By Jeffrey A. Trogolo, PhD
Since 1500 BC, when ancient Egyptians and Hindus first filtered water and medicines with activated carbon (AC), it has been used as a filter to remove impurities from water, air, gases, processed beverages and pharmaceuticals. Today, whether in block or granulated form, activated carbon is still a leading filtration medium in industrial, commercial and consumer water filters, providing end users with water free of undesirable tastes, odors, particulate matter and other impurities.
Activated carbon has been the premier water filtration medium for millennia; however, there is still room for improvement. Over the past 30 years, numerous scientific studies have demonstrated that activated carbon has a problem with bacterial contamination when used in water filtration applications. Simply put, carbon is an excellent medium for the growth of waterborne bacteria. Once bacteria colonize an activated carbon water filter, they will actually introduce more bacteria into the effluent than the waters entering the filter, often introducing the very impurities that filters are intended to remove. It is important to point out that filters containing an antimicrobial and achieving NSF-42 bacteriostatic certification are intended to limit the typical ‘amplification’ of HPC bacteria in carbon filter effluent from potable water influent, as opposed to a water purification filter that is intended to purify microbiologically unsafe water.
To solve this problem, technological innovators have employed a zeolite ‘smart material’ based on one of the oldest antimicrobial technologies known to humanity: silver. Silver zeolites provide outstanding antimicrobial protection to materials in countless applications, including medical devices, healthcare building products, food service and manufacturing equipment and consumer products. When bonded to an activated carbon filter, these zeolites release silver ions in an efficient, controlled manner that successfully inhibits bacterial growth throughout the effective life of the filter, protecting the filters from bacterial contamination. The mechanism of silver release is ion exchange; conditions that allow bacteria growth (moisture and nutrients, including electrolytes) are the conditions that trigger silver delivery. Upon reaching equilibrium designed to achieve an antimicrobial concentration, the silver will cease to be delivered. This marriage of ancient technologies and 21st century materials science represents the next generation of filtration technology.
The problem with carbon
Throughout the 1970s and 1980s, public complaints and concerns about the taste, odor and even health implications of impurities in drinking water gave rise to an explosion in the popularity of activated carbon filtration. Municipalities began using activated carbon to combat taste, odor and organic contaminant problems in their water supplies, while consumers and businesses installed POE and POU filters on their water lines. In the wake of these trends, many studies were undertaken in the 1980s and 1990s to evaluate the efficacy of AC filters, as well as their problem with bacterial contamination. During this time, Researchers from Montana, California and Ontario all published independent studies on the bacterial contamination of AC filters in the journal Applied and Environmental Microbiology. Almost universally, these research teams found significantly higher bacterial levels in water flowing out of activated carbon filters than in the corresponding influent water (Tobin, Smith and Lindsay, 1981). Over the normal useful lifetime of a water filter, naturally occurring bacteria present in the influent water colonized the activated carbon inside water filters. One significant cause of filter contamination is improper hygiene during filter installation. Using gloves to avoid hand contact with the filter and wetted housing components should be common practice during filter replacement.
This bacterial contamination is not surprising; the very characteristics that make activated carbon such an effective filter also make it hospitable for bacteria. Bacteria easily adhere to the porous surface of the activated carbon, which subsequently shelters it from fluid forces that might wash away the growing colony. The adsorptive properties of carbon enrich nutrient and oxygen concentrations in the carbon matrix, providing food for the colony, while at the same time binding with and neutralizing chlorine compounds that might otherwise kill the bacteria. In most cases, these organisms are non-pathogenic, of the sort found in almost any public water supply; however, the possibility remains that AC filters could also be a fertile breeding ground for pathogenic bacteria should they contaminate a water supply.
While these bacteria rarely pose a health risk to end users drinking, cooking, cleaning or preparing beverages with filtered water, they do reduce the efficacy of the filter as well as produce waste products that contaminate effluent water. When bacteria colonize a water filter, they produce a slimy biofilm—an aggregation of microorganisms and extracellular proteins, DNA and sugars secreted from the cells—that coats the carbon. The biofilm reduces the activated carbon’s adsorptive capacity, so it filters the water less effectively. Additionally, biofilm can actually slough off the filter, producing unpleasant tastes, odors and particulate matter in effluent that will ultimately be consumed as drinking water, ice or a prepared beverage, like soda or coffee.
