By Gary Hatch, Ph.D.
Summary: Several naturally occurring materials are used to produce activated carbon for use in drinking water filter cartridges. Each raw material offers unique advantages in specific applications, and is selected based on performance requirements and economic factors. In certain cases, coconut offers several benefits over coal. Following is an outline of these benefits, comparing inherent characteristics of coconut with other materials that ultimately affect the performance and, in some cases, the cost of a drinking water cartridge.
Activated carbons are amorphous charcoals obtained from heating or burning carbonaceous materials under controlled conditions. This process drives off volatiles and forms the basic char residue from which the activated carbon product is made. The usual carbonaceous starting materials are coal, oil, lignite, peat, wood and certain types of nutshells, the most common being coconut shells. After the charring process, further treatment—activation—is needed to give the final product its unique functional characteristics and properties. Activation is the process by which the internal pores are formed within the charred carbon particles. These narrow channels provide adsorption sites.
Activated carbon manufacturers utilize two primary activation processes—steam and chemical. Most of the carbon used today is processed by steam activation. In this process, the pores are opened and enlarged, forming intricate narrow channels of varying sized pores.
Each starting carbonaceous material possesses inherent properties for forming different size ranges of internal pores. In general, peat and lignite tend to form relatively large pores. Coal-based carbons tend to form a wide range of pore sizes (depending on the type and degree of activation). Coconut shell carbons tend to have much higher pore volume in the microporous region (<40 Angstroms, see Figure 1) and a slightly lower pore volume in the macroporous region. This is because coconut char is less amorphous (harder, more crystalline-like) than coal. The result is that coconut-based carbons adsorb smaller organic molecules, such as chloroform and other trihalomethanes (THMs), trichloroethylene (TCE), carbon tetrachloride and MTBE (methyl tertiarybutyl ether), while coal-based carbons more effectively remove larger molecules, such as color-producing compounds (tannins and humics).
In addition, because of the tenacity (high energy of adsorption) of the coconut-based carbon’s micropores, once the smaller organic molecules are adsorbed, they’re very tightly retained. Therefore, coconut shell carbons in general have a higher retentivity (see Figure 2) for these types of contaminants than do other types of carbons. For these reasons, coconut shell carbons are often selected for drinking water treatment where contamination is attributed to organic solvents or disinfection by-products (DBPs) such as THMs.
Figure 2 here (sent to David on May 25).
Another advantage of coconut shell carbon is that it’s typically harder than coal, lignite and peat-based carbons. This can be important in the initial shipping of the bulk material, and in the manufacturing/handling process and final shipping of the finished product.
Softer carbon granules may break down during bulk shipment and create dust and fines. During handling and assembly of cartridges, these fine particles may cause dust problems that in turn requires installation of venting and dust handling equipment, not to mention disposal of the waste dust. During shipping of the finished product (e.g., cartridge or system), constant vibration may also cause the carbon particles to grind against each other, again creating fines. Though not harmful, discharge of excess visible carbon fines during use could cause concern among end-users, i.e., customers. Utilizing a harder carbon such as a coconut carbon will minimize these concerns.
Carbon quality & purity
The overall quality of any activated carbon largely depends on the source of the raw material, as well as the quality assurance/quality control (QA/QC) program of the raw material supplier and the carbon manufacturer/processor. Likewise, the final quality of a finished product utilizing an activated carbon also should be verified. This can be substantiated in several ways, such as: 1) ISO certification of the carbon manufacturing process; 2) NSF, UL or other third-party testing and listing of the specific carbon product; and 3) NSF, UL or other third-party testing and listing of the finished product that uses the carbon.
Most carbon manufacturers and suppliers have their carbon products tested and listed with a third-party certifier of process media and equipment. Depending on the end-use (municipal treatment or use in a finished water treatment product/device), the carbon could be tested and listed under two different standards: ANSI/NSF Standard 42-Drinking Water Treatment Devices, Aesthetic Effects, or ANSI/NSF Standard 61-Drinking Water Additives. Both involve different methods of extraction testing to determine safety of the materials used in the product such that they do not impart anything above federal drinking water standards.
Process media used in large municipal water treatment applications usually are listed under Standard 61. A listing under Standard 42 is for either a specific volume/quantity application for media used in point-of-use/point-of-entry (POU/POE) devices or is for the specific device itself. The actual extraction testing procedure for each standard is different, and is more stringent under Standard 42.
