March 2003: Volume 45, Number 3
Material Extraction Testing of Activated Carbon: A Tale of Two Standards
by Rick Andrew and Richard A. Martin
One of the driving philosophies of all water product-related ANSI/NSF standards is the only way to ensure compliance is testing. These standards contain various requirements for many different types of products and, in almost all cases, conformance with a given requirement is established through testing according to a specific protocol.
The central issue of all these standards is safe water. Consumers and regulators in the United States demand high quality water with strict limits on residual contaminant levels. And, in keeping with the philosophy of testing to verify conformance, the standards have test protocols to verify water products don’t contribute unacceptable levels of contaminants to the water that’s in contact with them.
Activated carbon is no exception. There are material safety requirements for activated carbon used for drinking water purposes. Conformance with these requirements is verified through material extraction testing. There are actually two different standards with distinctly different requirements and material extraction test protocols—ANSI/NSF Standard 42 and ANSI/NSF Standard 61. This article examines why there are two standards, in which circumstances each standard applies, and what work is being done to try to harmonize the requirements of these standards.
Standard 42 applies to activated carbon intended for use in consumer drinking water treatment units (DWTUs). This includes carbon used in residential point-of-use and point-of-entry (POU/POE) systems. This intended end-use can result in long periods of stagnation with water just sitting in contact with the carbon, especially for POU systems. This water is often then consumed when someone turns on the water and fills a glass with the first water coming through the filter.
This prompted the Joint Committee on DWTUs to develop a material extraction testing protocol with minimal conditioning prior to exposure to the test water. Standard 42 requires carbon be exposed to water under 50 pounds per square inch (psi) pressure. To comply with this requirement, the test is conducted with carbon packed in a housing. The protocol specifies the carbon be flushed according to the manufacturer’s instructions, and then exposed to the test water for 24 hours. This water is saved. The carbon is then flushed again according to the manufacturer’s instructions, and exposed to the test water for another 24 hours. This water is also saved. The carbon is then flushed one final time according to the manufacturer’s instructions, and exposed to the test water for a final 24 hours. This water is saved, and composited with the two samples of water saved from the first two exposures. The final composite sample is the exposure sample that’s analyzed for residual contaminants.
Standard 61, Section 7, applies to process media products intended for the reduction of dissolved or suspended materials present in drinking water. Products covered include, but are not limited to, process media used in the following processes—ion exchange, adsorption, oxidation, aeration and filtration. One such adsorptive media is activated carbon.
Activated carbon, typically in granular or powdered (GAC or PAC) form, may be utilized in municipal water treatment applications. In comparison to POU/POE applications, very large volumes of carbon are commonly used in adsorption “beds” to capture contaminants during the water treatment process. This intended end-use involves constant flow with no stagnation.
Standard 61, Section 7 also outlines the general requirements for the evaluation of media including manufacturer use instructions, specifics for sample preparation, sample conditioning, and testing of products such as PAC and GAC. Before PAC or GAC media may be analyzed, the GAC sample must be wetted or completely immersed in tap water prior to conditioning for 16 hours +/-1 hour. PAC isn’t wetted unless specified in accordance with the manufacturer’s instructions. After the sample has been properly wetted, GAC undergoes a conditioning or backwashing phase for 30 minutes +/- 2 minutes, or in accordance with manufacturer instructions. PAC isn’t backwashed because of the small particle size.
Next, the media sample is exposed to the reagent water required to generate water samples that will be analyzed. The vessel is covered and placed on a magnetic stirrer for 60 minutes +/-5 minutes. Immediately following the exposure period, the liquid portion is filtered to separate the water and media for analysis.
The analytical testing for PAC or GAC involves analysis for formulation and process dependent analytes and at least the minimum analytes specified in table 7.1 of ANSI/NSF Standard 61-2002. Currently, the minimum list of analytes includes antimony, arsenic, barium, beryllium, cadmium, chromium, copper, lead, mercury, selenium, and thallium as well as GC/MS (base neutral acid scans) and radionuclides. After the data are generated, it’s normalized based on the testing situation and evaluated for compliance against the single product allowable concentration (SPAC) noted in Annex A of ANSI/NSF Standard 61.
Many carbon suppliers service both the DWTU and municipal treatment markets. As a result, they must demonstrate to their customers their carbon conforms to the requirements of both standards.
To simplify the situation for suppliers and purchasers of media, the Joint Committees on DWTUs and Drinking Water Additives have created a cooperative task group. This group is investigating the concept of harmonizing the materials’ requirements for media between the standards. They must first conduct testing to determine comparability of test results on the same media under both standards. Once that work is done, the group will proceed toward developing a harmonized protocol that will have as minimal an impact on currently conforming carbons and other media as possible, while meeting the goals of ensuring safe water for all usage patterns of related products.
Dual certification of carbon
While the harmonization effort continues, many carbon suppliers face the prospect of dual certification with standards 61 and 42. Standard 42 exposes the product under pressure; Standard 61 doesn’t. Standard 42 exposes various amounts of carbon media. Standard 61 exposes 25 grams +/-5 grams of carbon media per liter of exposure water. Due to distinctly different end uses, there may be no way to avoid testing the carbon to conform to both standards. There are ways, however, to make the rest of the dual certification as easy as possible. Only one formulation disclosure is required for certification to both standards. The certification audits to each standard are coordinated, therefore, maintaining one audit of the manufacturing facility.
There are other benefits to having activated carbon certified to both standards as well—reduced formulation review requirements and costs, and reduced material extraction testing costs for DWTU manufacturers using Standard 42 certified carbon are two of them. Another benefit is confidence that the carbon is safe for water contact in any end-use application. Safety for water contact is the primary issue addressed by the water product-related standards, and there’s no doubt carbon conforming to standards 42 and 61 is safe for a wide range of applications.
About the authors
Rick Andrew is technical manager of the DWTU Program at NSF International. He has been with NSF for four years. His previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a bachelor’s degree in chemistry and a master’s degree in business administration from the University of Michigan. He can be reached at email: email@example.com
Richard A. Martin is the business unit manager of the Water Distribution Systems Municipal Product Program at NSF International. He has worked for NSF for nine years in a variety of capacities. Martin has a bachelor’s degree in physiology from Michigan State University. He can be reached at email: firstname.lastname@example.org