By Evan E. Koslow, Ph.D., Shawn C. Nielsen and Michael J. Rook
Summary: As the water treatment industry tackles the threat of microbiological contamination, companies are continually searching for better and more effective means to address the issue. One company has discovered encouraging results through carbon block filtration.
In recent years, consumer demand has driven development of several point-of-use/point-of-entry (POU/POE) technologies that can provide broad microbiological reduction in potable water. For example, there has been significant growth in the sale of reverse osmosis (RO), ultraviolet (UV) light and ozone systems. These systems, however, are expensive, complex and may require regular maintenance and replacement of key components, which also may be expensive.
What if, though, you could offer a low-cost, highly effective microbial reduction device that was as simple as, say, an ordinary carbon block filter? Assume this new carbon block wasn’t only rated for cyst reduction, but also had the capability to provide 99.99 to 99.9999 percent (or 4 to 6 log) reduction of both virus and bacteria, respectively. Such performance would represent a full microbiological capability, not just bacteriostatic (controlling the proliferation of bacteria) performance. If an ordinary carbon block could achieve these results without a significant pressure drop penalty or a significant reduction in its chemical reduction performance, it would represent the Holy Grail of water purifiers.
A recent technological advancement not only provides microbiological interception (capture) but, at the same time, retains the standard chemical reduction capabilities normally associated with carbon block filtration. This technology is highly adaptable to existing systems, affordable and exhibits no significant impact on flow resistance through the carbon block. The technology involves the combined use of an appropriate carbon block pore structure and a chemical surface treatment process. Although the exact kill mechanism hasn’t been systematically confirmed, it appears that this surface treatment enhances the interception and/or inactivation of bacterial or viral particles.
This enhancement is the result of a synergistic interaction between two chemicals that, when combined into an organized surface-coating complex, provides broad-spectrum reduction of microbiological targets on contact. The only limitation is the rate of diffusion of the organism to the treated surface.
This diffusion and interception/kill process are governed by the time spent by the organism within the biocidal structure, pore size of the structure, physical size of the organism and number of “bumps” against the surface that the organism can withstand. Remarkably, performance of the surface treatment can be accurately predicted from these four parameters. The two chemicals used to form the treatment have little effect when used separately. They have essentially no mammalian toxicity and, once bonded to the carbon, don’t elute back into the water.
Current test methods
There are currently no official standards within the water treatment industry to assess the effectiveness of microbiological water treatment technologies on a comparative basis. While NSF International is currently in the process of developing a “Microbial Water Treatment” standard, specific test protocols are still in development, and haven’t yet been presented to the joint committee for approval as an ANSI/NSF standard. No date has been set for a new standard.
The U.S. Environmental Protection Agency (USEPA) Guide Standard and Protocol for Testing Microbiological Water Purifiers has long been the standard utilized within the industry to assess the performance of microbiological water “purifiers.” While this has been an industry benchmark, it should be understood that this document was originally prepared as only a guidance document for the development of a water purification standard. Table 1 provides a detailed comparison of the microbial reduction requirements for both the USEPA Guide Standard and the current draft ANSI/NSF Microbial Water Treatment standards. The proposed ANSI/NSF standard has two groups of acceptance criteria for microbiologically safe and unsafe water, but it’s far from clear that public health officials will accept two acceptance criteria.
Without an established standard, one company has embarked upon validation of the new filters using a modification of the current ANSI/NSF Standard 53 cyst-reduction test method combined with an adaptation of the USEPA protocol for testing microbiological purifiers. The following summarizes the results obtained when a variety of bacterial and viral organisms are substituted for Cryptosporidium oocysts in the NSF Standard 53 test protocol.
Modified Standard 53
Performance of the new carbon block microbiological reduction technology was evaluated utilizing a modified protocol based upon the cyst-reduction test found in Standard 53. The carbon block filters tested were 2.5-inch outer diameter (O.D.) x 1.25-inch inner diameter (I.D.) with a length of 7.2 inches. The filters were built to have the pore structure typically used within current cyst-reduction filters. Filters were individually challenged with four organisms—Escherichia coli (ATCC-11775), MS-2 bacteriophage (ATCC-15597-Bi), Bacillus subtilis (ATCC-9372), and Brevundimonas diminuta (ATCC-4335).
