By Rick Andrew

I am thrilled that the December, 2006 Water Matters column, “Minimum Claims Under the NSF/ANSI DWTU Standards” aroused a number of comments and questions. One in particular provided me with the impetus for this month’s column. A loyal WC&P reader asked, “How are products that contain multiple technologies, such as a carbon filter and a UV lamp, certified under the NSF/ANSI DWTU Standards?”

This is a great question. The answer is that there are a number of options and the decision on the exact course of action rests with the manufacturer. Let’s take a look at those options and the rationale behind them.

The NSF/ANSI DWTU standards are technology-specific
As discussed in a number of Water Matters columns, the NSF/ANSI Drinking Water Treatment Unit (DWTU) Standards are written specifically to address different treatment technologies. The concept is that test methods for assessing contaminant reduction performance must be appropriate to the technology being evaluated, so different technologies require different standards with different contaminant reduction test methods. See Figure 1 for an overview of Standards, technologies and required contaminant reduction claims.

This leads to a clear application of the Standards for water treatment systems that contain one technology only, such as a carbon filter or an ultraviolet (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.

Hence the question from the reader: which standard(s) apply when a UV lamp is combined with a carbon filter system?

A common scenario
First, let’s look at a very common example of systems that contain multiple technologies and how these are addressed. This example is reverse osmosis (RO) systems with carbon postfilters.

NSF/ANSI 58 requires that the RO systems be tested for contaminant reduction performance with any pre- or post filters removed. The RO contaminant reduction test methodologies in NSF/ANSI 58 are appropriate for evaluation of the membrane system, not for evaluation of an adsorptive or absorptive media filter. All RO systems must make a claim of TDS reduction, based on the membrane’s ability, when used in the system, 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.

The Standard also allows that claims for reduction of certain additional contaminants may be based on post- filters. 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 postfilters installed downstream of the product water storage tank. These two protocols are reflective of the different operating scenarios of these filters. Both of these protocols involve testing the postfilter in isolation from the membrane.

In summary, contaminant reduction claims on RO systems are based on the RO element alone, but additional claims may be made based on the post-filter alone. The assumption here is that when the entire system is used, with the RO element followed by a postfilter in series, that the contaminant reduction capabilities of each individually tested component will meet or potentially exceed the performance of those components when they were individually tested. This has become known as the ‘treatment train’ concept.

Treatment trains – a common approach to systems
RO systems are one very common example of treatment trains. But there are many others. Filtration systems that use a sediment prefilter followed in series by a carbon block filter are treatment trains. Utilizing the treatment train concept, it would be appropriate to evaluate contaminant reduction performance on the carbon block filter alone and then determine that a system incorporating a sediment prefilter upstream of the carbon block filter would have 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 pre- filtration ‘wrap’ could be thought of as 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’ (see Figure 2).

Figure 2: Carbon block filter element and carbon block filter element with “wrap”

Finally, the reader’s question!
A system consisting of a carbon filter followed by a downstream UV system is a treatment train. Per the discussion above, the carbon filter and the UV system may be evaluated independently to the appropriate NSF/ANSI DWTU Standards. The combined system could then carry the contaminant reduction claims tested on the individual technologies within the system.

It is ultimately the decision of the manufacturer as to which (of the claims) will be certified claims. The manufacturer may choose not to certify the system at all, or may decide to certify all of the contaminant reduction claims that will be made on the system. This could involve certification of the system to three different standards – NSF/ANSI 42, 53, and 55.

Another option 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 prominent among these factors would be marketing decisions. Does the manufacturer view the system primarily as a filtration system, with the added benefit of UV technology? Or as a UV system with a carbon prefilter? This kind of analysis 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 claims options. (For those of you who may be curious, there are seven different possible combinations of Standards in this scenario.)

More questions, please!
My goal in authoring Water Matters is to enlighten WC&P readers on the requirements, new developments, rationales, and background information behind the NSF/ANSI DWTU Standards. Feedback from readers is highly beneficial to me as I strive to achieve this goal. Please contact me via e-mail at if you have any questions, as I’m more than happy to provide answers. And who knows, your question may lead to a topic for a future Water Matters column!

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 1-800-NSF-MARK or email:


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