Silver as a Residual Disinfectant To Prevent Biofilm Formation in Water Distribution Systems▿

+ Author Affiliations

¹Department of Agricultural and Biosystems Engineering, Room 403, Building 38, The University of Arizona, Tucson, Arizona 85721
²Department of Soil, Water and Environmental Science, Room 429, Building 38, The University of Arizona, Tucson, Arizona 85721
³Department of Agricultural and Biosystems Engineering, Room 403, Building 38, The University of Arizona, Tucson, Arizona 85721
⁴Arizona Materials Laboratory, Department of Materials Science and Engineering, The University of Arizona, 4715 E. Fort Lowell Road, Tucson, Arizona 85712

ABSTRACT

Biofilms can have deleterious effects on drinking water quality and may harbor pathogens. Experiments were conducted using 100 μg/liter silver to prevent biofilm formation in modified Robbins devices with polyvinyl chloride and stainless steel surfaces. No significant difference was observed on either surface between the silver treatment and the control.

The materials used in drinking water distribution systems are readily colonized by bacteria (5). The rates of biofilm formation and release into a distribution system (DS) can be affected by many factors (14). Although few biofilm organisms pose a threat to humans, many opportunistic pathogens are able to survive and proliferate (40).

Chlorination is a commonly used water treatment in the United States and Europe (41). Chlorine is also used to provide a residual disinfectant in the DS to prevent water recontamination and to maintain the standards achieved at the first point of disinfection (4). Once a biofilm is established, however, bacteria are more resistant than planktonic populations to disinfectants, including chlorine (16, 20, 32, 44), and antibiotics (25).

Factors affecting survival in biofilms in chlorinated water include low-nutrient conditions, strain variation, bacterial attachment to surfaces with concomitant metabolism changes, and bacterial encapsulation (1, 19, 43). Biofilm growth can lead to pipe corrosion (24, 27), deterioration in water quality (24) and aesthetics (27, 36), and other undesirable effects (24). Chlorine also produces harmful disinfectant by-products (46), particularly with high levels of organic matter. Free chlorine creates problems in older DSs by causing pitting corrosion. Precipitation of ferric hydroxide accelerates corrosion and represents a demand on residual free chlorine aside from that of organic matter (39). The identification of safe alternative disinfection methods is therefore desirable.

Silver's antimicrobial effect has been demonstrated in numerous applications against different types of microorganisms (7, 10). The bactericidal efficacy of silver is through its binding to disulfide or sulfhydryl groups in cell wall proteins (11, 35). Silver also binds to DNA (38). Through these binding events, metabolic processes are disrupted, leading to cell death (21).

Silver has been reported to delay or prevent the formation of biofilms in medical catheters (8, 13, 15, 33), prosthetic heart valves (3, 17), vascular grafts, and fracture fixation devices (6, 9). Silver has also been used in water filters (31), cooling towers (22), and DSs (23, 26, 29). Silver exerts its antimicrobial effect by progressive elution from the devices.

Silver is effective against planktonic bacteria (34) and has been used for water disinfection in Europe (18, 31). In addition, silver, in combination with copper, has proven effective against Legionella pneumophila in hospital DSs for more than a decade (37). Silver is not believed to react with most organics in DSs or to produce toxic by-products (46). The objective of this study was to determine if silver inhibits biofilm formation on two very different surfaces to evaluate its potential as a residual disinfectant in DSs.