E. coli was selected because it’s one of the most widely used, and industry accepted, surrogate organisms available for microbiological evaluations. B. subtilis, the standard U.S. Army surrogate for a similar form of anthrax (B. anthracis), is known within the defense industry by its designation, BG. B. diminuta is broadly used in the pharmaceutical industry for testing sterilizing-grade filter elements. It’s one of the smallest and most penetrating bacteria available, with a diameter of approximately 0.3 microns (µm). MS-2 bacteriophage is a virus that infects E. coli. It’s an extremely small, 25-nanometer (0.025 µm) in diameter, particle with a negative charge at neutral pH and widely used in the filter industry to test filters because of its greatly enhanced capacity to move through porous structures. To assay these various organisms, industry standard methods were used to collect, dilute and measure the number of organisms (as colony-forming or plaque-forming units) in both the influent and effluent of the test filters.
A separate filter was used in each test involving each test organism. Filters were flushed with RO/deionization (DI) water for a period of five minutes, and the test stand was calibrated to an initial flow rate of 0.5 gallons per minute (gpm). Influent and effluent blank samples were drawn to ensure no cross contamination within the test stand. The filters were then challenged with four liters of water containing a high concentration of the various microorganisms. Initial influent and effluent microbiological samples were drawn following a minimum two-liter introduction of the challenge solution, allowing an adequate distribution of the influent challenge water throughout the test stand. To simulate extended service, a dechlorinated municipal water supply—60 parts per million (ppm) of total dissolved solids (TDS), 6.8 pH—containing nominal test dust (0-5 µm particles) was used to clog the filter and cause incremental 25 percent reductions in flow during each subsequent challenge of the filter.
Upon achieving the desired flow reduction, the system was flushed with four liters of RO/DI water to remove any residual nominal test dust from the influent lines. A microbiological challenge was then initiated and the system operated for a period sufficient to fully flush the test stand. Influent and effluent samples were then taken for analysis.
Summary of results
For larger bacteria (0.8 to 2 micrometers), such as E. coli and B. subtilis, the reduction across the filter was greater than 8-log (99.999999 percent) with no detection of these organisms downstream of the filter. This is especially interesting because of the thick protective shell that surrounds B. subtilis, which can enhance this organism’s resistance to disinfection. For the smaller bacterium, B. diminuta, an average of 6-log reduction was observed (99.9999 percent). This result is in concert with the small size of the organism in relation to pore structure of the carbon block, which results in less efficient contact. For the viral challenge, MS-2 bacteriophage, the average reduction was 5-log (99.999 percent), which again tracks with the even more minute size of this organism compared to the pore structure of the filter. Results are summarized in Table 2.
Microbiological filtration is envisioned to be the next leap forward in the consumer water treatment industry. It’s believed that one cost-effective means to achieve this will be to provide a broad-spectrum microbiological reduction carbon block. The resulting device could provide a combination of chemical (chlorine, taste, odor, lead, mercury and various organic compounds), particulate (asbestos, Class 1 particulate reduction), cyst (Giardia and Cryptosporidium), bacterial, and viral interception that meet both existing USEPA guidelines and anticipated NSF targets. Such filters could easily retrofit the enormous number of existing filtration systems adapted to hold carbon block water filters.
The authors would like to thank Caroline E. Nielsen, a KX Industries research and development (R&D) laboratory director; Meedia Kareem, a staff microbiologist, and Richard Kendrick, R&D director, for their assistance on this project.
About the authors
Dr. Evan E. Koslow (email: [email protected]) is chief executive officer at KX Industries, of Orange, Conn. He’s written over 100 articles and papers related to the water treatment industry and holds over 30 patents. Shawn C. Nielsen (email: [email protected]) and Michael J. Rook (email: [email protected]) are technical project managers with responsibility for microbiological testing, regulatory compliance and R&D management. All of the authors also can be contacted at (203) 799-9000, (203) 799-7000 (fax) or website: www.kxindustries.com