Pathogen-free drinking water is a priority for the safety of human health concerned with waterborne infectious diseases. Increasing urbanization has aggravated the problem of microbial contamination in most of drinking water sources resulting into outbreaks and sporadic incidences of water transmitted diseases mainly gastroenteritis, cholera, dysentery, typhoid, poliomyelitis and hepatitis. Although there are a number of popular methods such as filtration, ozonisation, reverse osmosis and UV radiation, chlorination is the most popular globally used method for drinking water disinfection, particularly in piped supplies at community level. Recent analytical studies have revealed that chlorination of water produces numerous disinfection by-products (DBPs) after the reaction of residual chlorine with natural organic compounds such as humic and fulvic acids in water. Many of such DBPs have been reported to be mutagenic/ carcinogenic. To overcome this problem, there is a pressing need to replace chlorination with a safer and appropriate alternative process.
Certain metals like mercury, silver and copper with an oligodynamic property have been found to be biocidal with the capability to disinfect the water. Among these, silver is more appropriate as it is non-toxic and an efficient water disinfectant. Silver has been known as an effective biocide against viruses, bacteria, protozoa, algae, yeasts and moulds. Silver has strength, malleability, ductility, high reflectance of light, temperature resistance and electrical as well as thermal conductivity. Silver is used for electroplating, currency, ornaments, utensils, mirror plating, sweet coating, photography, electrical/electronic instrumentation, solar energy, medical (dental) applications and scientific research. The use of pots and pitchers made up of silver for storage of drinking water is an age-old tradition which indicates that its bactericidal property was well known to our ancestors.
2. SILVER AS DISINFECTANT
The antimicrobial effects of silver (Ag) have been recognized for thousands of years. In ancient times, it was used in water containers and to prevent putrefaction of liquids and foods. In ancient times in Mexico, water and milk were kept in silver containers. Silver was also mentioned in the Roman pharmacopoeia of 69 B.C.
In 1884, silver nitrate drops were introduced as a prophylactic treatment for the eyes of new-borns, and this became a common practice in many countries throughout the world to prevent infections caused by Neisseria gonorrhoeae transmitted from infected mothers during childbirth. In 1928, the “Katadyn Process” based on the use of silver in water at low concentrations, was introduced.
Silver ions have the highest level of antimicrobial activity of all the heavy metals. Gram-negative bacteria appear to be more sensitive than gram-positive species. Kawahara et al. posited that some silver binds to the negatively charged peptidoglycan of the bacterial cell wall. Because gram-positive species have a thicker peptidoglycan layer than do gram-positive species, perhaps more of the silver is prevented from entering the cell.
Generally speaking, the observed bactericidal efficacy of silver and its associated ions is through the strong binding with disulphide (S–S) and sulfhydryl (–SH) groups found in the proteins of microbial cell walls. Through this binding event, normal metabolic processes are disrupted, leading to cell death. The antimicrobial metals silver (Ag), copper (Cu), and zinc (Zn) have thus found their way into a number of applications.
2.1. Applications And Uses
a) Drinking Water
Chlorine has been used as the principal disinfectant for drinking water since the early 1900s. In the 1970s, it was discovered that chlorination caused the formation of numerous chlorinated compounds in water, including trihalo-methanes and other disinfection by-products (DPB), that are known to be hazardous to human health. There is therefore a need to assess alternative disinfectants.
Silver electrochemistry experiments suggest that silver may have potential as a chlorine alternative in drinking water disinfection in applications in which chlorine may be considered too hazardous. Silver has been used as an effective water disinfectant for many decades, primarily in Europe. It has also been used to treat recycled water aboard the MIR space station and aboard NASA space shuttles.
