IODINE RANGE AND OTHER KEY FACTS
The use of granular activated carbon (GAC) for water purification became common in 1906 when the “activation” process was applied to charcoal (which had been used for centuries). Thermal activation of charcoal greatly improves its pore volume, surface area and structure making it an excellent workhorse for water treatment.
THE ADSORPTION CONCEPT
A clean carbon surface is oleophilic, which means “oil loving” and the opposite of hydrophilic or “water loving”. It also has a strong attraction for organic compounds and other non-polar contaminants and adsorbs them onto the carbon surface where they are held by van der Waals forces. Adsorption is the primary mechanism by which GAC works and the main reason it is widely used to reduce undesirable taste, odor and color. It improves the safety of drinking water by also effectively removing common disinfection byproducts (THM’s), organic contaminants like chlorinated solvents and other industrial pollutants, pesticides, and select heavy metals such as lead and mercury. Current GAC products are made from coconut shell, coal, wood, lignite and/or petroleum products. The selection of carbon source is often guided by the contaminant reduction performance targeted by the manufacturer.
Each carbon source produces a GAC with a unique pore structure made up of micro-pores, mesopores and macro-pores. Micro-pores are very small—often smaller than a typical molecule (5-1000 Angstroms) as a reference, an Angstrom (Ȧ) is 1/10,000,000 of a millimeter. To get a better visual example, the period at the end of this sentence is about 5,000,000 Ȧ. Coconut shell carbons have a higher percentage of micro-pores which makes it a good choice for small organics and disinfection byproducts. With wood carbons they have more macro-pores making them better for de-colorization and removal of larger organics. Coal bases offers an intermediate pore structure making them a good choice for general purpose organic reduction. GAC’s capacity for organic removal is derived from its very high surface area, for example, a single gram of GAC can have a surface area exceeding 1000 m2 and a pound of carbon has more than 35 acres of surface— almost 100 football fields. It is not possible to use the complete surface area of GAC nor is it necessary. For instance, if you were to load a one molecule thick, filmed layer of an organic onto the entire surface of GAC, you would have only loaded about 6.25 ml of liquid~~ about 1/4 teaspoon per pound, but since the organic is soluble in itself, the film thickens and the adsorbed weight increases. Systems in GRANULAR ACTIVATED CARBON (GAC) FACT SHEET International Headquarters & Laboratory Phone (630) 505-0160 WWW.WQA.ORG, a not-for-profit organization WQA Technical Fact Sheet series and running to saturation (outlet contaminant level equals inlet level) it is possible to load as much as 0.1 lbs or 45 ml on a volumetric basis per pound of GAC.
The key is to have accessibility to the adsorption sites within the GAC pore structure and surface area available. This is a function of (1) the degree of activation of the carbon base for providing the pathways and is measured by its carbon tetrachloride number (CTC). A value of 50 or more is considered good for water treatment, and (2) the relative surface area of the carbon measured by Iodine Number (Io N) which for potable water treatment should be in the range of 900-1050.
An important consideration in selecting a GAC product is its Abrasion Number (AN) which is a relative measure of the media’s ability to withstand abrasion (or size reduction) when physically tumbled (such as during backwashing). Shell carbons have the highest abrasion resistance at about 90 AN while Coal bases are average at about 70 AN. Ratings below 70 may not hold up well to active backwashing.
The mesh size of a particular GAC may not have an affect on its ultimate adsorptive performance but it does affect hydraulic performance (pressure drop) and kinetics (rate of reaction). An 8×30 mesh carbon is approximately 2mm in diameter, and a 12×40 measures about 1mm and a 20×50 mesh is about use shallower beds. The rate of reaction of a 12×40 mesh is twice of an 8×30 mesh of the same product and a 20×50 mesh is twice as fast as the 12×40. Deep beds of 6-10 feet deep may choose the 8×30 product. Cartridges would use a 20×50 and most with general purpose applications should consider the 12×40 mesh.
