The purpose of the study that is discussed in this article was to compare the performance of silverized GAC and silver zeolite treated GAC in preventing the growth of bacteria in water filters. Granular activated carbon (GAC) has been widely used in water filtration for many years. Although overall effective, one issue with using carbon as a filtration media is that it can be an excellent media for supporting bacterial growth. This bacterial growth, and the resulting formation of bio-films on the carbon, can reduce the filtration efficiency and lead to issues surrounding quality of the effluent water. Introducing silver treatments with carbon is clearly a method of controlling bacterial growth. This study was undertaken to compare the performance of two filters, one using standard silverized GAC, and the other using an alternate technology which bonds carbon with a silver zeolite. The design parameters of the filters (flow, use life, etc.) were the same to accurately assess the differences in performance.

Materials and testing

Sample filters were prepared using a standard silverized GAC (0.2 percent) and GAC prepared using a silver zeolite based antimicrobial technology. The silverized GAC filters were filled with a blend of 20×50 acid washed coconut shell activated carbon and 1.05 percent 20×50 mesh silver impregnated coconut shell carbon at a ratio yielding 0.2 percent silver. The filters containing silver zeolite were filled with a modified carbon block CL2 base media containing a silver zeolite antimicrobial component. The filters were designed with a maximum usage life of 2,500 gallons.

The filters were installed in a water manifold allowing the same inlet water to flow through both filters, ensuring the source water was equal. A wound fiber pre-filter was installed prior to the manifold. This was run for several days prior to the initiation of testing to allow bacteria to grow as a source of inlet bacteria. This was done to challenge the filters with bacteria so that their performance in controlling bacteria could be measured in the time frame that the test was being run. City tap water was used as the feed water for the system in an effort to replicate standard use conditions.

The system had inline flow meters to measure the water flow rate through each filter. Needle valves were used to balance the flow rate through the filters at 0.42 gpm. The meters were electronically controlled so that they could be turned off and on without having to adjust the flow rate through the filters.

For this test protocol, the filters were connected to the system and initially flushed for 10 minutes. A sample of water was taken at this time for silver analysis. The water was then allowed to run for an additional 50 minutes and then turned off. The filters were allowed to sit for one hour and then water flow was resumed. This alternating one hour on one hour off was continued throughout the day. The water flow was then turned off after eight hours.

In the morning, the water was again turned on for one hour intervals followed by one hour off for an additional eight hours. This cycling was continued throughout the test. The filters were not run on the weekends. The test was terminated after 17 days of operation, with approximately 1,700 gallons of water run through the filters. At the flow rate used in this test, approximately 100 gallons of water were run through the filters each day.

Each morning after the test was started, samples were taken for analysis. Before the system was turned on, the inlet to the manifold was disconnected and turned on. This was allowed to run for one minute, and then a sample was taken for heterotrophic plate count (HPC) analysis. The water flow was turned off immediately after the sample was taken, and was reconnected to the manifold. The water flow was then initiated through the filters. After one minute of flow through the filters, a water sample equal to the open volume of the filter was taken for HPC and silver analysis.

Silver and HPC analysis were run each day and charted. The silver content of the effluent water samples were analyzed using a Perkin Elmer GFAA with a minimum detection limit of 1 ppb. HPC measurements were made using membrane filtration (47 mm, 0.45 µm filter from Millipore) based on Method 9215 D Standard Methods for the Examination of Water and Waste Water, 20th Edition.

Results

A graph of the silver release results is shown in Figure 1. The silverized GAC does show silver release, but it is variable and low, never exceeding 2 ppb. The silver zeolite treated sample, on the other hand, shows silver release in the 10-25 ppb range for the initial 1,000 gallons run. The silver release does decline after 1,000 gallons, but even at the termination of the test, the sample was showing almost 5 ppb release.

The silver release values material treated with silver zeolite based antimicrobial technology show that silver is present at concentrations that are effective in controlling bacterial growth, while staying well below any regulatory limits.

The HPC data for the two samples, along with the inlet water, is shown in Figure 2. To consider a filter bacteriostatic according to NSF 42, it must not increase the HPC of the filter by more than 20 percent over the lifetime. The data is graphed to show the geometric mean. This is the standard way that HPC data is analyzed in NSF 42 in order to minimize the effect of fluctuations in the bacterial counts.

As Figure 2 shows, the silverized GAC quickly develops a much higher HPC count than the inlet water. The silver zeolite treated sample maintains extremely low HPC throughout the trial. Based on these results, silverized GAC would not be a suitable choice for a bacteriostatic filter. The performance of the silver zeolite treated GAC indicates that it has the ability to control the bacterial growth needed to obtain a bacteriostatic claim.

Discussion

The silver zeolite treatment clearly shows better performance in controlling bacterial growth on carbon surfaces. Silver zeolites provide outstanding antimicrobial protection to materials in applications such as medical devices, carbon block filters and textiles. 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 activated carbon filters from bacterial contamination. And, they are more efficient and effective over time when compared to silverized GAC.

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 product protection throughout the useful life of the filter. The crystalline structure of a silver zeolite traps silver ions inside an interconnected internal pore structure.

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. Silverized GAC, on the other hand, is just a silver salt solution deposited onto the GAC. There is no control over the dissolution of the salt to allow it to release over the lifetime of the filter.

Silver zeolite technology or filters containing the same have been registered by the EPA, FDA, European Food Safety Authority, NSF International and California Department of Pesticide Regulation. A filter that has received NSF certification provides long-lasting, highly-effective antimicrobial protection.

Many manufacturers already make claims about the bacteriostatic properties of their water filters. However, most of these claims are not backed by rigorous certification processes. These filters fall short of certification because their performance is not consistent throughout the life of the filter. Silver zeolites have been certified or listed by all these organizations, as well as by the European Food Safety Authority. They provide long-lasting, highly effective antimicrobial product protection of the activated carbon filter.

This extended antimicrobial protection confers several additional benefits to silver zeolite AC filters over traditional silver nitrate impregnated AC filters. Naturally, silver zeolite AC filters provide the same high quality filtration and purification 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, producing taste- and odor-free drinking water. Silver zeolite AC filters require less maintenance time, less frequent replacement and a lower total cost of ownership, providing users with superior performance and value and effluent water quality.

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 nitrate impregnation solves the problem. Clearly the data in this particular study supports the premise that silver zeolite filters outperform silver nitrate treated filters.

Additionally, many other types of activated carbon filters, including granulated activate carbon filters, can benefit from the application of this antimicrobial technology. Silver zeolite AC filters, providing longer useful lifespans and reduced levels of bacterial contamination, can advance the performance of filtration products to achieve higher value for the end product and consumer.


A chemical engineer with a strong background in product development, David Pickering is manager of engineering at Sciessent LLC, makers of the Agion® brand of antimicrobial solutions for consumer, industrial and healthcare markets. David serves both as project manager and development engineer for Sciessent’s clients, combining hands-on laboratory work with high-level planning and project team leadership. Previously, David held engineering and sales roles at W. R. Grace & Co.’s Specialty Vermiculite Products and Cement, Concrete and Masonry Products Research divisions. He holds a BS ChE from the Massachusetts Institute of Technology in Cambridge, MA.