By Gary Battenberg
When the topic of activated carbon is opened for discussion, the first thought that comes to mind for many in the water-treatment industry is remediation for chlorine, taste, and odor. Whether the application is for drinking water point-of-use products or point of entry, commercial, industrial, or institutional use, activated carbon is generally accepted as the best all-around adsorbent available for improving the taste and aesthetics of water.
There are many contaminants found in water that activated carbon has affinity for, such as color, detergents, odor, organics, pesticides, phenols, tannins, toxic organic compounds, and trihalomethanes. However, there are substances for which carbon is not a proven application.
This article series will break out those contaminants categorically to provide a reference for applications in which activated carbon will and will not yield desired results. It will address the proven efficacy of granular activated carbon in media tank configurations, which typically yields excellent remediation results under proper operating conditions.
First Things First
As with any water-treatment application, the first order of business is to obtain an accurate water analysis to ascertain the treatability of the challenge water. After interpreting the analysis, the target contaminants will then be categorized relative to which treatment technology or technologies will be required to yield the customer’s desired water quality.
Primary considerations when specifying activated carbon for water treatment include temperature, pressure, pH, and other characteristics that may inhibit peak performance of activated carbon. Contact time is very important to keep in mind when treating water for specific contaminants.
For example, it is widely known that activated carbon can scrub chlorine from water very quickly with flow rates of 15 gallons per minute (gpm) per square foot (sq ft) or 5 gpm per cubic foot (cu ft), whereas chloramine requires about half that of chlorine: 6 gpm per square foot or 2.6 gpm per cubic foot of activated carbon. Toxic organic contaminants, however, require a much longer contact time of 0.7 gpm per square foot or 0.9 gpm per cubic foot of activated carbon. These flows, both per square foot and cubic foot, have a bed depth component.
High Activated-Carbon Affinity
Let’s look at a partial list of contaminants for which activated carbon has an extremely high affinity and their uses.
- Amyl acetate, amyl alcohol: Used in paints and thinners.
- Benzene: Part of the original dry-cleaning chemicals referred to as BTX.
- Bleach: 5.25% to 7.5% chlorine (sodium hypochlorite) for typical household use.
- Butyl alcohol, butyl acetate: Industrial solvents.
- Calcium hypochlorite: Generally used in swimming pools and often to sanitize drinking water, such as when used in pellet feeders on water well heads.
- Chloral: Solvent.
- Chloroform: Used as a solvent and formerly in medicine as an anesthetic.
- Chlorine: See bleach. Also available in gas phase for industrial use, such as in municipal water utilities.
- Chlorobenzene: Solvent used in making pesticides.
- Chlorophenol: Pesticide.
- Cresol: Disinfectant and deodorizer.
- Defoliants: Chemical compounds that remove leaves from trees and plants; also used in warfare, such as Agent Orange.
- Diesel fuel: Used in oil heating furnaces and diesel engines for commercial trucks and passenger vehicles.
- Dyes: Used to impart color to tiles, paper, leather, and clothing.
- Ethyl acetate, ethyl acrylate: Solvents and paints.
- Gasoline: Used as fuel for the internal combustion engine.
- Glycols: Used in antifreeze.
- Herbicides: Used as weed killer.
- Hydrogen peroxide: Sometimes used instead of chlorine to reduce increase in total dissolved solids in water or a reaction with background organics that may produce trihalomethanes.
- Hypochlorous acid: Used in skin care products.
- Insecticides: Kills insects or prevents their destruction.
- Iodine: Used in medicine to support thyroid health and help heal wounds, and added to table salt.
- Isopropyl acetate, isopropyl alcohol: Solvent, health care disinfectant.
- Ketones: Created and used by the body as an alternate fuel source in the absence of carbohydrates or glucose.
- Methyl bromide: Pesticide.
- Methyl ethyl ketone: Industrial solvent.
- Naphtha: Used in oil-based paints and gasoline blending.
- Nitrobenzene: A U.S. Environmental Protection Agency (EPA) priority pollutant, typically present in oil-refining wastewater.
- Nitrotoluene: Used in the production of dyes.
- Odors (general): Used in aqueous-phase installations to remove various odors.
- Oil, dissolved: Highly dispersed in water to create a homogenous liquid.
- Organic esters: Used as synthetic base-fluids for lubricant formulations.
- Oxalic acid: Used as an acid rinse in laundries to remove rust and ink stains from fabrics and scale from automobile radiators.
- Oxygen: Depleted from air by wet activated carbon, so workers must take precautions when working inside a vessel.
