemical spill or an algal bloom, to remove trace synthetic chemicals as well as taste and odor producing chemicals like geosmin. It is intended for one-pass use. It can be added as needed to raw water, or at rapid mix, and before filters; and it can be removed by filtration and then disposed of.
GACs have larger particle sizes and are used in fixed beds and post contactors. They are removed from the beds when they become exhausted for the task; and then they are either replaced with virgin carbon or sent to a reactivation facility, and returned to service either in that drinking water facility or in a less sensitive application. In the U.S., many conventional water treatment plants with concerns about trace organics or undesirable tastes from their source water may replace the sand or anthracite coal in their filters with GAC providing a GAC depth of perhaps two to three feet. These are called filter adsorbers because they provide both functions. They are backwashed regularly like other granular media. These are also used to dechlorinate the water.
Filter adsorbers may be in service for years before replacement or reactivation. They are not optimized for removal of organics because of short water contact times and infrequent reactivation, since the logistics of removal and replacement from those types of filters do not allow frequent cycling. There are also several deeper-bed GAC contactor systems in the U.S. (such as at the Cincinnati, Ohio, water works) where it is reactivated on-site and many more in Europe. These are amenable to more frequent removal and reactivation, e.g., annually, so they will be more effectively removing trace organic chemicals; and they can be replaced or reactivated based upon water quality specifications.
Some water suppliers utilize a treatment called biological activated carbon (BAC), wherein the incoming water may be pretreated with ozone to chemically react with some organic chemicals and oxygenate the water on the GAC so that it encourages microbial growth on the filter that converts more of the natural and synthetic chemicals to biodegradable products. This approach usually has much longer operating times between replacements. Carbon contactors are also used in beverage water processing and other industrial processes where they can remove trace organics as well as to decolorize and dechlorinate the water.
What can it do as a water treatment and what can’t it do?
Adsorption is a complex physical/chemical process in which the chemical in the water is transported to the carbon surface where it resides in equilibrium between the surface and the chemical in the water. Components of mixtures may compete with each other for adsorption. There are physical chemical phenomena where the accumulated chemical may be desorbed by numerous factors such as temperature changes, displacement and solution composition changes. Adsorption isotherms are used to quantify the relative capacities of various carbons to accumulate and retain chemicals.
In general, hydrophobic nonpolar chemicals are more effectively retained than hydrophilic polar chemicals, and larger molecules are more effectively retained than smaller. Lower molecular weight alcohols, aldehydes and amines are generally not well adsorbed because of their polarities. Some examples that are determined by experimental isotherm constants show PCBs and heptachlor are orders of magnitude more effectively removed from water than chlorinated solvents like chloroform and small miscible molecules like dimethylnitrosamine.
Thermally reactivated carbon does not return to its exact virgin state. Spent carbon before reactivation might have lost about 40 to 60 percent of its surface area and pore volume; regenerated carbon will recover back to about 90 to 95 percent of its pore volumes and surface area. Some makeup GAC will also be added to restore the original quantity.
How is GAC used in a POU or POE system?
Extruded activated carbon (EAC) is produced by mixing PAC with a binder and extruding it into a cylindrical block. These types of units are used in POU systems for both chemical removals and dechlorination. In fact, the dechlorination capacity of GAC far exceeds the chemical removal so the device can dechlorinate a considerably larger volume of water even though organic chemical removal has been curtailed.
AC can be used in both POU and POE systems for various purposes. In a POU unit the purpose is primarily for dechlorination, which removes the chlorine taste. Some organic reduction might also occur but the contact times are so short and the quantity of the GAC is limited so organics removal is probably limited and temporary. In POE systems the quantity of GAC and the contact times are much greater, so it can be used for reducing organic contaminants as well as to dechlorinate. The latter could also be employed as a smaller cartridge in either reverse osmosis (RO) systems or ion exchange units to protect the membrane or resin from the degradation that free chlorine can cause.
GAC both dechlorinates the water and accumulates organic material and also produces a chemical reducing environment. That combination indicates that an environment conducive to microbial regrowth has been produced. This bacterial regrowth is measurable as heterotrophic plate count (HPC). These are generally not considered to be pathogenic, but they can be generated to very high counts and produce slimes and off tastes. For that reason, these GAC or carbon block systems are usually impregnated with silver salts to depress the HPC regrowth. It is important to understand the purpose of the silver is to help protect the unit and the water from excessive regrowth and not as a water treatment disinfectant to reduce pathogens.
AC is an extremely versatile purifying medium, including gas phase treatment, a drinking water and wastewater treatment component that can remove organic chemicals, dechlorinate and remove tastes, odors and colors, as well as improve the quality of beverage products and other fluids. It is produced from readily available carbon containing materials by heating in the absence of air, then activation of the carbonized material by additional heating under oxidizing conditions. The non-powdered AC forms used in water treatment are recycled by thermal regeneration. Many POU and POE devices contain GAC or extruded carbon blocks and provide dechlorination and taste and odor reduction, as well as reduction of many chemicals.
Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.