By Rick Andrew
Most aspects of the NSF/ANSI DWTU Standards address POE treatment systems, POU systems designed and marketed for residential applications, and POU systems designed for commercial food service applications. However, NSF/ANSI 53 Drinking Water Treatment Units – Health Effects also includes requirements for testing cyst reduction claims on filters used in bottled water plants.
Bottled water filters defined
NSF/ANSI 53 requires that bottled water filters being evaluated must have a redundant sealing mechanism such as 222 or 226 double O-ring seals. Additionally, the manufacturer must specify a rated service flow for operation of the filter, as well as a maximum recommended pressure drop at the rated service flow. Typical bottled water filters evaluated under NSF/ANSI 53 are single, open-end 10-inch (254-mm) or 20-inch (508-mm) pleated filter cartridges with 222 or 226 double O-ring seals and a variety of closed end configurations, fin or flat.
In addition to the cyst reduction test, filters conforming to NSF/ANSI 53 must be evaluated for material safety through review of material formulations and extraction testing, similar to other POU and POE products covered by the standard.
Testing using microspheres
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 when evaluated under the protocol specified in NSF/ANSI 53. 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 adsorption. 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, so that they are visible under an epifluorescent microscope. Each sample of challenge water and filtered water must be examined under the microscope so the number of microspheres per liter can be accurately counted.
Laboratories conducting cyst reduction testing on bottled water filters must have an apparatus that can deliver water containing 50,000 microspheres per liter at an inlet pressure of up to 90 psig to two test filters, with water supply sufficient to provide constant flow at the manufacturer’s rated service flow for up to 16 hours per day. The test apparatus must also include appropriate housings for the filters being tested to fit and seal into. Also, the laboratory must be able to change the influent water during the test to two other types of water besides the microsphere challenge water—general test water and test dust loading water are also introduced to the filter throughout the test (see Figure 1).
The test protocol itself is designed to mimic conditions typical of a bottling line in a bottled water facility. It starts with conditioning two bottled water filters per the manufacturer’s instructions, using the general test water. Testing begins using the polystyrene microsphere challenge water at the rated service flow specified by the manufacturer, using a dynamic test manifold inlet pressure of up to 620 kPa (90 psig) with continuous flow. The manufacturer’s rated service flow ± 10 percent is maintained throughout the test using a control valve located downstream of the test filters. The polystyrene microsphere challenge water is introduced until the collection of the start-up sample is completed.
The laboratory then changes the water being introduced to the filters from the polystyrene microsphere challenge water to the test dust loading water. 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 stops introducing the test dust loading water and changes to the general test water for 10 minutes. The laboratory then stops introducing the general test water and changes to the polystyrene microsphere challenge water for 20 minutes. At the end of the 20-minute period, a pressure pulse is administered to the filters, as challenge water and filtered water samples are collected.
After sampling, the introduction of the polystyrene microsphere challenge water is stopped and the laboratory changes the water being introduced to the filters from the polystyrene microsphere challenge water to the test dust loading water. The introduction of the test dust loading water is continued until the next sampling point, at which point the procedure is repeated.
The influent (polystyrene microsphere challenge water) and effluent (filtered water) samples are collected and measured at the start of the test, and at 25, 50, 75, 100 and 150 ± 10 percent of the manufacturer’s recommended maximum pressure drop at the rated service flow. Immediately prior to collection of the effluent samples, the laboratory administers a pressure pulse to the filters under test 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 this kind of pulsation and still provide 99.95-percent removal of cysts.
For bottled water filters that have maintenance procedures, which include reuse, backwashing, cleaning, sterilization, etc., the manufacturer’s maintenance procedures are followed, the filtration elements(s) returned to service and the test repeated. In addition to the collection of the influent and effluent samples specified, a sample of effluent is collected immediately upon resumption of flow to the filters under test.
The standard requires 99.95 percent removal of the microspheres. This requirement applies at all sample points for both filters. So, at each sample point, the number of microspheres in one liter of water collected from each filter is compared to the number of microspheres in one liter of the challenge water collected at that sample point. 99.95 percent of the microspheres must be removed. For example, if the challenge water contains exactly 50,000 microspheres per liter, the water collected from each filter can contain no more than 25 microspheres per liter to demonstrate 99.95-percent removal.
Standards to provide assurance
The purpose of the NSF/ANSI DWTU Standards is to provide assurance of the safety and contaminant reduction performance of POU and POE products. In addition to the typical types of products that most of us think about, such as residential and commercial food service, NSF/ANSI 53 also provides criteria to measure the safety and contaminant-reduction performance of filters used in bottled water plant applications. These criteria allow bottlers to choose filters that have been evaluated for the integrity of their media and design to assure 99.95-percent removal of cysts under the operating conditions typical of a bottling line.
About the author
Rick Andrew is the General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. He has previously served as the Operations Manager and, prior to that, Technical Manager for the pro- gram. 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.