By Jim Lukanich, Senior Technical Specialist,
Buckman Laboratories International
This article appeared recently on the VerticalNet, Inc. web site Chemical Online (www.chemicalonline.com). Design and operating personnel in the municipal wastewater industry with responsibilities for biological treatment processes should find it educational.
The development of biofilms and the role they play in corrosion and deposition processes may be the most misunderstood and underestimated factor cooling water and other industrial water systems. Ask any water treatment professional about calcium carbonate scale or to explain the major attributes of a typical corrosion cell, and you may get a reasonable explanation provided with some confidence. Ask about biofilms, and you get a different response. This review is designed to provide a basic understanding of what biofilms are, the problems they can cause, and what might be done to deal with them. Part 1 explains why biofilms cause system problems; Part 2 will provide solutions.
What is a biofilm?
Simply stated, a biofilm consists of microbial cells (algal, fungal, or bacterial) and the extracellular biopolymer they produce. Generally, bacterial biofilms that are of most concern in industrial water systems, since they are generally responsible for the fouling of heat transfer equipment. This is de in part to the minimal nutrients that are required for many species to grow. One should keep in mind that the more nutrients available in the form of useable organic carbon, the greater the diversity and numbers of organisms that can be supported.
When dealing with cooling towers and spray ponds, algal biofilms are also a concern. Not only will algal biofilms foul distribution decks and tower fill, but algae will also provide nutrient (organic carbon) that will help support the growth of bacteria and fungi. Algae do not require organic carbon for growth but instead utilize CO2 and the energy provided by the sun to manufacture carbohydrate. So, a cooling water system with little organic carbon can generate its own nutrient through the growth and dispersal of algal cells.
Microorganisms can be found in both the bulk water and on the surfaces of industrial water systems. Bacteria attach to surfaces by proteinaceous appendages referred to as fimbriae (Fig. 1). Once a number of fimbriae have "glued" the cell to the surface, detachment of the organism is very difficult. One reason bacteria prefer to attach to surfaces is the organic molecules adsorbed there provide nutrient. Once attached, the organisms begin to produce material termed extracellular biopolymer, or slime. The amount of biopolymer produced can exceed the mass of the bacterial cell by a factor of 100 or more. The slime tends to provide a more suitable protective environment for the organism.
Slime consists primarily of polysaccharide and water. In fact, biofilms are generally >90% water. The polysaccharides vary depending on the species but are typically made up of repeating oligosaccharides, such as glucose, mannose, galactose, xylose, and others. An often cited example of a bacterial-produced biopolymer is xanthan gum, produced by Xanthamonas campestris (Fig. 2). This biopolymer is used as a thickening agent in a variety of food and consumer products.
Gellation of some biopolymers can occur upon addition of divalent cations, such as calcium and magnesium. The electrostatic interaction between carboxylate functional groups on the polysaccharide and the divalent cations results in a bridging effect between polymer chains. Bridging and crosslinking of the polymers help to stabilize the biofilm, making it more resistant to shear.
Problems associated with biofilms
Once bacteria begin to colonize surfaces and produce biofilms, numerous problems begin to arise, including reduction of heat transfer efficiency, fouling, corrosion, and scale. When biofilms develop in low-flow areas, such as cooling-tower film fill, they may initially go unnoticed, since they will not interfere with flow or evaporative efficiency. After time, the biofilm becomes more complex, often with filamentous development. The matrix provided will accumulate debris that may impede or completely block flow (Fig. 3).
Figure 3a. Initial Development
Figure 3b. Maturation
Figure 3c. Accumulation of debris
Biofilm structure is often imagined as a coating of microbial cells and biopolymer that is spread evenly across a surface. In reality, biofilm structure is much more complex. Biofilms may be patchy and highly channelized, allowing nutrient-bearing water to flow through and around the matrix.
Algal biofilms may foul cooling tower distribution decks, tower fill, and basins. When excessive algal biofilms develop, portions may break loose and transport to other parts of the system, causing blockage as well as providing nutrient for accelerated bacterial and fungal growth. Biofilms can cause fouling of filtration and ion exchange equipment . Fig. 4 shows a 75-micron cartridge filter fouled with biofilm. In photograph A (300X) the filter fibers are shown to be fouled with an unknown material. Closer examination of the foulant exhibited in photograph B (9,000X) shows the foulant to be bacterial cells slime.
