The presence of biofilms in public drinking water systems is a topic that has been widely discussed and studied in the last decade. While biofilms themselves may not constitute a serious health risk, when they're combined with inadequate or absent disinfectant residuals, they set the stage for opportunistic bacteria to develop into a true health hazard.
Biofilm PropertiesThree primary factors are necessary for the growth and reproduction of bacteria:
- Carbon and energy source
- Nutrient source
- Nitrogen source
Many water supplies derive the carbon and energy source from a carbonate species, which is naturally occurring or added as part of a treatment process. The nutrient levels in most water can vary significantly and are essential to biofilm growth. Nitrogen is the most problematic factor in many supplies.
The secondary factors involved in biofilm growth and regrowth:
- Ambient water temperature
- Flow rate
- Concentration of inorganic material
- Surface/substrate type
These secondary factors will not contribute to biofilm growth unless a nutrient source as described by the three primary factors is present. However, when the nutrient source is present, these secondary factors are very important. Water temperature can significantly affect bacterial growth, and usually increases exponentially. The flow rate of water in the pipe, type of surface, substrate (type of pipe and deposits or tubuculation present) and concentration of inorganic materials work together to determine the amount of available nutrients for cell growth.
Contamination SourcesBacterial contamination necessary for the formation of biofilm generally comes from one of three sources:
1. Breakthrough that occurs when the concentration of available disinfectant is not able to adequately eliminate or inactivate bacteria that are present;
2. Cross-connection contamination, which allows bacteria to enter the distribution system due to back pressure or backsiphonage; or through infiltration, which allows bacteria to enter the distribution system due to aspiration; and
3. Contamination during installation or repair of the distribution system itself or of a plumbing system connected to the public water supply distribution system.
Contamination often is caused by the entrance of only a few free "floating cells" (planktonic cells) into the distribution system. Minimum or inadequate disinfectant residuals or other disinfectant demands can allow these cells to attach to the wall of the pipe or other matter in the pipe. Once attached, these bacteria begin to grow rapidly, secreting a mucopolysaccaride (slimy) substance that envelops or surrounds the cell. This secreted slime substance serves as a physical barrier to disinfection and allows bacteria to continue to reproduce rapidly. This slimy substance is known as a glycocalyx, and it can be resistant to normal operating or extremely high chlorine residuals.
Indicators of Biofilm GrowthAny one of the following symptoms indicate that a biofilm may be present:
1. Typical or atypical growth during bacterial analyses (presence of colonies or confluent growth - Too Numerous to Count [TNTC])
2. Lack of disinfectant residual or difficulty in maintaining a consistent disinfectant residual
3. Presence of a slimy growth in water drawn from the tap or on pipe walls
4. Musty or yeasty odor to the water
5. A decrease in flow velocity
When any of these systems are present, samples should be collected and analyzed for bacterial culture and speciation to determine if a biofilm or biological regrowth is present. The specific organisms usually associated with biofilm include:
These bacteria all produce a slimy sheath. The first five are not typically enumerated or identified in the coliform testing procedures required by current federal regulations. The next two bacteria (Enterobacter and Escherschia) are coliform bacteria and would require investigation by the water supply. The last two organisms are iron-reducing bacteria and are found in both wells and distribution systems where a source of iron is abundant. Because the slime protects the bacteria by coating it with a sheath, conventional disinfection using chlorine is not always able to both penetrate the slime and eliminate or inactivate the bacteria. Oftentimes, the chlorine removes only the outer slime sheath, without inactivating or eliminating the bacteria.
One species of Pseudomonas - p. aeruginosa - is a very common organism found in biofilms. This organism is the causal agent for "swimmers ear," and also can cause skin irritations and lesions. It is the most common culprit of swimming pool and spa closings. Recent research has linked P. aeruginosa to respiratory infections and related illnesses in both the very young and elderly populations, and in persons with compromised immune systems, which includes persons undergoing dialysis, chemotherapy or treatment of other chronic illnesses. Contact usually is made when the water is in an aerosol form, through showers or vaporizers. A pink or rusty colored slimy substance usually accompanies the presence of this organism.
There is a long history of treatment to prevent biofilm growth in water distribution systems. Much of the early information comes from water well rehabilitation when wells were found to be contaminated with a slime-forming iron reducing bacteria. These organisms also are resistant to chlorine due to the protective sheath formed. Initially, wells were treated with acids and chrorine, then purged to attempt to physically remove the organisms. In more recent years, specific polyphosphates have been used effectively as a dispersant. This process allows the dispersion and destruction of the slime, which then allows the disinfectant to reduce or eliminate bacterial growth.
Many in the water industry expressed initial concern regarding polyphosphate uses, as phosphates are considered by some to be a nutrient. Several studies have indicated that the presence of poly and orthophosphates in water, under normal operating conditions, do not provide the needed nutrient demand, and in some cases, may provide a bacteriostatic (inhibiting) factor to growth. However, the use in the well rehabilitation field is waning. The main reason is the inability to remove all of the phosphate from the groundwater formation, which does allow the additional nutrient to possibly maintain or increase growth. In terms of using polyphosphate in a distribution system as a biofilm dispersant, care must be taken not to feed in doses exceeding 2.0 mg/l. At these, or higher levels, polyphosphates can begin to cause lead levels to rise.
A water distribution system presents many unique challenges as opposed to a well. In many cases, continuous use of water is necessary, making isolation of a specific section of the distribution system for aggressive treatment difficult, if not impossible. Many supplies have reduced biofilm growth through vigorous flushing, although in many instances it is not possible to achieve the scouring velocity and pressure needed to physically remove the biofilm.