The current Standard 42 extraction procedure utilizes a moderately aggressive low pH (6.5-7.0) and low total dissolved solids (TDS, 45-55 mg/L) water. The cartridge or system is flushed according to the manufacturer’s recommended flushing instructions and is allowed to soak for 24 hours. A “unit volume” of water is then collected, and the unit is flushed again and allowed to soak another 24 hours. After the second 24-hour soak, another unit volume sample is collected and the unit is flushed once more. This is repeated a third time. Each of the three 24-hour stagnation samples is composited into one sample and analyzed for extractables. All established extraction parameters must meet the Maximum Drinking Water Level (MDWL) as listed in the standard. The MDWLs are based on the U.S. Environmental Protection Agency’s primary or secondary maximum contaminant levels (MCL) for drinking water.
A recent report by a major carbon supplier1 indicates some sources of raw carbon materials may contain trace levels of arsenic. Though this report and a related article2 do show coconut carbon is typically somewhat cleaner than coal-based carbon—depending on lot selection and rinsing procedures—either source can provide a safe, high quality carbon for use in drinking water applications. If coal-based carbons, however, become more susceptible to lot selection and clean-up costs, coconut shell carbons may become more price competitive. Also, this report shows how important a reliable QA/QC program is for allowing equipment/cartridge manufacturers to consistently acquire high quality carbon and for eventually meeting the ANSI/NSF extraction requirements.
There has been some concern that coconut shell carbons aren’t consistent in quality. Again, the previously mentioned study on varying quality of different carbon sources forewarns users of the importance of a rigorous QA/QC testing program to verify consistent quality. For equipment manufacturers utilizing coconut shell carbon, selecting a supplier should include the “three Rs.” Is the source/supplier reputable, reliable and repeatable, i.e., do you get the same carbon quality in every shipment? This can only be verified if the equipment manufacturer creates a last line of defense by establishing its own strict QA/QC program for testing the incoming carbon not only for purity but also for performance parameters of its intended use. Only then can the carbon and the final product in which it’s used be assured of having consistent quality.
A ‘greener’ approach
One feature not mentioned and that yet may offer attractive selling appeal to the end-use customer is that a coconut shell carbon could be considered a “renewable” product. Coconut carbon originates from a naturally grown coconut tree, is usually activated by steam, and typically isn’t acid washed nor otherwise chemically treated. Coal, lignite and peat-based carbons, though originating from natural vegetation, are mined from the ground and usually require acid washing to reduce undesirable extractables.
A drinking water treatment product that utilizes “renewable” coconut shell carbon has many real advantages. Dealers may be able to transform those advantages into sales. And, for the end-use customer, the biggest advantage will be a higher quality product that produces higher quality drinking water.
Though other sourced-based carbons have their place in broad-spectrum use for general water treatment, a number of advantages exist for selecting coconut shell carbon specifically for drinking water treatment. Basically, these advantages center around adsorptive and physico-chemical properties.
In the end, pore structure, performance, hardness and more consistent quality and purity are all important factors that must be weighed when selecting the type of activated carbon utilized in drinking water cartridges. And in many cases, coconut shell may offer many advantages, but it’s up to the water treatment equipment manufacturer to consider all the factors and their impact on the cartridge’s performance and costs before making a decision between coconut shell and other possible sources.
This article is intended to provide general information only and should not be construed as advice with respect to any particular applications or matter. The effectiveness of coconut shell carbons may vary depending on source, water conditions and certain other operating variables.
- “Impact of New Arsenic Standards on Point-of-Use Carbon Filtration,” Barnebey Sutcliffe Corporation, 2001.
- Bayati, Mohammed and M. Stouffer, “Impact of Proposed New Arsenic Standards on POU Carbon Filtration,” WC&P, July 2000.
About the author
Dr. Gary Hatch is the director of research and development for the USF Consumer & Commercial Group, Sheboygan, WI. He’s responsible for new product development and new materials qualification for use in filtration products manufactured for Culligan, American Plumber and USFilter brand water treatment products. Hatch has a doctorate degree in analytical/inorganic chemistry from Kansas State University. He can be reached at (800) 222-7558 or email: email@example.com.