By Ellen Van Buren
Certified products and standards
The use of activated carbon for drinking water treatment is common in both residential filtration and municipal water treatment. Examples of activated carbon media that are certified for drinking water treatment applications include granular activated carbon (GAC), powdered activated carbon (PAC) and reactivated GAC/PAC. These are process media that are intended for the reduction of dissolved chemicals or suspended materials present in drinking water. The scope of this article includes loose media only and does not include carbon blocks or carbon filter cartridges, although these products may also be certified.
The certification options for activated carbon media include:
- NSF/ANSI 61 for activated carbon media used in a municipal/community water system. This is a system for providing water for human consumption through pipes or other constructed conveyances that has at least 15 service connections or regularly services at least 25 individuals.
- NSF/ANSI 42 for activated carbon media used in a POU system. A POU system is used to treat the water supply at a single tap for drinking purposes.
- NSF/ANSI 42 for activated carbon media used in a POE system. A POE system is used to treat the water supply at a building or facility for drinking, washing and flushing or for other non-consumption, water supply purposes.
The product manufacturer selects the applicable certification to obtain for their product based on the intended end use of the product and target market they are seeking. If the manufacturer is targeting multiple markets, activated carbon can certainly be certified to multiple standards and/or uses.
Material extraction testing differences
Both NSF/ANSI 42 and NSF/ANSI 61 require material extraction testing to ensure the product does not contribute contaminants at unacceptable levels into the drinking water. NSF/ANSI 42 includes a test protocol specific to POU end uses and also references the protocol and criteria in NSF/ANSI 61 for POE products. NSF/ANSI 61 also includes a different protocol for activated carbon used in municipal water treatment. The test protocols (including conditioning, exposure water and exposure method), however, differ based on the differences in the end use of the activated carbon. For example, activated carbon packed tightly into a POU filter with low flowrates and longer stagnation times versus activated carbon used in a water treatment facility added directly to large-volume treatment tanks with continuous flow of water.
Conditioning the media
For activated carbon media certified to NSF/ANSI 42 for POU end use, no conditioning wash process is required. Manufacturers may submit use instructions with conditioning instructions to be incorporated into the test. Or they may elect to test without flushing the media. NSF/ANSI 61 also allows for manufacturers to submit use instructions to be followed for conditioning. If use instructions are not provided, the standard requires that adsorption media >/= 0.25-mm diameter be conditioned for 30 minutes. All other media is not required to be conditioned.
After conditioning (if elected), the NSF/ANSI 42 material extraction testing begins by flushing the product in accordance with the manufacturer’s instructions and then immersing the product in a pressure vessel in the exposure water outlined in Figure 1 for 24 hours. A water sample is then collected. This procedure is repeated for two more 24-hour periods, using fresh exposure water each time. The three samples are composited and a full chemical analysis of the composited exposure water sample is conducted based on the formulary review of wetted materials. Any contaminant detected in the water must not exceed maximum concentration values, as indicated in the standard.
Media < 0.25 mm in diameter. The sample is immersed in the exposure water outlined in Figure 1. The amount of media per volume of exposure water must meet or exceed the weight-per-volume ratio shown in Figure 2 and be able to produce enough exposure water to complete the analysis. The vessel is covered and placed on a magnetic stirrer for 60 ± 5 min. Then the liquid portion is passed through a Whatman #41 filter as well as a 0.45 μ-filter and the resulting filtrate is collected. The solid portion remaining on the filter is dried, weighed and used to calculate the evaluation dose.
Media of ≥ 0.25 mm in diameter. The sample is immersed in the exposure water outlined in Figure 1. The amount of media per volume of exposure water must meet or exceed the weight-per-volume ratio shown in Figure 2 and be able to produce enough exposure water to complete the analysis. The contents are mixed to ensure that the entire sample is in contact with the exposure water. The vessel is then sealed with polytetrafluoroethylene (PTFE) and the sample is exposed according to the schedule outlined in Figure 3. The weight-to-volume ratio is recorded at the time of exposure and represents the evaluation dose.
NSF/ANSI 42 references NSF/ANSI 61 for POE media. The sample is immersed in the exposure water outlined in Figure 1. The amount of media per volume of exposure water must meet or exceed the weight-per-volume ratio shown in Figure 2 and be able to produce enough exposure water to complete the analysis. After conditioning according to the manufacturer’s instructions, the exposure vessel is refilled with the exposure water in Figure 1 and maintained for 24 hours at a temperature of 23 ± 2°C. The exposure vessel is then flushed with five unit volumes, refilled and maintained for a second 24-hour period. The exposure vessel is then again flushed with five unit volumes, refilled and maintained for a third period of 24 hours at 23 ± 2°C. At the end of the third 24-hour exposure, the extraction water sample is collected for analysis.
Under NSF/ANSI 61 for municipal end use, the concentration of analytes present in the extraction water is multiplied by calculated normalization factors to account for differences between the actual laboratory evaluation ratio and the weight-per-volume ratio. Materials in contact with drinking water must not impart levels of compounds that exceed the total allowable concentration (TAC), maximum contaminant level (MCL) or maximum acceptable concentration (MAC), as specified in the standard.
Different test methods appropriate to varying end uses
The end uses for activated carbon are quite different from each other in terms of usage level. Think about free-floating PAC used in municipal treatment versus tightly packed GAC used in a POU filter, as well as the potential for stagnation (municipal treatment has no stagnation and continuous flow, compared to POU, which has stagnation almost all of the time except when someone is dispensing drinking water).
The differences in the extraction testing protocols for POU, POE and municipal end uses were developed to account for these different end uses of activated carbon media. There is not one certification that is necessarily more stringent than the others, or right or wrong; rather, different options available based on how the activated carbon would actually be used to ensure the highest level of public health and safety in terms of assurance against excessive contaminant leaching.
Ellen Van Buren is NSF’s Senior Business Development Manager-Water Systems. She has a Bachelor’s Degree in chemistry from Michigan State University and can be reached at (800) NSF-MARK or email: Vanburen@nsf.org