Evaluation of Cyst Reduction for Bottled Water Filters
By Rick Andrew
The NSF/ANSI Drinking Water Treatment Unit (DWTU) standards include evaluation criteria for a wide variety of technologies and products. Requirements for a spectrum of products ranging from system components to multi-stage POU RO systems and whole-house filters are specified in these standards. Technologies including membrane filtration, active media, ion exchange, UV, RO and more are considered part of the wide-ranging landscape covered by these standards.
One of the more unique products and end uses covered by the NSF/ANSI DWTU Standards is cartridge filters used in bottled water plants. Requirements for bottled water filters were added to NSF/ANSI 53 Drinking Water Treatment Units – Health Effects back in the late 1990s. Basically, the manufacturers of these filters were seeking a way to provide buyers with assurance that their filters were of high quality and would perform under conditions typical of a bottled water treatment and packaging facility.
Evaluation of material safety through formulation review and extraction testing, plus evaluation of cyst reduction performance, was determined to address the request for an assurance of quality for these filters. The way this evaluation is conducted, however, has a few nuances to be considered.
Nuances of evaluation
The first nuance is that these filters are sold as cartridges to be used in industry standard housings. With this in mind, evaluation of bottled water filters under NSF/ANSI 53 is limited to filters with -222 or -226 double O-ring seals or a similar redundant sealing mechanism. This type of seal is industry standard and the double O-ring redundant seal helps assure sealing with industry standard housings. Housings for testing purposes must be appropriate to the configuration of the filter (fin end or flat end, etc.) and also sized according to the sizing on the double O-ring seal of the filter being tested (i.e., -222 or -226).
The other nuance of the evaluation is the test method for cyst reduction. The operation of typical POU filters involves users periodically flowing water through the filter, opening and closing valves as they obtain filtered water throughout the day. Bottled water facilities, however, operate with continuous flow, filling bottles on a bottling line for hours at a time. With these operational modes in mind, the test method under NSF/ANSI 53 for typical POU and POE systems making cyst reduction claims that involve on/off cycling of flow is not appropriate for evaluation of filters to be used in a bottled water facility. So, a new cyst reduction protocol was developed.
Cyst reduction protocol
Polystyrene microspheres are used as a surrogate for Cryptosporidium and other protozoan cyst organisms. In order to make a claim of cyst reduction, bottled water filters must be capable of removing 99.95 percent of 3-µm polystyrene microspheres at all sample points, when tested according to the protocol in the standard. The microspheres themselves must have a tightly controlled size specification, with 95 percent of them in the range of 3.00 ± 0.15 μm.
Additionally, the spheres must have a low surface charge to assure that the mechanism of removal is purely mechanical filtration, as opposed to electrostatic adhesion. For this reason, the spheres must have a surface charge content of less than 2 uEq/g. The microspheres must also contain a fluorescein isothiocyanate (FITC) dye or equivalent, which makes them visible under an epiflourescent microscope. The test method requires that each sample of challenge water, as well as each sample of filtered water, must be examined under the microscope to accurately count the number of microspheres contained in the water.
To conduct the test, two bottled water filters are installed in housings per the manufacturer’s instructions. They are then conditioned using the general test water described in Figure 1. After conditioning, cyst microsphere challenge water (see Figure 1) is introduced to the filters at the rated service flow specified by the manufacturer. A dynamic test manifold inlet pressure of up to 620 kPa (90 psig) is used to achieve this flowrate. The test proceeds with continuous flow of challenge water. The manufacturer’s rated service flow (± 10 percent) is maintained throughout the test using a control valve located downstream of the test filters. The cyst microsphere challenge water is introduced until the collection of the start-up samples is completed.
The laboratory then changes the water being introduced to the filters from the cyst microsphere challenge water to the test dust loading water (see Figure 1). This water is introduced to the filters until the pressure drop across the filters is 25 percent of the manufacturer’s maximum recommended pressure drop. At this point, the laboratory changes to the general test water for 10 minutes.
The laboratory then changes to the cyst microsphere challenge water for 20 minutes. At the end of the 20-minute period, a pressure pulse is administered to the filters by causing a rapid interruption and resumption of flow typical of a fast-acting valve located downstream of the filters under test. The pressure pulse is included in the test protocol to assure that the filter integrity is sufficient to withstand an event of this nature, yet continues to provide 99.95-percent removal of cysts. Immediately following the pressure pulse, challenge water and filtered water samples are collected.
After sampling, the laboratory changes the water being introduced to the filters from the cyst microsphere challenge water to the test dust loading water. The introduction of the test dust loading water is continued until the next sampling point (50 percent of manufacturer’s maximum recommended pressure drop), at which point the procedure is repeated, starting at the point of the laboratory changing to the general test water for 10 minutes. The test continues in this manner, ultimately with cyst microsphere challenge water and filtered water samples collected and microspheres counted under the microscope in samples from the start of the test and at 25, 50, 75, 100 and 150 percent (± 10 percent) of the manufacturer’s recommended maximum pressure drop at the rated service flow.
If the manufacturer specifies filter maintenance procedures, such as reuse, backwashing, cleaning, sterilization, etc., then the filters are tested a second time after the manufacturer’s maintenance procedures are followed. These filters must pass the test both times.
Manufacturer and buyer confidence
The bottom line is that the NSF/ANSI DWTU Standards are tools to help assure confidence in products. Manufacturers, distributors and end users all gain confidence from products that are third-party certified to meet these standards. These criteria and methods for evaluation of bottled water plant filters for cyst reduction performance and safety for contact with drinking water ultimately provides additional quality assurance at two levels in the supply chain. First, it allows manufacturers of these filters to assure the quality of their product and second, it helps bottlers to assure the quality of their product by using filters that conform to these requirements.
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: Andrew@nsf.org