By Rick Andrew
Activated carbon (AC), which plays a major role in POU/POE water treatment, has been successfully employed in water treatment for many years. The treatment capabilities and limitations of AC (whether derived from coal, wood, coconut shells or other sources) have been widely researched and are generally well understood. It is used in granular, powdered and carbon block forms, as well as some other, more novel configurations.
Given this history, it is no surprise that the two earliest of the NSF/ANSI DWTU Standards for POU/POE systems—NSF/ANSI 42 Drinking Water Treatment Units – Aesthetic Effects (first adopted in 1973) and NSF/ANSI 53 Drinking Water Treatment Units – Health Effects (adopted in 1980)—address activated carbon systems. Included in these two standards are requirements for the material safety, structural integrity, contaminant reduction performance and product literature for POU/POE activated carbon systems. The two standards share common requirements for material safety and structural integrity and are separated in scope by the types of contaminant reduction claims addressed by each. NSF/ANSI 42 includes requirements for claims of aesthetic treatment of drinking water; NSF/ANSI 53 includes requirements for claims of treatment of water for contaminants with health effects.
Claims under these standards can be either chemical reduction, which is achieved by adsorption of an active media (such as AC), or mechanical reduction, which is achieved by a physical sieving mechanism. Although activated carbon can contribute to mechanical filtration, especially when combined with other ingredients and formed into a carbon block, many people think of the adsorption capabilities associated with AC. These capabilities are tested in POU/POE systems according to chemical reduction test methods specified in NSF/ANSI 42 and NSF/ANSI 53.
Chemical reduction testing methods
NSF/ANSI 330 Glossary of Drinking Water Treatment Unit Terminology defines chemical reduction as, “The removal of a chemical contaminant contained in the influent water by a water treatment system or process.” The list of potential chemical contaminants is a very long one, as over time more and more chemicals are used for industrial and manufacturing processes, cleaning processes, fuels and a broad range of other purposes, and can end up in the environment and in bodies of water used as source water for drinking purposes.
Activated carbon performs its chemical reduction function through the mechanism of adsorption. It has broad capabilities but ultimately limited contaminant adsorption capacity, meaning that as more and more water is treated and more and more contaminants adsorbed, the media eventually becomes saturated to the point where it is less and less effective. With this in mind, it is important for test methods for POU/POE systems for chemical reduction capabilities to address treatment capacity. In these systems, there are multiple factors that influence treatment capacity, including:
- Design of the POU/POE system
- Chemical properties of the contaminant
- Concentration of the contaminant
- The maximum allowable effluent concentration
- Operational cycle
As one might expect, all of these factors are addressed in the test methods for chemical reduction in the standards.
Design of the system. NSF/ANSI 42 and NSF/ANSI 53 require that plumbed-in POU systems be tested with a valve to control on and off cycling, either upstream or downstream of the test system, depending on whether the system will be installed upstream or downstream of the faucet in end-use applications.
Chemical properties of the contaminant. In most cases, testing of the specific contaminant being claimed is required because the chemical properties (such as molecular weight, water solubility, electrical charge and others) vary depending on the molecular structure of the contaminant. In a few cases, scientific studies have demonstrated that testing for one specific contaminant can support claims that other contaminants will also effectively be treated by AC systems. The most significant of these is the VOC surrogate claim under NSF/ANSI 53, which specifies that chloroform can be used to verify reduction of a specific list of organic contaminants, based on extensive research studies conducted by NSF and other laboratories.
Concentration of the contaminant. The contaminant concentration in the influent challenge water under NSF/ANSI 42 is set at a typical value based on occurrence. Under NSF/ANSI 53, the concentration of the contaminant in the influent challenge water is usually set at the 95th percentile of occurrence. This occurrence information is typically derived from US Geological Survey data or other similar databases of contaminant concentrations in source water. The 95th percentile of occurrence means that 95 percent of those water sources identified to be contaminated with the contaminant in question have a concentration equal to or lower than the challenge concentration used in the standard. If use of occurrence data suggests a very low concentration for the influent—or when occurrence data is not available—a concentration of three times the maximum allowable effluent concentration is used.
Maximum allowable effluent concentration. This refers to the concentration of the contaminant allowed to remain in the treated water. Aesthetic contaminants under NSF/ANSI 42 is set based on US EPA secondary maximum contaminant levels or other aesthetic thresholds. The maximum allowable effluent concentration for health contaminants under NSF/ANSI 53 is set at regulated levels based on US EPA, Health Canada levels or at other health effects concentrations when contaminants are not regulated by these agencies.
Flowrate. NSF/ANSI 42 specifies operating the test system at the manufacturer’s specified flowrate. NSF/ANSI 53 requires operating the test system at the highest achievable flowrate with an initial clean system inlet pressure of 60 psi.
Operational cycle. To simulate field usage involving intermittent starting and stopping of flow through the system, the test methods under NSF/ANSI 42 and NSF/ANSI 53 require cycling the flow on and off using valves. This cycle is typically a 10-minute-on and 10-minute-off cycle known as 50/50-cycling. At the manufacturer’s discretion, a 10/90-cycle can be used, which is typically two minutes of flow followed by 18 minutes without flow. Both of these test cycles are more aggressive than would be demonstrated in real-world applications.
Establishing treatment capacity
Testing includes sampling of the influent and effluent water at specified intervals throughout the test, usually at six different sample points. Effluent concentrations for aesthetic contaminants under NSF/ANSI 42 must be below the maximum allowable effluent concentration to 100 percent of the manufacturer’s rated capacity based on volume. Effluent concentrations for health contaminants under NSF/ANSI 53 must be below the maximum allowable effluent concentrations to 200 percent of the manufacturer’s rated capacity based on volume. For those systems incorporating a performance indication device (PID, end-of-life indicator), which must be a volume-based filter change indicator, effluent concentrations must be below regulated levels to 120 percent of the manufacturer’s rated capacity based on volume.
Comprehensive and proven testing methods
The NSF Joint Committee on Drinking Water Treatment Units continues to develop NSF/ANSI 42 and NSF/ANSI 53 based on new developments in POU/POE technology, regulatory initiatives, concerns regarding specific contaminants and developments in testing methodologies. The basic concepts of both activated carbon technology and the NSF/ANSI 42 and NSF/ANSI 53 test methods, however, are well established. With origins in the 1970s, the chemical reduction and mechanical filtration test methods used in these standards have proven to be very effective in qualifying the capabilities of AC filters that have long been a tried-and-true tool in the water treatment professional’s toolbox.
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