By Rick Andrew
The standards for filtration systems are structured based on the technology and end use of the product being evaluated. Likewise, the standards requirements are developed based on the capabilities, operating mode, and limitations of these technologies and their associated end uses. An excellent example of this structure can be seen in membrane filtration technologies.
The structure of the standards stems from several membrane technologies used widely for the treatment of drinking water. These technologies have various capabilities, end uses, and operating modes. The main ones to consider include the following five types:
- Pleated membrane filter systems providing filtration of particles in the micron range.
- Microfiltration membrane filter systems intended for filtration of particles into the submicron range.
- Ultrafiltration membrane systems, typically in hollow-fiber configuration, used for filtration of particles down to the 25 nanometers to 100 nanometers size range.
- Nanofiltration membrane systems, typically utilizing a crossflow element configuration, effective in filtration at the molecular levels.
- Reverse osmosis (RO) membrane systems used in crossflow element configuration and effective at rejecting particles, as well as charged ions in water.
These various types of membrane filtration systems help accomplish different filtration tasks based on their respective pore sizes. Membranes with larger pore sizes utilize a direct flow path through the pores of the membrane, whereas nanofiltration and RO membrane systems typically operate in crossflow mode, meaning there is a product water stream for the filtered water and a separate reject or waste stream that carries contaminants away from the membrane. To extend module life, some direct flow path membrane filter systems use backwashing or another alternate flow mode to clear accumulated debris and particles from the membrane surface and out of the system.
Within the realm of drinking water treatment, there are a number of possible end-use applications for these membrane filtration systems. Examples include centralized treatment for public water supplies, point-of-entry water treatment, and point-of-use water treatment. Filtration for nonpotable water is another end use, as is recreational water filtration for pool and spa applications.
Given this landscape, there are a number of different NSF/ANSI water treatment standards covering various aspects of membrane filtration of water. A summary of this landscape is provided in Figure 1, including an overview of the NSF/ANSI water treatment standards relevant to membrane technologies and membrane systems, and two important protocols that are not NSF/ANSI standards but are necessary to understanding the scope of membrane filtration standards. Figure 1 includes a description of the standard or protocol number, title, scope, and a brief summary of the types of requirements included based on the technology, capabilities, and end use. Additional pertinent notes and comments to help overall perspective round out the content of Figure 1.
Requirements for Products Used in Contact with Drinking Water
All of the standards related to drinking water require an evaluation for safety of materials that come in contact with drinking water. Material safety is a cornerstone of all NSF/ANSI drinking water standards and protocols. This requirement specifies that all materials in contact with drinking water must be subject to technical review to determine, based on the material formulation, what contaminants may leach out of them. This review takes into account the ingredients, impurities, processing aids, and other chemicals associated with each material. It establishes the analytical parameters for the extraction testing.
The exact approach to extraction testing varies by standard due to differences in end use. It is conducted according to several different yet similar test methods—each specific to and appropriate for the end use of the product and product type. In each case, sophisticated instrumental techniques for analysis, including gas chromatography–mass spectroscopy and inductively coupled plasma mass spectroscopy, are deployed to detect, identify, and accurately quantitate trace concentrations of a large number of potential contaminants.
Results for these extraction tests are typically adjusted through a mathematical normalization based on the amount of water expected to be flowing through the product when in use, which depends on where and how it is used in the distribution system. Finally, an assessment of the potential toxicity of any detected contaminants and their concentrations is conducted to make a determination of pass or fail for the test.
Tests for Reduction of Contaminants
The purpose of membrane treatment products is to reduce contaminants in the water. Accordingly, many of these standards include criteria and test methods for evaluating contaminant reduction performance. The types of contaminants evaluated under each standard depend on the treatment system’s end use, as well as the type of membrane technology being evaluated. For example, NSF/ANSI 58 includes test methods for evaluating the reduction of heavy metal ions in drinking water, whereas NSF/ANSI 419 is focused on testing the effectiveness of microbiological treatment.
The microbiological treatment effectiveness test methods in these standards typically involve the use of actual microorganisms. These microorganisms can be either the specific ones being evaluated (such as Cryptosporidium), or surrogate organisms selected because they are useful in a laboratory and representative of the organism being evaluated. These surrogate organisms are selected to be conservative test representatives (for example, smaller than the organisms being treated), safe to work with in the laboratory, and reasonable in terms of cost. Well-defined particles, such as polystyrene microspheres, are also used as surrogates in some tests.
The test methods vary significantly. They are designed and developed to achieve repeatable and reproducible tests that provide conservative evaluations. Ultimately, the purpose of these tests is to provide end users with confidence of performance in real-world conditions. For example, NSF/ANSI 58 requires a seven-day test operation with a variety of system usage patterns incorporated into the test, including partial drawdown of the treated water storage tank, complete storage tank drawdown, and a weekend stagnation period.
Broadly Comprehensive Standards
Each of the standards referenced in this article includes a great degree of detail related to each of its requirements, such as test methods, test results acceptance criteria, and quality assurance of laboratory results. There is enough detail in these standards to provide ample material for entire articles to be written on a variety of topics. By instead sharing this broad overview, it is possible to see the overall landscape of standards that cover this broad category of membrane water filtration system standards.
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]