Water is an excellent solvent for small amounts of just about everything including organic solvents, the forces by which water holds an organic in solution are quite strong. For the GAC to pull an organic from solution, the forces of attraction must be stronger than the forces of solubilization. Components that enhance organic adsorption onto GAC are: lower pH (organics are more soluble at higher pH), slower flow-measured in minutes of empty bed contact time (EBCT), relative molecular weight (MW) and solubility of the organic. Low MW alcohols such as methanol and ethanol are very soluble in water and not removed by GAC. Although organics are generally less soluble in colder water, temperature also plays a role in mobility (transport) of the contaminant into the pores. Accordingly, cold water may actually inhibit the removal of that contaminant. Additionally, some organics are volatile and can be desorbed (released) from the GAC in hot water, using a hot water source to backwash the GAC will actually do a partial regeneration on it.
Color bodies such as tannins are high MW organics that can be removed by GAC, although a proper product must be selected to have the right pore structure for the task. Wood based GAC products have been known to have a long history of use for de-colorization.
Empty Bed Contact Time (EBCT), represents the hydraulic flow rate through a GAC bed and is usually reported in “minutes”. In other words, it is the volume of media (in gallons, m3, ft3) divided by the flow volume (in gallons, m3, ft3) per unit of time (minutes, hours). A one cubic ft. GAC filter is 7.5 gallons of GAC and a flow of water through that filter at 2.5 gpm has an EBCT of (7.5/2.5) = 3 minutes. Take in account you need to use the same volume units to do the calculation. A GAC filter must have adequate EBCT to do the job plus sufficient safety margin to cover variations in the quality of the water being treated. Organics such as pesticides and THM’s require long EBCTs because of the safety margin needed, think of the “consequences of failure” when sizing your design. A second concept in retention time is that of “Half Lengths”, which states that if it takes x number of seconds (or minutes) to reduce a particular contaminant by 50%, it will take x number of seconds to reduce the residual level of contaminant by an addition 50% (of the remaining). Therefore, if one half-length removes 50%, two half lengths will remove 75% and three will remove 87.5, then about seven half lengths are required to remove 99% and ten to reach 99.9%. This is the reason why many municipal GAC systems used for reducing the level of THM’s to single digit ppb levels will have EBCT’s of 25 minutes or more. Also, to the contact time, there are hydraulic flow considerations for GAC. Filters can’t be too large nor too small. Large filters are prone to channeling when run too slowly and small filters have a superficial (surface) flow of 4-10 gpm/ft2 (minimum to maximum) with temporary peaks not exceeding 12-15 gpm/ft2. Volumetric flows can be 1-10 gpm/ft3 for (minimum to maximum). Take note that the EBCT range for that volumetric flow is 7.5 minutes down to about 45 seconds.
GAC has the ability to catalytically reduce chlorine from an oxidizer (NaOCl) to a salt (NaCl) by taking away the reactive oxygen, which is a very fast reaction requiring an EBCT of only 30-40 seconds. With a 10 inch cartridge with a flow of 0.5 gpm is about 25 seconds EBCT and can remove 95% of incoming chlorine for 2500 gallons. A cubic foot of the same GAC flowing at 5 gpm (EBCT = 1.5 minutes) can do the same job for 1,000,000 gallons and can make about 40 cartridges (with an implied capacity of 40×2500 = 100,000 gallons). The longer EBCT allows the GAC to work better for a longer time because the longer EBCT allows more of the GAC to take part in the process. More communities are switching from chlorine to chloramine (usually NH2Cl, the mono-chloramine) in order to reduce the tendency for chlorine to form THM’s when used as the primary disinfectant. The deactivation of chloramine can be accomplished with GAC although the filter has to be sized about four times the size of a regular GAC filter used for chlorine. To increase the EBCT, filter units may be strung in series. Catalytic carbon, which has an oxidizing capability is more effective for chloramine reduction with EBCT’s of an average of about 5 minutes.
