Silver occurs naturally in the environment, mainly in the form of its very insoluble and immobile oxides, sulfides and some salts. Because silver ions are bacteriastatic, silver is used both as an emergency drinking water disinfectant and impregnated in some water filters to prevent microbial regrowth.
Silver has been used in the United States for water disinfection since the 1950s, and numerous commercial water filters using silver are currently being marketed today.
Household filter systems using ceramic filters coated in the anti-bacterial agent colloidal silver have been shown to be effective in eliminating bacteria in contaminated household water in various locations around the world.
In particular, positive results for a low-cost filter technology being used in flood-prone areas of Southeast Asia were recently noted in an International Development Enterprises report on filter technology trials to the International Federation of Red Cross and Red Crescent Societies.
Antibacterial capability
The specific antibacterial mechanism of silver is not clearly understood, but research with E. coli and S. aureus has shown that silver treatment of these microorganisms has resulted in DNA losing its replication ability.
Research on drinking water disinfection systems has shown that silver can be used successfully to control bacterial growth. It has also been found that the addition of copper and silver to water systems may allow the concentration of free chlorine to be reduced while still providing comparable sanitary quality of the water.
Still other research has shown that silver can be used in hospital water systems to control Legionella and in cooling towers to control bacterial growth. Silver has also been used to enhance the effectiveness of ultraviolet (UV) radiation in inactivating viruses in water.
Many commercial POU water filters make use of silver impregnated activated carbon to remove contaminants in household water supplies.
Bacteria can multiply on carbon media that does not contain silver and, subsequently, be released into the water supply. Silver impregnation prevents contamination of the active carbon filter and provides additional bactericidal effects.
Health concerns
The World Health Organization’s (WHO) guidelines for drinking water quality indicate that there are no adequate data with which to derive a health-based value for silver in drinking water.
These guidelines state that, "where silver salts are used to maintain the bacteriological quality of drinking-water, levels of silver up to 0.1 mg/liter can be tolerated without risk to health."
Silver is regulated by US Environmental Protection Agency (EPA) National Secondary Drinking Water Regulations.
The secondary maximum contaminant level in public water supplies is 0.1 mg/liter, which is a non-enforceable guideline based on possible cosmetic effects, such as skin discoloration.
References
- Bayati, M. A. 1997. Even Carbons Have a Silver Lining: Silver impregnated filters offer extra protection against bacteria. Water Technology, July, 1997.
- Bullard, M. 2002. Low-Cost Household Water Purifiers for Flood-Prone Areas: International Development Enterprises (IDE) report on filter technology trials to the International Federation of Red Cross and Red Crescent Societies (IFRC). http://www.potpaz.org/Mekong.doc
- Butkus M. A., Labare M. P., Starke J. A., Moon K., Talbot M. Use of aqueous silver to enhance inactivation of coliphage MS-2 by UV disinfection. Appl Environ Microbiol. 2004 May;70(5):2848-53.
- Codony F, Domenico P, Mas J. 2003. Assessment of bismuth thiols and conventional disinfectants on drinking water biofilms. J Appl Microbiol. 2003;95(2):288-93.
- Feng, Q. L., J. Wu, G. Q. Chen, F. Z. Cui, T. N. Kim, J. O. Kim. 2000. A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus. J Biomed Mater Res, 52, 662-668, 2000.
- Kim J., Cho M., Oh B., Choi S., Yoon J. 2004. Control of bacterial growth in water using synthesized inorganic disinfectant. Chemosphere. 2004 May;55(5):775-80.
- Stout J. E., Yu V. L. 2003. Experiences of the first 16 hospitals using copper-silver ionization for Legionella control: implications for the evaluation of other disinfection modalities. Infect Control Hosp Epidemiol. 2003 Aug;24(8):563-8.
- USEPA. 1992. Secondary Drinking Water Regulations: Guidance for Nuisance Chemicals. EPA 810/K-92-001. http://www.epa.gov/safewater/consumer/2ndstandards.html
- WHO. 2004. Draft third edition of the WHO Guidelines for Drinking-Water Quality. Geneva, World Health Organization. http://www.who.int/ water_sanitation_health/dwq/guidelines3rd/en/
- Yahya MT, Landeen LK, Messina MC, Kutz SM, Schulze R, Gerba CP. 1990. Disinfection of bacteria in water systems by using electrolytically generated copper:silver and reduced levels of free chlorine. Can J Microbiol. 1990 Feb;36(2):109-16.
Gregory D. Jennings is an extension specialist in biological and agricultural engineering for the North Carolina Cooperative Extension Services at NC State University, Raleigh, NC.
