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.

Bedbugs are small, oval, brownish insects that live on the blood of animals or humans. Adult bedbugs have flat bodies about the size of an apple seed. After feeding, however, their bodies swell and are a reddish color.
Bedbugs do not fly, but they can move quickly over floors, walls, and ceilings. Female bedbugs may lay hundreds of eggs, each of which is about the size of a speck of dust, over a lifetime.
Immature bedbugs, called nymphs, shed their skins five times before reaching maturity and require a meal of blood before each shedding. Under favorable conditions the bugs can develop fully in as little as a month and produce three or more generations per year.
Although they are a nuisance, they do not transmit diseases.

Where Bed Bugs Hide

Bedbugs may enter your home undetected through luggage, clothing, used beds and couches, and other items. Their flattened bodies make it possible for them to fit into tiny spaces, about the width of a credit card. Bedbugs do not have nests like ants or bees, but tend to live in groups in hiding places. Their initial hiding places are typically in mattresses, box springs, bed frames, and headboards where they have easy access to people to bite in the night.
Over time, however, they may scatter through the bedroom, moving into any crevice or protected location. They may also spread to nearby rooms or apartments.
Because bedbugs live solely on blood, having them in your home is not a sign of dirtiness. You are as likely to find them in immaculate homes and hotel rooms as in filthy ones.

When Bedbugs Bite

Bedbugs are active mainly at night and usually bite people while they are sleeping. They feed by piercing the skin and withdrawing blood through an elongated beak. The bugs feed from three to 10 minutes to become engorged and then crawl away unnoticed.

Most bedbug bites are painless at first, but later turn into itchy welts. Unlike flea bites that are mainly around the ankles, bedbug bites are on any area of skin exposed while sleeping. Also, the bites do not have a red spot in the center like flea bites do.
People who don't realize they have a bedbug infestation may attribute the itching and welts to other causes, such as mosquitoes. To confirm bedbug bites, you must find and identify the bugs themselves.

Signs of Infestation

If you wake up with itchy areas you didn't have when you went to sleep, you may have bedbugs, particularly if you got a used bed or other used furniture around the time the bites started. Other signs that you have bedbugs include:

Blood stains on your sheets or pillowcases
Dark or rusty spots of bedbug excrement on sheets and mattresses, bed clothes, and walls
Bedbug fecal spots, egg shells, or shed skins in areas where bedbugs hide
An offensive, musty odor from the bugs' scent glands