By Rick Andrew
There are several membrane technologies that are widely used for treatment of drinking water. The NSF/ANSI Standards address these membrane technologies in a variety of ways and in a number of different standards, including:
- Pleated membrane filter cartridges providing filtration of particles in the micron range
- Microfiltration membrane filter cartridges intended for filtration of particles into the sub-micron range
- Ultrafiltration membranes, typically in hollow fiber configuration, used for filtration of particles down to the 25-100 nanometer size range
- Nanofiltration membranes, typically in cross-flow element configuration, effective in filtration at the molecular level
- Reverse osmosis membranes used in cross-flow elements and effective at rejecting particles, as well as charged ions in water
As briefly described above, these membrane types are incorporated into various forms of filtration components, based on the pore size, including filter cartridges, bag filters, hollow-fiber membrane modules and spiral-wound crossflow elements. Membranes with larger pore sizes utilize a direct flow path through the pores of the membrane, whereas nanofiltration and RO membranes typically are used in cross-flow mode with a product water stream and a reject or waste stream that carries contaminants away from the membrane. Some direct-flow-path systems use a backwashing or other alternate flow mode to clear accumulated debris and particles from the membrane surface and out of the system, to extend module life.
There are a number of possible end uses for these membranes and membrane treatment technologies related to water treatment. These include various applications for drinking water (such as treatment for public water supplies), POE water treatment and POU water treatment. Water purification for non-potable water is another end use, as is recreational water treatment.
These numerous membrane technologies, product configurations and end uses are addressed via a number of different NSF/ANSI water treatment standards covering various aspects of membrane treatment. Figure 1 provides an overview of these standards relevant to membrane technologies and membrane systems. Also included in Figure 1 are two important protocols that are not NSF/ANSI standards, but are important to note for a look at the complete picture. Figure 1 describes the standard or protocol number, title, scope and a brief description of the types of requirements included. Some additional pertinent notes and comments are also included.
Most of the drinking-water-related standards require an evaluation for material safety, which is a cornerstone of all of the NSF/ANSI drinking water standards and protocols. This requirement specifies that all materials in contact with drinking water must be subject to a review to determine what contaminants may leach out of them based on the ingredients, impurities, processing aids and other chemicals associated with each material. This review establishes the analytical parameters for the extraction testing. Extraction testing 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 analytical techniques, including gas chromatography/mass spectroscopy (GC/MS) and inductively coupled plasma/mass spectroscopy (ICP/MS), are employed to detect, identify and accurately quantitate low 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 flowing through the product, 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.
Contaminant reduction testing
The purpose of membrane treatment products is to reduce contaminants in the water, so many of these standards include criteria and test methods for establishing contaminant reduction performance. The types of contaminants evaluated under each standard depend on the end use being evaluated, as well as the type of membrane technology being evaluated. For example, NSF/ANSI 58 includes, among others, test methods for evaluation of reduction of heavy metal ions in drinking water, whereas NSF/ANSI 419 is focused on microbiological treatment.
The microbiological treatment test methods in these standards typically involve the use of microorganisms, either the actual microorganisms intended to be treated (such as Cryptosporidium), or surrogate organisms useful in a laboratory environment. These surrogate organisms are selected to be conservative test representatives (for example, smaller than the organisms being treated), as well as safe to work with in the laboratory and reasonable in terms of cost. Particles such as polystyrene microspheres are also used as surrogates in some tests.
Contaminant reduction test methods vary significantly, with each designed and developed to achieve repeatable and reproducible tests that provide conservative evaluations. Ultimately, the purpose 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 draw down of the treated water storage tank, complete storage tank draw down and a weekend stagnation period.
Many more details
Each of the standards described in this article includes a great degree of detail related to requirements, test methods, acceptance criteria, quality assurance and more. Entire articles could be written about each of them or even about various aspects of each of them. Detailed articles about one standard or a portion of a standard can be highly informative, but they may be missing a broad view of the whole spectrum of standards applicable to various types of membrane technologies, end uses and product configurations. The information presented here is intended to provide a high level overview of these standards to provide insight into the overall picture of the standards applicable to membranes and membrane products used to treat water.
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]