By Rick Andrew

Hollow fiber, ultrafiltration technology is a major growth area within the POE and POU water treatment industry. The technology has become more commercialized in recent years and manufacturers are beginning to leverage its capabilities in their product lines. The high flow rates, low-pressure drop, durability and ultrafiltration capability add to the appeal of these systems.

Ultrafiltration is loosely defined in industry terms as separation of particles in the range of about 25 nanometers to about 1,000 nanometers from fluids. This size range correlates roughly with the size range for viruses and with particles with a molecular weight of roughly 2,500 to 250,000. In the spectrum of filtration, this is between nanofiltration and microfiltration (see Figure 1).

The current technology for accomplishing ultrafiltration is a hollow fiber configuration, with flow occurring from inside the hollow fiber through to the outside. The hollow fibers are embedded in potting material at both ends of the ultrafiltration module.

Different modules have different types of designs. Often, POU ultrafiltration modules are designed to have ‘dead end’ flow, meaning that there is no backwashing or forward flushing of accumulated filtered material from within the hollow fibers.

Typical POE modules are incorporated into systems with sensors, electronic controls and valves to facilitate backwashing, forward flushing or both. Although the technology to allow for this arrangement is more expensive than a simple, dead-end flow system initially, the increased life of the hollow fiber module obtained by clearing the filtered material from the hollow fibers offsets the additional initial costs over the life of the system.

Standards for certification
Many people are surprised to learn that these systems are not certified to NSF/ANSI 58, but rather to NSF/ANSI 42 and/or NSF/ANSI 53. There are several reasons for this.

First, NSF/ANSI 58 is limited in scope to reverse osmosis systems only. Because the scope is limited to reverse osmosis only, the test methods all assume a crossflow arrangement. As described above, ultrafiltration modules do not use crossflow, but rather are either dead end or use backwashing and/or forward flushing.

Further, NSF/ANSI 58 requires all systems to reduce 75 percent of TDS as 750-mg/L sodium chloride in water. With the pore size range of ultrafiltration, this level of ionic rejection is not achieved.

The typical claims that are potentially applicable to ultrafiltration systems are described in Figure 2. Note that they are all mechanical filtration claims.

The ultrafiltration modules themselves do not have any absorptive or adsorptive capabilities, so they do not perform metals reduction or organic chemical reduction functions. There are some systems on the market that do incorporate activated carbon or other media along with the hollow fiber ultrafiltration technology to achieve additional performance and these systems can be certified for additional claims under NSF/ANSI 42 and/or NSF/ANSI 53.

Mechanical reduction testing
Mechanical reduction testing on hollow fiber ultrafiltration systems is conducted per the requirements of the applicable standard, i.e., NSF/ANSI 42 or 53. Test dust of various specifications is used to build pressure drop and reduce flow rates through the system. For nominal particulate reduction and turbidity reduction tests, the test dust is used as the actual contaminant challenge.

In nominal particulate reduction testing, a laser particle counter is used to count particles in the appropriate size range in the influent and effluent samples, with an 85 percent reduction required. In turbidity reduction testing, a nephelometric turbidimeter is used to measure turbidity in the influent and effluent samples, with a reduction in turbidity from 11 NTU in the influent to <0.5 NTU in the effluent samples required.

For asbestos and cyst reduction testing, test particles are introduced at sample points. Asbestos reduction requires 99 percent reduction of a mixture of both anthophyllite and chrysotile asbestos fibers, with analysis conducted utilizing either transmission electron microscopy or x-ray diffraction.

Cyst reduction requires 99.95 percent reduction of either three-micron polystyrene microspheres or live Cryptosporidium parvum oocsyts. Analysis is performed utilizing epifluorescent microscopy. Both asbestos reduction and cyst reduction tests require analysis of influent and effluent samples at the various sample points to establish that sufficient percent reduction of the influent concentration has been achieved.

Additional requirements
In addition to testing of the specific mechanical reduction claims made by manufacturers, hollow fiber ultrafiltration systems must conform to the material safety, structural integrity and product literature requirements of NSF/ANSI 42 and/or NSF/ANSI 53. Material safety is established through a complete formulation review of all materials in contact with drinking water, followed by whole product extraction testing to assess the potential for any harmful compounds to leach from the system.

Structural integrity is determined by cyclic testing with repeated cycling between zero and 150 psig, as well as a separate test of elevated hydrostatic pressure for 15 minutes. The product literature must include a permanent system data plate, an installation and operation manual, a performance data sheet and replacement element packaging, if applicable.

This literature must contain a whole host of required information to help consumers better understand the benefits, requirements and limitations of the products.

Reliable confirmation of new technology
Although widespread commercialization of POE and POU hollow fiber ultrafiltration systems is a phenomenon of the last few years, the methods for determining their material safety, structural integrity and contaminant reduction performance are tried and true methods that have been used to evaluate other POU and POE mechanical filtration technologies for many years. Consumers can rest assured that systems conforming to NSF/ANSI 42 and 53 are safe, durable and perform as advertised despite the relatively recent emergence of the product category.

About the author
Rick Andrew is Operations Manager of the NSF Drinking Water Treatment Units Program for certification of POE and POU systems and components. Prior to joining NSF, his previous experience was in the area of analytical and environmental chemistry consulting. 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: Andrew@nsf.org.

 

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