Both the Environmental Protection Agency (EPA) and the World Health Organization (WHO) regard silver as safe for human consumption. Only argyria (irreversible skin discoloration) occurs with the ingestion of gram quantities of silver over several years or by the administration of high concentrations to ill individuals. There have been no reports of argyria or other toxic effects caused by silver in healthy persons (World Health Organization 1996). Based on epidemiological and pharmacokinetic data, a lifetime limit of 10 grams of silver can be considered a No Observable Adverse Effect Level (NOAEL) for humans (World Health Organization 1996). In the United States, no primary standards exist for silver as a component in drinking water. The EPA recommends a secondary non-enforceable standard of 0.1 mg/L (100 ppb) (Environmental Protection Agency 2002). The World Health Organization (1996) has stated this amount of silver in water disinfection could easily be tolerated because the total absorbed dose would only be half of the NOAEL after 70 years. Silver has been used as an integral part of EPA- and National Sanitation Foundation (NSF)-approved point-of-use (POU) water filters to prevent bacterial growth. Home water purification units (e.g., faucet-mounted devices and water pitchers) in the United States contain silverized activated carbon filters along with ion-exchange resins (Gupta et al. 1998). Today, some 50 million consumers obtain drinking water from POU devices that utilize silver (Water Quality Association 2001). These products leach silver at low levels (1–50 ppb) with no known observable adverse health effects. Such filters have been shown to prevent the growth of Pseudomonas flu-rescens and Pseudomonas aeruginosa in water; however, several studies have raised questions about their efficacy. Reasoner et al. (1987) established that bacterial colonization of such devices occurs within a matter of days and may result in a large number of bacteria in the product water.
b) Cooling Towers/Large Building Water Distribution Systems
Cooling towers provide cooling water for air compressors and industrial processes that generate heat. They provide an ideal environment and a suitable balance of nutrients for microbial multiplication. Chlorine is a popular method for controlling such bacterial growth, but there are difficulties in maintaining disinfection efficacy, particularly at a high temperature or pH. Chlorination can also cause corrosion of cooling tower facilities.
Ag/Cu ionization has been used in cooling towers to control bacterial growth. In a study by Martinez et al. (2004), an appreciably reduced chlorine concentration of 0.3 parts per million (ppm or mg/L) was combined with 200 ppb Ag and 1.2 ppm Cu. This method had an appreciable impact on levels of coliform bacteria, iron-related bacteria, sulphate-reducing bacteria and slime-forming bacteria in a cooling tower.
Large hot water distribution systems in hospitals and hotels have also often been attributed as a source of contaminating bacteria. Contaminated systems are usually treated by either superheating the water with flushing of the distal sites (heat-flush), by hyper chlorination, or by installing Ag/Cu ionization units. Greater bacterial reductions have been observed with Ag/Cu ionization than with the heat-flush method. Ag/Cu ionization is known to provide long-term control and may be used in older buildings in which the pipes could be damaged by hyper chlorination. Such systems are easy to install and maintain, are relatively inexpensive, and do not produce toxic by-products.
One microorganism that has been commonly isolated from cooling towers is Legionella pneumophila, the causative agent of Legionnaires’ disease. Many outbreaks have been linked to cooling towers and evaporative condensers. L. pneumophila is also commonly isolated from the periphery of hot water systems in large buildings such as hospitals, hotels, and apartment buildings where temperatures tend to be lower. Ag/Cu systems have been in common use in hospitals to control Legionella for more than a decade. Mietzner et al. reported that one such ionization system maintained effective control of L. pneu-mophila for at least 22 mon. Legionella may develop a tolerance to silver after a period of years, requiring higher concentrations to achieve the same effect.
c) Recreational Waters
Bacteria, protozoa, and viruses may occur naturally in recreational waters or be introduced into swimming pools by bathers or through faulty connections between the filtration and sewer systems. Species carried by bathers include the intestinal Streptococcus faecalis and Escherichia coli, as well as skin, ear, nose, and throat organisms such as Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus salivarius, Pseudomonas aeruginosa, and Mycobacterium marinum (Singer 1990). Mild to serious illnesses caused by inges