Mechanical cleaning can remove biofilms from filter components, providing a short-term solution to this problem, but often the components of a filtration system in a water cooler, refrigerator or in-line filter are difficult to access. Routine maintenance and frequent replacement of commercial- or industrial-grade filters can incur significant materials and staff-time cost, while consumers may forget or ignore manufacturer recommendations for cleaning and replacement of filters. Whatever the case may be, the result is the same: impurities are introduced into the effluent water by the filter itself. To solve this problem, manufactures needed to develop or deploy a technology to prevent the bacterial colonization of AC water filters in the first place.
Smart materials, smart solutions
To provide AC water filters with long-lasting antimicrobial protection, filter manufacturers turned to another ancient technology: antimicrobial silver. Silver has been used for its antimicrobial properties for almost as long as activated carbon has been used for filtration. As far back as the 6th century BC, Cyrus, the King of Persia, decreed that his water must be transported only in silver vessels in order to keep it pure. Silver is a broad-spectrum antimicrobial that eliminates bacteria through multiple modes of action: it interferes with the bacteria’s metabolism, attacks the proteins that make up the bacteria and inhibits bacterial replication. Even in very low quantities, it effectively eliminates bacteria with little chance for the development of resistance. In comparison to other readily available ions (metallic and otherwise), silver inhibits microbial growth at levels near 20 ppb, far lower than similar substances like copper, zinc or antimicrobial chemicals (see Figure 2).
Silver zeolites have proven to be the most effective means to protect activated carbon filters from bacterial contamination, as they release silver ions in a controlled manner, providing antimicrobial protection throughout the rated lifetime of the filter. The crystalline structure of a silver zeolite traps silver ions inside an interconnected, internal pore structure. This microscopic structure makes a treated surface a smart material, as the silver ions are released only when the conditions for bacterial colonization are present. Therefore, when a surface is wet, the zeolite-containing surface becomes active, releasing silver to an antimicrobial concentration. Once the concentration reaches anti-microbial levels, the zeolite then turns off, preserving the silver reservoir for a longer useful life. Silver zeolites are already used in many anti-microbial applications requiring the controlled release of silver ions and therefore, are registered with the 
US EPA for use in contact with food and drinking water, and used in medical devices approved by the Food and Drug Administration (FDA). Silver zeolite technology also meets the high standards set for food and water contact safety set by US EPA, European Food Safety Authority, National Sanitation Foundation and California Department of Pesticide Regulation. While consumers may not be aware of the rigorous nature of NSF bacteriostatic certification, filter manufacturers understand that a filter that has received this certification provides long-lasting, highly effective antimicrobial protection. While silver ions are bacteriocidal, in terms of filter type, a bacteriostatic filter is designed so that the effluent water contains approximately the same concentration of bacteria as the influent water. The term bacteriocidal would refer to a water purification context in which the filter renders microbiologically contaminated water safe for consumption.
A new paradigm
Silver zeolite activated carbon filter technology provides an excellent opportunity for filter manufacturers looking to differentiate their products in a mature water filtration marketplace. There is little market share to be gained by pitting one unprotected activated carbon filter against another; however, educated consumers and filter distributors will recognize the antimicrobial protection incorporated into AC filters as a key point of differentiation. Consumers and enterprises that use activated carbon filters clearly care about the aesthetic benefits of filtered water, and are willing to pay for filters that provide demonstrable protection from bacterial contamination.
Many manufacturers already make claims about the antimicrobial properties of their water filters. Most of these claims, however, are not backed by the rigorous certification processes used by organizations such as NSF. These filters fall short of certification because their performance is not consistent throughout the rated lifetime of the filter. Silver zeolite filters have been certified by NSF for their bacteriostatic function and durability. This extended antimicrobial protection confers several additional benefits to silver zeolite AC filters over traditional silver-impregnated AC filters. Naturally, silver zeolite AC filters provide the same high-quality filtration that other AC filters provide, but the effective life of the filter is extended as the damage caused by bacteria and biofilms is reduced. The lack of biofilms and bacterial fouling allows a higher level of filter efficiency, which produces taste- and odor-free drinking water that is beneficial and desirable for ice, coffee and other food service applications. Whether deployed in industrial, commercial or residential settings, silver zeolite AC filters will require less maintenance time, less frequent replacement and a lower total cost of ownership, providing users with superior performance, value and effluent water quality.