ACTIVATED CARBON FOR RADON REMOVAL
Radon (Rn) is a radioactive gas that does not transmit taste or odor to water. It emerges from the natural decay of uranium to radium and radium to radon (radon is known to increase the risk of lung cancer when inhaled and is the second leading cause of lung cancer in the US today). Ingestion of radon contaminated water can lead to an increased risk of stomach cancer. Radon in drinking water is not federally regulated, although EPA has a longstanding proposal for an MCL of 300 pCi/L, or an alternative MCL of 4000 pCi/L. Nonetheless, limits in drinking water differ by state and range from 300 to 20,000 pCi/L. GAC’s radon reduction performance will be reliant on carbon source, bed design, radon loading, and to some extent water chemistry. Radon performance is directed by the establishment of a steady state rate constant (Kss) where the adsorption and subsequent decay to radon daughters comes into equilibrium with the radon being loaded from the influent water. GAC has been exhibited to achieve greater than 90% reduction of radon from treated drinking water. Yet for POE, or small system applications the EPA suggests using this technique only when the radon content is < < 5000 pCi/L to establish that there are no excessive gamma emissions from the carbon and to restrict the buildup of long rived radionuclides (i.e. 210 lead) which can generate handling and disposal concerns. It is suggested that media should be replaced at least on an annual basis. The US EPA Carb dose program can be used to determine anticipated radon progeny activity at end of media life. Ion exchange pretreatment, or backwashing can be used to avoid fouling of the media. At low influent radon activities (< 5000 pCi/L), EPA considers GAC to be a favored treatment technology for POE, provided that the water being treated does not consist of long-lived radionuclides. At higher influent levels (> 5000 pCi/L) aeration techniques are favored. NSF/ANSI Standard 53 includes an analysis protocol for evaluating radon reduction for POU devices. The protocol limits use to influent levels of radon below 4000 pCi/L and requires a minimum 90% reduction of radon in the treated water, the Standard also limits filter life to one year.
There will come a time when the GAC becomes exhausted and you would need to re-bed the media. If the GAC was used for potable water and is deemed non-hazardous, there are no concerns regarding disposing of it to an industrial landfill. If the use was industrial waste of known hazardous contaminants, the spent carbon must be tested to determine if it would be classified as hazardous or non-hazardous and this may depend upon the concentration of contaminants contained. Hazardous media is classified by toxicity, ignitability, corrosivity or reactivity. For more information, guidelines can be found on the EPA website under: EPA Hazardous Waste Generators, Title 40, Part 261.
WASTE GENERATOR DEFINED
The US EPA classifies “waste generators” by size. For abundance of “haz” waste under 100 Kg/month (220 lbs), generators may be labeled as “Conditionally Exempt Small Quantity Generators” (CESQG). No permit is needed, and they may accrue up to 1000 Kg (2200 lbs) at any time. They MUST, however, turn over such waste to someone authorized to manage it and more than likely involving a fee. Larger generators have greater limits but require permits and inspections. A substitute to landfill disposal is returning the spent GAC to the manufacturer for re-activation, this is usually reserved for large scale carbon users with a substantial quantity of media. Reactivation uses a special high temperature furnace to vaporize most contaminants and reestablishes the clean pore structure of the original carbon. Take note that re-activated carbon cannot be used for potable water applications unless it is the same GAC previously used in the process by the same user. Reactivated GAC may still contain toxic inorganics that may be changed in their degree of solubility by the reactivation process. Re-activation should be considered only as a means of disposal. Another removal technique is using the spent carbon as fuel in a cement kiln. The spent carbon has a high recoverable fuel amount and is demolished in the process, this option is best reserved for larger quantities of hazardous media. As part of treatment system installation procedures, system performance characteristics should be verified by tests conducted under established test procedures and water analysis. Consequently, the resulting water should be monitored periodically to verify continued performance. The operation of the water treatment equipment must be controlled diligently to assure that acceptable feed water conditions and equipment capacity are not exceeded. Consumers should get in contact with a water treatment professional as described above or follow the equipment manufacturer’s installation instructions and contact the manufacturer to confirm usage and capacity. To ensure the manufacturer can administer the most accurate recommendations, have test results for lead and iron on hand for review.