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Biofilms                   

Problems associated with biofilms

Once bacteria begin to colonise 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 packing, they may initially go unnoticed, since they will not interfere with flow or evaporative efficiency. After a time, the biofilm becomes more complex, often with filamentous development. The resulting matrix provided, will trap debris that may impede or completely block flow 

 

1. Initial Development 

2. Maturation 

3. 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 far more complex. Biofilms may be patchy and highly channelised, allowing nutrient-bearing water to flow through and around the biofilm matrix.

Algal biofilms may foul cooling tower distribution channels, tower fill, and basins. When excessive algal biofilms develop, portions may break loose and be transported to other parts of the system, causing blockage as well as providing nutrient for accelerated bacterial and fungal growth. Biofilms can also cause fouling of filtration and ion exchange equipment .

 

Table 1

Thermal conductivity comparison of deposit-forming compounds and biofilm substance. 

Substance Thermal Conductivity
(W m-1K-1)
CaCO3 2.6
CaSO4 2.3
Ca3(PO4) 2.26
Fe2 2.9
Analcite 1.3
Biofilm 0.6

Bacterial biofilms may also foul heat exchange equipment. Bacterial fouling of heat exchangers can occur quickly due to 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 with rapid accumulation of biofilm. 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

Deposits in the form of biofilm and biofilm with entrapped debris are generally easy to comprehend, but biofilms may often lead to the formation of mineral scales as well. Calcium ions are fixed into the biofilm by the attraction of carboxylate functional groups on the polysaccharides. In fact, divalent cations, such as calcium and magnesium, are integral in the formation of 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 by-products from the bulk water that will act as crystal growth sites.

A typical biofilm-induced mineral deposit that we are all familiar with is the calcium phosphate scale the dental hygienist removes from our teeth. When biofilms grow on tooth surfaces, they are referred to as plaques. If these plaques are not continually removed, they will accumulate calcium salts, mainly calcium phosphate, and form tartar (scale). One could make a comparison between rinsing your mouth twice daily with an antiseptic mouthwash to control plaque, with feeding microbicides and biodispersants to control biofilm related deposition in heat exchangers. If biofilms in heat exchangers are not controlled, then, like dental plaques, mineral scale may result.

The growth of bacteria and formation of biofilms may also result in another problem, that of corrosion. Microbiological corrosion may be defined as corrosion that is influenced by the growth of micro-organisms, either directly or indirectly. To understand microbiological corrosion or MIC for short, it helps to have a basic understanding of corrosion chemistry. This document is not intended to provide that information, but any water treatment training manual will. 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. 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. Microbiological corrosion is electrochemical corrosion where in some manner the presence of the micro-organisms is having some influence in the creation or acceleration of corrosion processes.

Micro-organisms can influence corrosion in a variety of ways. Formation of localised differential cells, the production of mineral and organic acids, ammonia production, and sulphate reduction are just a few of the mechanisms by which bacteria, fungi, and algae can influence corrosion. The formation of localised cells is the primary mechanism of corrosion caused by iron oxidising bacteria, such as Gallionella sp. and Siderocapsa sp. When iron and manganese oxidising organisms colonise a surface, they begin to oxidise available reduced forms of these elements and produce a deposit. In the case of iron oxidising organisms, ferrous iron is oxidised to the ferric form (Fe++ ---> Fe+++ +1e-) with the electron lost in the process being utilised 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 oxidise. The porous encrustation (tubercle) may potentially become an autocatalytic corrosion cell or may provide an environment suitable for the growth of sulphate-reducing bacteria. 

Corrosion may also develop when localised 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 localised corrosion cell.

There are numerous factors that will contribute to localised corrosion on metal surfaces. The production of ammonia by the reduction of nitrates or nitrites may lead to severe localised loss on copper based metals. The production of organic acids, such as acetic, butyric, or citric acid, may help solubilise protective metal oxide films. Inorganic acid, such as sulphuric 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 sulphate-reducing bacteria, such as Desulfovibrio sp., are the bacteria most often considered when discussing microbial corrosion. These organisms can seek out and colonise 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 recognisable from the characteristic sulphide by-product present within the corrosion cell. Sulphate reducing bacteria primarily cause corrosion by utilising the molecular hydrogen produced at the cathode, thereby depolarising it . Since the rate of corrosion is under cathodic control, removal of cathodic reduction products will increase the rate of corrosion. Systems that are sulphate limited will have less of a tendency to be attacked by SRB.

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