Conclusion
Innovative manufacturers are already producing carbon block filters that incorporate silver zeolites for antimicrobial protection for use in numerous commercial and consumer applications, but there is still room for more growth in the development and popularization of these filters. Many end users of AC filters are unaware of the problem of bacterial contamination, and of those who are aware, many mistakenly believe that silver impregnation solves the problem . Additionally, many other types of activated carbon filters, including granulated activate carbon filters, can benefit from the application of this exciting antimicrobial technology. Silver zeolite AC filters, providing longer useful lifespans and reduced levels of bacterial contamination, will undoubtedly grow both in variety of applications and in sales volumes. The water filtration industry may finally be on the verge of solving a 3,500-year-old problem.
Silver nitrate… Fool’s Gold
The simplest way to incorporate chemicals into activated carbon is through a process known as impregnation. Impregnated activated carbon is often used to remove contaminants from natural gas. For water filtration applications, the activated carbon filter is impregnated with silver nitrate and then heated to convert the nitrate to metallic silver—the theory being that the metallic particles will release silver ions when in contact with water, providing antimicrobial protection to the carbon filter.
As is often the case, however, the simplest solution is often not the best. Experiments have revealed that silver nitrate-impregnated AC filters do not possess superior antimicrobial protection (Tobin, Smith and Lindsay, 1981). Most likely, water flow through the filter flushes away the silver early in the useful life of the filter, perhaps even during the pre-conditioning process. Therefore, silver ions are unavailable to provide antimicrobial protection to the carbon when the risk of bacterial contamination arises during normal filter operation.
Recently, a series of privately conducted experiments by companies active in the field of water filtration tested for the amount of heterotrophic plate count (HPC) bacteria in 100 mL of water before and after filtration by a silver-impregnated GAC water filter. At the start of each day, the inlet feed was detached and water was run for one minute. At that point, a water sample (the inflow sample) was taken and measured for HPC bacteria. The water feed was reattached to the filter and run again for one minute. At that point, a second water sample (the outflow sample) was taken from the filter output. Subsequent inflow and outflow samples were taken at 100-gallon intervals.
In over 80 percent of the data points, more bacteria were present in the outflowing water than in the inflowing water, and in over half of the data points, there were at least five times as many bacteria present in the outflowing water than in the inflowing water (see Figure 3). The increase is most likely the result of biofilm sloughing off the contaminated filter, producing high bacteria counts in the effluent water. The results of this testing are clear. Silver zeolite-bonded activated carbon filters provide consumers and commercial filter users with the antimicrobial protection they want where silver-impregnated filters do not.
About the author
Jeffrey A. Trogolo, PhD., Chief Technology Officer of Sciessent, directs the company’s technology operations and product development, and was an original member of the team that founded Agion Technologies. This work has earned him the 2006 Technology Pioneers Award from the World Economic Forum. Trogolo joined Agion in 1998, after serving as Staff Scientist and Research Director for orthopedic device applications at Spire Corporation. While there, he developed silver antimicrobial applications using ion-beam assisted deposition onto metal and polymer substrates and surface modification processes for improving artificial joint performance. Prior to Spire Corporation, Trogolo operated Niche Microstructural Corporation, a materials characterization and consulting firm. In addition, he is on the Board of Directors of Solutions Benefiting Life (SBL), a nonprofit organization with a mission to bring cleaner, safer water to the developing world. Trogolo has published papers in various journals and earned his BS Degree and Ph.D. in materials science and engineering from Rensselaer Polytechnic Institute. He is an inventor on 11 issued and eight pending US patents.
About the company
Sciessent, located in Wakefield, MA, is a leader in providing customized solutions that enhance the value of partners’ products. Sciessent’s Agion and Agion Active brands are based on silver and other naturally occurring elements and have been incorporated into a wide range of consumer, industrial and healthcare products, including cell phones, shoes, keyboards, water filters, medical catheters and ice machines. For more information about Sciessent, please visit www.sciessent.com; for the Agion brand, visit www.agion-tech.com and www.agionactive.com and follow the Agion brand’s exciting consumer campaign on Facebook and/or Twitter.