This article originally appeared in WQP April/May 2021 issue as "Step-by-Step Guide to Extraction Testing"
One of the most misunderstood certification requirements is materials safety testing, otherwise known as the extraction test. Even for those with years of experience in the industry, the nuances and details of the extraction testing protocol and associated formulation review can be intimidating. By breaking things down step-by-step, the requirements become a little bit clearer. For this article, the discussion will be limited to drinking water treatment unit (DWTU) products.
What is the Purpose of an Extraction Test?
The overall intent of the extraction test is quite simple: to ensure that any materials contacting drinking water do not leach out harmful contaminants back into the water. This is particularly important in the water treatment industry, since the overall goal is to provide consumers with water that is improved compared to what they receive at the source. In the most basic form, this testing is accomplished by soaking the device in water such that all the normally wetted surfaces are covered. After a set amount of time, this water is removed and then analyzed to see what contaminants have now been introduced.
For filtration devices that contain carbon media, many times testing is required to be conducted both with and without media present. This is because the carbon can adsorb some of the contaminants that are leached out. Since it may be possible for the system to be operated without the media in place, it is important to test in this manner to ensure that the media does not mask an issue with the materials used in the system. However, it is also important to test with the carbon media to ensure that the media itself is not responsible for leaching any contaminants.
The formulation, or toxicology review, is a little less intuitive than the testing itself. This review is conducted prior to the testing and is done to ensure that the proper contaminants are being looked for during the analysis phase of the extraction test. Each wetted material is reviewed to determine which contaminants are most likely to leach from it. This provides a roadmap of sorts for the analytical lab to follow while analyzing the water, also known as the analytical test battery.
This step is important because there are thousands of potential chemicals for which labs can analyze. If labs were required to analyze for every single one, the extraction testing would be prohibitively expensive and take long to complete. By having a toxicologist review the materials in advance, the analysis becomes much more targeted. This is also why certifiers will ask for a detailed list of the wetted parts up front, since this is needed to complete the formulation review.
The conditioning of the device is sometimes an overlooked aspect of the testing. However, this can be a critical factor in being able to pass the test. During the manufacturing process, residual chemicals can exist on the surface of the parts. Conditioning is the procedure used to wash these chemicals away prior to exposing the device to the extraction water. Since most devices are directly connected to plumbing, the most common way to accomplish this is by running water through the unit for several minutes.
Conditioning performed in the lab must match the instructions that consumers are given to perform upon installing the device. As such, the procedure should be reasonably easy for the average person to complete, to ensure they are able to similarly wash away any residual chemicals prior to use.
Within the DWTU standards there are some similarities and differences between the protocols for conducting the extraction test. This can get confusing for individuals because, ultimately, each protocol is attempting to meet the same goal, but the path to get there varies. The process for the formulation review mentioned above is the same for all extraction tests. Once the analytical test battery is established, the actual exposure testing is performed. While there are a few different methods used depending on the product type, there are two main differences based on the end use application of the product being tested: point-of-use (POU) and point-of-entry (POE). It is important to note the procedures discussed below are applicable to complete systems. Materials, components and media that are evaluated outside of a system may have different requirements that do not follow these guidelines.
For POU products, extraction testing requires that 2L of water be gathered at each point water is drawn. Many POU units do not have a holding volume this large, so to accomplish this, multiple samples (not to exceed 16) are exposed to obtain the expected volume. The product(s) are installed as they would be in the field and conditioned per the manufacturer’s instructions with the extraction water. The solution is made up of tap water that is manipulated to ensure a consistent pH of 6.75 + 0.25, temperature of 73 + 3°F, total dissolved solids in the range of 50 + 5 mg/L, and free available chlorine (except when testing reverse osmosis systems) of 0.5 + 0.05 mg/L.
For those products that are connected to the plumbing system, the extraction test is conducted under a constant pressure of 50 psi. The water is held in the product for 24 hours. When that time has expired, the 2L of water is collected and preserved and the remaining, if applicable, is discarded. The same procedure is followed two more times. All three collections are then combined for analysis.
While POE products may be tested and certified under many of the DWTU standards, the extraction requirements in the DWTU standards guide a user to NSF/ANSI/CAN 61 section 8 for the testing protocol. Testing under NSF/ANSI/CAN 61 is different and requires the exposure to be done with multiple pH waters depending on what types of materials from which the product is made.
For DWTU products, the most common pH waters used are the pH 5 for the metals analysis and pH 8 for the metals and organics analysis. Two pH waters are used for the metals because metals react differently depending on the pH, so this helps to capture the two ends of the spectrum. The pH 5 water is altered to maintain the following characteristics: pH of 5 ± 0.3, 2 ± 0.5 mg/L free available chlorine, and 100 mg/L hardness. The pH 8 water has the following: pH of 8 + 0.3, alkalinity of 500 ± 25 mg/L, free chlorine of 2 ± 0.5 mg/L, and dissolved inorganic carbon in the amount of 122 ± 5 mg/L.
POE products are still conditioned first per manufacturer instructions and tested under a 50-psi static pressure. From here, the product goes through a series of 10 dump-and-fills over the course of 14 days. Then on days 15 and 16, the product is filled, held for 24 hours, and dumped. On day 17, the exit water is preserved for analysis.
While some may jump to think that the 61 protocol is so much easier than the POU protocol because excessive amounts of water flush the product and sample water is not accumulated, it is important to note that neither one nor the other is necessarily easier. They are different because the end use is different. It is important to recognize that a POE unit also treats water that is not intended for human consumption for uses such as bathing, dishwashing and showering. When thought about in this way, the dump-and-fill allowance tends to make more sense.
After the exposure, the collected water is sent through the proper analytical instrumentation to be evaluated per the analytical test battery. Because the testing equipment that delivers the exposure water to the products could leach chemicals into the water, this water is analyzed, and the results are corrected for anything that is present from the equipment and not the product. This ensures that the reported results are associated only with the product tested.
While the intent of the extraction testing is straightforward and important to protect the health of consumers, it is easy to see that the associated standards can become quite complex in their details. Hopefully, this information begins to decode some of those complexities and offers some clarity into why the different protocols exist. Armed with this knowledge, the next time a certifier calls for extraction testing to be completed, it may be ever so slightly less daunting.