By Rick Andrew

Drinking water treatment technologies have specific capabilities, strengths and weaknesses. For example, activated carbon is very effective for treating organic contaminants in drinking water, although it does not have any ability to reduce water hardness. UV treatment can produce great results in disinfection of drinking water, although the water must be clear and free of UV-absorbing compounds in order for the treatment to be maximized. A similar analysis of capabilities, strengths and weaknesses can be performed for other types of water treatment technologies. These unique advantages and limitations can make it advantageous to have product configurations that include more than one treatment technology in series. In NSF parlance, these systems are called treatment-train systems. Much like a train with an engine, freight cars and caboose serving different functions, the various stages in a treatment-train water treatment system can be effective on different types of contaminants, and can also serve to enhance the effectiveness of downstream treatment technologies. The evaluation of treatment-train systems to the NSF/ANSI Drinking Water Treatment Unit (DWTU) standards might seem at first to be quite complicated, because of the various technologies and standards. It is, however, actually more straightforward than one might think.

Technology-specific standards
The DWTU standards each address different treatment technologies. Test methods for evaluating contaminant reduction performance must be appropriate to the technology being tested, so different technologies require different standards with different contaminant reduction test methods. Figure 1 includes an overview of standards, technologies and required contaminant reduction claims. This paradigm leads to a clear choice regarding which standard applies to a given water treatment system that contains one technology only, such as a carbon filter or a UV system. For treatment of aesthetic contaminants, carbon filters are evaluated to NSF/ANSI 42. For treatment of health effects contaminants, carbon filters are evaluated to NSF/ANSI 53. UV systems fall under the scope of NSF/ANSI 55. Treatment-train systems, however, include multiple technologies in series, which leads to the question regarding which operative standard(s) is/are most appropriate.

POU RO systems
POU reverse osmosis systems most often include other types of filtration, be they mechanical filtration and/or carbon filtration. The configuration of a POU RO system with prefiltration options (possibly mechanical prefiltration for well systems and carbon prefiltration for public water supplies) and a carbon postfilter is very common. Because of this common configuration, NSF/ ANSI 58 directly addresses treatment trains. The standard specifies that these systems be tested for contaminant reduction performance with any pre- or postfilters removed. The RO contaminant reduction test methodologies in NSF/ANSI 58 are appropriate for evaluation of the membrane system, but they are not designed to test an adsorptive or absorptive media filter. Based on the membrane’s performance in the system, all reverse osmosis systems must make a claim of TDS reduction to reduce TDS concentration of an incoming challenge stream of 750 mg/L NaCl in deionized water. Additional claims may be based on contaminant reduction performance of the membrane in the system.

NSF/ANSI 58 also includes claims for reduction of certain additional contaminants based on performance of activated carbon postfilters. There are actually two different test protocols for postfilters; one for those installed upstream of the product water storage tank (i.e., extended contact postfilters), and the other for those installed downstream of the storage tank. These two protocols are reflective of the different operating conditions for postfilters installed in two different locations in the system. Both protocols involve testing the postfilter in isolation from the membrane.

Contaminant reduction claims on RO systems are based on the performance of the reverse osmosis element in the system, without pre- or postfilters, and additional claims may be made based on the postfilter alone, without the element. The assumption here is that when water is treated by the entire system treatment train, with the RO element followed by a postfilter in series, the contaminant reduction capabilities of each individually tested component will meet or potentially exceed the performance of those components when they are individually tested. This has evolved into the treatment-train concept, which can be more broadly interpreted to evaluate other treatment-train systems.

Other treatment-train systems
Reverse osmosis systems are probably the most common example of treatment trains, but there are many others. Two- stage filter systems that use a sediment filter cartridge followed in series by a carbon block filter cartridge are treatment trains. Utilizing the treatment-train concept, it would be appropriate to evaluate contaminant reduction performance on the carbon block filter cartridge alone, and then determine that a system incorporating a sediment filter cartridge upstream of the carbon block filter cartridge would have at least equivalent contaminant reduction performance. The two technologies in a treatment train do not actually have to be in separate elements or sumps within the system. A filtration element consisting of a carbon block combined with a prefiltration ‘wrap’ could be considered a treatment train. The logical conclusion regarding this type of system is that any contaminant reduction claims tested on the carbon block alone could also be applied to an element consisting of the carbon block and an added wrap. A system consisting of a carbon filter followed by a downstream UV system is also a treatment train. In keeping with the treatment-train concept, the carbon filter and the UV system may be evaluated independently to the appropriate standards. The combined system could then carry the contaminant reduction claims tested on the individual technologies within the system.

Certification considerations
It is ultimately the decision of the manufacturer as to which of the claims will be certified. The manufacturer may choose not to certify the system at all. Or, the manufacturer may decide to certify all of the contaminant reduction claims that will be made on the system. For the carbon filter/UV system described above, this could involve certification of the system to three different standards: NSF/ANSI 42, 53 and 55. Another option for the manufacturer would be to certify selected claims on the system to the appropriate standards. The selection of the claims to certify would depend on a number of factors, although a major consideration would be in the marketing area—the decision regarding product positioning. Does the manufacturer view the system primarily as a filtration system, with the added benefit of UV technology? Or does the manufacturer view the system as a UV system with a carbon prefilter? The answers to these questions could lead to a number of different options in terms of evaluation and certification to any combination of Standards 42, 53 and 55 for multiple-claim options. Ultimately, with a product consisting of a carbon filter and UV in series, there are seven different possible combinations of standards to which the product could be certified.

About the author
Rick Andrew is the Operations Manager of the NSF Drinking Water Treatment Units Program. Prior to joining NSF, his previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: Andrew@nsf.org.

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