By Rick Andrew
The contaminant reduction test methods in the NSF/ANSI Drinking Water Treatment Unit (DWTU) Standards are carefully and scientifically crafted to provide robust assurances that products tested in accordance with these standards will work effectively under most real-world scenarios. As anyone familiar with the complexities and nuances of water treatment can attest, this is no small feat considering the wide variation in water quality and contaminants and the impact on the treatment efficacy of various treatment technologies caused by those variations.
A case in point is regarding total organic carbon (TOC) in the challenge water when testing reduction of trace organic contaminants by active media technologies. TOC is a common constituent in drinking water, derived from various organic materials that get into the source water and decompose. Items such as fallen leaves, dead wood, other plant material, etc., are constantly being deposited in surface water and decompose, leaving residual organic compounds behind that remain in the finished drinking water.
TOC is a relevant consideration in testing because it interferes with active media adsorbing trace organic contaminants. The performance of an active media water filter for reduction of trace organic contaminants can be substantially better in terms of treatment capacity and efficiency when there is little or no TOC in the water compared to when there is a significant concentration of TOC. This situation is recognized and addressed in the NSF/ANSI DWTU Standards by requiring that the test water for testing trace organic contaminant reduction of active media filtration systems must include at least 1 ppm of TOC.
A Clear Requirement – Or Is It?
This requirement of at least 1 ppm of TOC in the test water on the surface appears to be very simple and very clear. And for those laboratories using water with a surface water source, typically there is going to be at least 1 ppm of TOC in the tap water that they have available to create the test water. So in this case, it is simple and clear.
However, it is less simple and clear when the laboratory is using water with a well water source. These laboratories may have considerably less TOC in their tap water, so they are in a position where they must add TOC to meet the requirement of the standard. But what compounds to add, and how to add them?
Not All TOC Is Created Equally
This question of TOC addition is a good one. There are some organic compounds that could be added to the water to bring the level of TOC up above 1 ppm that would act not at all like the natural TOC derived from decomposition of leaves, plants, etc. For example, sugar could be added to the test water, and would technically be TOC, but it doesn’t have the same adsorptive interference with active media and trace organic contaminants as the natural TOC that exists in the real world. So, it becomes necessary to specify more precisely how TOC is added and what compounds are added to achieve test results reflective of what happens in the real world with drinking water derived from a surface water source that has natural TOC in it.
New Specification for Addition of TOC
To effectively address this situation, the NSF/ANSI DWTU Standards have recently been updated to more precisely describe the process and reagent chemicals used to supplement the TOC in laboratory test water as necessary. Figure 1 describes the general test water used for trace organic contaminant reduction, including the TOC concentration. Note that the footnote in Figure 1 states that the primary source of TOC is a natural presence in the source water, with a method for addition of TOC described in the standard, if addition is necessary.
NSF/ANSI 53 specifies that chlorinated tannic acid is to be used to supplement the TOC in the test water, with a specific preparation method indicated in Annex N-7. According to Annex N-7, tannic acid is slowly dissolved in 12% bleach to partially chlorinate the tannic acid to improve the stability of the TOC and simulate chlorinated natural organic matter (NOM) in natural waters.
The preparation of this chlorinated tannic acid is detailed in a procedure written not only to help provide accuracy in the preparation, but also to foster safety in the laboratory. The reaction between the bleach and the tannic acid is very exothermic (gives off a lot of heat), and caution must be taken to control this heating. Figure 2 describes the preparation method.
As many of us know all too well, water treatment is a complex endeavor with many twists, turns, and surprises along the journey. This reality comes well into play when devising test methods to evaluate effectiveness of various technologies in treating different contaminants. The NSF/ANSI DWTU Standards embrace this complexity through a wide variety of contaminant reduction testing methods reflecting these nuances through specifications of different characteristics in test water and testing protocols. The precise details associated with assuring the presence of appropriate concentrations and makeup of TOC in water for testing reduction of organic contaminants by active media technologies is one excellent example of such characteristics.
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. 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: [email protected]