- Polychlorinated biphenyls (PCBs): Highly carcinogenic, used in capacitors of submersible water pumps and was banned under the EPA Toxic Substances Control Act.
- Pesticides: A group of substances that kill or control pests such as insects, weeds, fungi, rodents, or bacteria, that harm crops, animals, and humans.
- Phenol: Used in the production of phenolic resins.
- Plastic taste: Removed from water by activated carbon.
- Rubber hose taste: See plastic taste.
- Sodium hypochlorite: See bleach.
- Trihalomethanes (THMs): A group of organic chemicals formed in water when chlorine reacts with natural organic matter, such as humic acids from decayed vegetation, which creates chemicals suspected of being carcinogenic.
- Toluene: Dry cleaning chemical; when combined with benzene and xylene, makes up dry-cleaning chemical compound BTX.
- Toluidine: Pesticide precursor to metolachlor and acetochlor.
- Trichloroethylene: Spot cleaning solvent.
- Turpentine: Used in varnishes and for thinning oil-based paints.
- Xylene: Dry cleaning chemical; when combined with benzene and toluene, makes up dry-cleaning chemical compound BTX.
Building an Activated Carbon Contactor
It is very important to keep in mind that media bed depth must not be overlooked. Industry standards for activated carbon recommend minimum bed depths of 36 inches for optimal performance. Toxic organics require much longer empty bed contact time (EBCT) than chlorine or chloramine. Activated carbon contactors are typically placed in the third stage of a treatment array because they are not backwashed like a typical backwash carbon tank. Since contactors don’t require freeboard for backwash, the tank can be filled to capacity. This will increase both media volume and service flow rate.
Proper contactor preparation includes setting up the tank and riser assembly. Next, add enough water to cover the lower collector, and then add the gravel. Rock the tank side to side and to and fro to level the gravel. Then add water to fill the tank by two-thirds, and pour in the activated carbon. As the carbon drops through the water, most of the carbon fines will float, which will make it easier to flush away on startup. This also helps to prevent trapped air in the carbon bed.
Note of caution: Some EPA filter projects stipulate that a standard riser assembly is not permitted because of potential bypass around the riser O-ring seal in the head, and therefore require face piping for the influent and effluent circuits with only a lower collector to prevent contamination leakage to service.
After soaking for 24 hours, the tank can be rinsed up by flushing the bed from the bottom. This is done by routing the water into the outlet port and flushing the bed in upflow condition, flushing the carbon fines until the water is clear and total dissolved solids are stabilized before being placed into service. Then connect the feed water to the inlet and outlet ports for downflow operation. The contactor will have test ports on inlet and outlet piping for sample collection to test for breakthrough of the organics. Tip: To minimize carbon fines when building a contactor, specify water-washed or acid-washed carbon. These contain very low fines and will require significantly less water for rinse-up.
Empty Bed Contact Time
EBCT essentially means a slowdown of the service flow to ensure sufficient residence time between the water and carbon. It is equal to the media volume of the empty bed divided by the flow rate.
Example: One cubic foot of activated carbon is equal to 7.48 gallons. Toxic organic removal requires a service flow of 0.9 gpm per cubic foot of media. A tank that is 10 inches in diameter and 48 inches tall will hold 2 cubic feet (44-inch bed depth) of activated carbon. Therefore, 7.48 x 2 = 14.96 gallons capacity in 2 cubic feet.
0.9 gpm x 2 = 1.8 gpm, so 14.96/1.8 gpm = 8.3 minutes EBCT
For reference, use the following EBCT time for designing a contactor for toxic organics:
8-10 minutes/0.7 gpm per sq ft – 0.9 gpm per cu ft
About the author 
Gary Battenberg is a senior business development manager for Argonide Corporation. His four decades of experience in the water industry covers mechanical and adsorptive filtration, ion exchange, UV sterilization, reverse osmosis, and ozone technologies. He has worked in the domestic, commercial, industrial, high-purity, and sterile water treatment arenas. A contributing author to WC&P and a member of its Technical Review Committee since 2008, Battenberg was voted one of the magazine’s Top 50 most influential people in the water treatment industry in 2009. He can be reached by email at [email protected] or by phone at (407) 488-7203.
About the company
Founded in 1994, Argonide Corporation is a family-owned business focused on water filter manufacturing. Today, Argonide sells filtration cartridges and systems all over the world, commercially and residentially, across many applications and markets. Products include NanoCeram, DEAL carbon blocks (diatomaceous earth media coated with aluminum oxide-hydroxide), and the COOLBLUE Microbiological Water Purifier System.