Biofilms can develop on almost any surface exposed to an aqueous environment. The biofilm systems that result can be used beneficially, as exemplified by some waste-water treatment processes. However, biofilms can be quite problematic in certain engineering systems. In hot and cold water distribution systems and cooling systems, for example, biofilms can cause under-deposit corrosion and increased risk of the presence of Legionella or Pseudomonas aeruginosa bacteria.
Biofilm development on a surface exposed to a fluid flow is the net result of several physical, chemical, and biological processes including the following:
•Transport and adsorption of organic molecules to the surface,
•Transport of microbial cells to the surface,
•Microorganism attachment to the surface,
•Microbial transformations (growth and exopolymer production) at the surface resulting in the production of biofilm, and
•Partial detachment of the biofilm caused by fluid shear stress.
Numerous mechanisms have been postulated for the process of microbial attachment to the surface. Most agree that the production of a polysaccharide binding material is necessary. Biofilm production is the combined effect of cellular reproduction and extracellular polymer production. The rate of biofilm production depends on the diffusion of nutrients into the biofilm followed by their synthesis into attached biomass. Nutrient or oxygen depletion in lower layers of the biofilm can significantly limit the overall production process.
At any point in the development of a biofilm, portions of biofilm are sheared away and re-entrained in the fluid flow. Detachment, a continuous biofilm removal process, is highly dependent on hydrodynamic conditions. In addition to shearing, sloughing also can significantly contribute to detachment. Sloughing refers to a random, massive removal of biofilm attributed to nutrient/oxygen depletion deep within biofilms. Sloughing is more frequently witnessed with thicker, less dense films that develop under low fluid shear conditions.
Biofilm is composed of extracellular polymeric substances produced by bacteria, aerobic and anaerobic bacteria, fungi, algae (if light is present), nutrients and other organisms.
Direct observations of bacteria growing in a wide variety of natural industrial and pathogenic situations have shown that these organisms grow predominantly in multi-species biofilms attached to available surfaces. The biofilm performs several functions vital to the survival of its constituent organisms. The polysaccharide anchors the micro-organisms to a surface. The biofilm traps nutrients from the bulk solution and makes them available to the bacteria. The biopolymer also shields the bacteria from predators and bacteriophages.
When biofilms develop on metallic surfaces they can create conditions conducive to corrosion. The presence of adjacent micro-colonies of different types of bacteria contributes to the formation of local anodes and cathodes due to differences in pH, or other ions.
The polysaccharides from different bacteria can have differing chelating abilities that produce concentration cells at the metal surface. When contaminated industrial systems form biofilms, the heterotrophic population dominates the upper layers. The lower layers are comprised of bacteria that can remove metabolic end products as we have seen in degradative processes.
The bacteria that perform this function in nature often also have the capacity to utilize hydrogen produced by other bacteria. In corrosive biofilms, the hydrogenase positive organisms are positioned in the bottom of the biofilm, where hydrogen is being formed as the cathodic reaction product. Removal of this cathodic hydrogen depolarizes the corrosion cell, greatly accelerating the corrosion rate.
Biofilms can lead to bacteria hiding from the temperature and biocides we use to try to remove these bacteria. A proposed mechanism for biofilm resistance to biocidal agents is that biofilm-associated cells grow significantly more slowly than planktonic cells and, as a result, take up biocidal agents more slowly.
Bacterial biofilm, or slime as it is more commonly called, causes several problems in cooling systems. Losses in heat transfer translate to losses in production or increased energy costs. Increases in corrosion result from biofilm directly or indirectly through the promotion of anaerobic bacteria, “Desulfovibrio desulfuricans”. Bacterial biofilm may also harbour pathogenic organisms such as Legionella.
It is very important to prevent biofilms forming in systems and strategies used to remove biofilms include using biodispersants (but these cannot be used in potable water so are mainly directed at cooling systems) or biofilm penetrating biocides such as chlorine dioxide, silver hydrogen peroxide or some non-oxidising biocides. The product selection is key as not all can be used in all applications.
Collaton Consultancy Limited are expert Legionella consultants working for both water treatment companies and end users alike, Expert Witness services are also offered should a legal case arise. If you have any specific issues relating to the above you would like help with then contact Collaton Consultancy Limited
Legionella Risk Management 