Photograph A (300x)
Photograph B (9,000x)
Bacterial biofilms may also foul heat exchange equipment, especially after a process leak or influx of nutrient. The sudden increase in nutrient in a previously nutrient-limited environment will send bacterial populations into an accelerated, logarithmic growth phase. The biofilms that develop will then interfere with heat transfer efficiency. Table 1 demonstrates the thermal conductivity of a variety of deposit-forming compounds compared to biofilm. A lower number indicates a greater resistance to heat transfer (See Reference).
| CaCO3 || 2.6 |
| CaSO4 || 2.3 |
| Ca3(PO4)2 || 2.6 |
| Fe2 || 2.9 |
| Analcite || 1.3 |
| Biofilm || 0.6 |
Mineral scale is another form of biofilm-caused fouling. As noted earlier, calcium ions are fixed into the biofilm by the attraction of carboxylate functional groups on the polysaccharides, and form gels in some extracellular polysaccharides. If we can imagine these calcium ions being fixed in place by the biofilm at the heat transfer surface, then it would make them more readily available to react with carbonate or phosphate anions that are present. This would then provide nucleation or crystal growth sites that would not normally be present on a biofilm free surface. Additionally, biofilms may entrap precipitated calcium salts and corrosion byproducts from the bulk water that will act as crystal growth sites. This scale is not unlike the plaque that a dental hygienist removes from our teeth. One could liken antiseptic mouthwash rinses to control plaque to the microbicides and biodispersants to control biofilms in industrial equipment.
The growth of bacteria and formation of biofilms may also result in corrosion. Microbiological corrosion (MC) may be defined as corrosion that is influenced by the growth of microorganisms, either directly or indirectly. In essence, corrosion occurs on a metal surface due to some inherent or environmental difference between one area on that surface and another. These differences will create anodic and cathodic areas, setting up a basic corrosion cell (Fig. 5). The anode is the area at which metal is lost. The electrons given up by the metal flow to the cathode to be consumed in a reduction reaction. MC is electrochemical corrosion where microorganisms create or accelerate corrosion processes.
Formation of localized differential cells, the production of mineral and organic acids, ammonia production, and sulfate reduction are just a few of the mechanisms by which bacteria, fungi, and algae can influence corrosion. The formation of localized cells is the primary mechanism of corrosion caused by iron oxidizing bacteria, such as Gallionella sp. and Siderocapsa sp. When iron and manganese oxidizing organisms colonize a surface, they begin to oxidize available reduced forms of these elements and produce a deposit. In the case of iron oxidizing organisms, ferrous iron is oxidized to the ferric form (Fe++ ---> Fe+++ +1e-) with the electron lost in the process being utilized by the bacterium for energy production. As the bacterial colony becomes encrusted with iron (or manganese) oxide, a differential oxygen concentration cell may develop, and the corrosion process will begin. The ferrous iron produced at the anode will then provide even more ferrous iron for the bacteria to oxidize. The porous encrustation (tubercle) may potentially become an autocatalytic corrosion cell or may provide an environment suitable for the growth of sulfate-reducing bacteria. Figs. 6, 7, and 8 demonstrate the stages of localized corrosion cell development resulting from the growth of iron oxidizing bacteria.
Figure 6. Bacterial growth and iron oxidation
Figure 7. Corrosion cell development
Corrosion may also develop when localized cells are formed, due simply to biofilms developing on metal surfaces. The oxidation of iron or manganese is not a requirement for the development of a localized corrosion cell.
There are numerous factors that will contribute to localized corrosion on metal surfaces. The production of ammonia by the reduction of nitrates or nitrites may lead to severe localized loss on copper-based metallurgy. The production of organic acids, such as acetic, butyric, or citric acid, may help solubilize protective metal oxide films. Inorganic acid, such as sulfuric acid produced by Thiobacillus sp., can also have detrimental effects. As biofilms develop, they will eventually achieve a thickness at which oxygen concentration is either very low or completely excluded. At a thickness of just 200 microns, the oxygen concentration within the biofilm is reduced to near zero ppm. When this occurs, facultative and obligate anaerobes can flourish.
Anaerobic sulfate-reducing bacteria, such as Desulfovibrio sp., are the bacteria most often considered when discussing microbial corrosion. These organisms can seek out and colonize areas deficient in oxygen, such as those found within porous corrosion tubercles, within biofilms, and under debris. These bacteria are responsible for rapid and severe metal loss in industrial water systems. This type of corrosion is easily recognizable from the characteristic sulfide by-product present within the corrosion cell. Sulfate reducing bacteria primarily cause corrosion by utilizing the molecular hydrogen produced at the cathode, thereby depolarizing it (Fig. 9). Since the rate of corrosion is under cathodic control, removal of cathodic reduction products will increase the rate of corrosion. Systems that are sulfate limited will have less of a tendency to be attacked by SRB.
For more information: Buckman Laboratories International, 1256 N. McLaen Blvd., Memphis TN 38108. Tel: 901-278-0330.
Characklis, W.J. and Marshall, K. C., Biofilms, John Wiley and Sons, 1990, pp. 316.
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