By Rick Andrew
Desalination is a process for reducing the level of salt in seawater to transform it into drinking water. Desalination tends to be energy intensive, with the process being economical typically only in arid areas that are near the sea and not at high elevations.
Although distillation can be used to achieve desalination, most municipal desalination plants utilize reverse osmosis (RO), treating the seawater with specialized desalination RO elements.
Because these desalination RO elements have significant contact with drinking water treated through this process, it is important to have assurance that the materials in contact with the drinking water are safe and will not contaminate the water.
Evaluation for Safety of Materials
NSF/ANSI/CAN 61 Drinking Water System Components – Health Effects includes criteria and test methods for evaluating a wide range of products, materials, and technologies used in municipal drinking water treatment, storage, and distribution. The evaluation consists of preconditioning, exposure to extraction water, analysis of contaminants that may leach, normalization of results to real-world conditions, and toxicological assessment of concentrations of any contaminants that may be detected. One category of products covered by the standard is reverse osmosis elements, including desalination elements.
Conditioning and Exposure
Reverse osmosis elements, including desalination elements, are categorized as “mechanical devices” under NSF/ANSI/CAN 61. As mechanical devices, they are exposed to water as a whole product unless they are very large in size (holding a volume of more than 20 liters or weighing more than 34 kg), in which case the standard allows for testing of individual material samples. Representative samples from a family of similar elements may be tested when they are manufactured of the same materials.
The elements are first washed to remove any debris associated with shipping, and then conditioned for up to two weeks. The conditioning period lasts up to 14 days and may be shorter if the manufacturer requests. During the conditioning period, the elements are filled with water, and the water is changed at least 10 times with a minimum period of 24 hours between water changes. After the 14-day conditioning period, the water in the elements is discarded.
After the conditioning period, the elements are exposed to the extraction water for 24 hours. The water is then changed for fresh water, with another exposure for 24 hours. Once again, the water is changed for fresh water, and the elements are refilled and exposed for 16 hours. This water is then collected for analysis of potential contaminants.
Analysis
The analysis for potential contaminants leaching into the drinking water is extensive. Certain materials classified as “acceptable materials” have an established analytical test battery included in the standard. Test batteries include analyses specific to certain chemicals, and also broad scans such as GC/MS and LC/MS. Acceptable materials have established material formulations, so the nature of the material and potential contaminants resulting from the material being in contact with drinking water are well studied and documented. As such, for devices that have a high flow of water through them when in use at public water treatment facilities such as RO/desalination elements, material formulation disclosures from the material supplier for these materials are not required. Materials in this category that are often used in RO and/or desalination membrane modules include PVC, ABS, polysulphone, polyurethane, and various rubber materials such as NBR and EPDM. Once these materials have been tested and analyzed according to the test battery in NSF/ANSI/CAN 61, they are acceptable.
Other materials that may be more specialized and proprietary are required to have a formulation disclosure from the material manufacturer that includes the identity by CAS number or chemical name of each component of the formulation, including but not limited to the activators, antioxidants, antimicrobials, cosolvents, fillers, initiators, peroxides, pigments, plasticizers, process aids, solvents, stabilizer, surfactants, and terminators; and percent or parts by weight for each chemical in the formulation. This information is reviewed by technically competent individuals to determine the analytes included in the test battery. This review considers characteristics of each ingredient in the formulation such as:
- known or suspected toxicity of the substance or its byproduct(s);
- high water solubility of the substance;
- monomer(s) of polymeric ingredients;
- solvents and cosolvents used in the polymerization process or those used in the material formulation;
- antioxidants, antimicrobials, curing agents, initiators, peroxides, pigments, plasticizers, process aids, stabilizer and terminators and their impurities, degradation, and hydrolysis products;
- high probability of extraction of a substance or its byproduct(s) at toxicologically significant concentrations; and
- extraction or migration information for the substance provided by the manufacturer or that present in the public literature.
Like the “acceptable materials” with established analytical test batteries included in the standard, these other materials are acceptable once they have been tested and analyzed according to the test battery developed as described here.
Considering this approach to analysis, NSF/ANSI/CAN 61 is considered to be a performance-based standard. Essentially, materials are established to conform based on their performance in a test. Other approaches could include “positive lists” of materials that are deemed acceptable if they are manufactured according to specific processes and meeting certain quality control elements during their manufacturing. But in order to best facilitate innovation and provide a path to new material types being introduced to the market, NSF/ANSI/CAN 61 is structured to be a performance-based standard.
Normalization
NSF/ANSI/CAN 61 includes a mathematical adjustment process to account for differences in the end use of a product compared to how they are exposed for testing in a laboratory. For instance, a desalination membrane element would see considerable flow through the element on a continuous or nearly continuous basis when deployed in real-world situations, as opposed to being soaked in exposure water as described above in a laboratory for testing. By exposing in this manner to allow detection and identification of potential contaminants more easily, and then adjusting the results to real-world conditions to evaluate possible toxicity, the standard allows for a high degree of confidence that contaminants are detected and end users are protected.
Toxicological Evaluation
NSF/ANSI/CAN 61 requires that any contaminants detected must be evaluated for potential toxicity, whether those contaminants are regulated or non-regulated. As such, it includes a process for toxicological risk assessment. Additionally, it refers to NSF/ANSI/CAN 600 that includes established toxicological risk values for regulated and many non-regulated contaminants that have been detected in tests over the years. This comprehensive approach to evaluation of contaminants provides a highly robust and protective test to assure protection of end users.
Assurance
Ultimately, NSF/ANSI/CAN 61 is about assurance of safety of products in contact with drinking water. Regulators in North America rely on it, as do specifiers and regulators in other regions of the globe who respect the rigor and level of assurance provided by the standard. The scientific approach to testing and evaluation is based on years of refinement of a standard first published in 1987, with continuous improvement occurring now and into the future. Consumers and end users can have a high degree of confidence in products that are certified to conform to NSF/ANSI/CAN 61.
About the author
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]