How+does+it+occur?

= Biofilm Development =

To understand the causes and development of biofouling, it is essential to be familiar with the elementary processes such as colonization. In general, colonization is a sequence of accumulation and growth. Biological accumulation is most simply described as the buildup of organisms and is a result of immigration and attachment of plant material. More specifically, immigration can be explained as the transport by current followed by settlement. Once the organisms have attached to the hard surface, growth takes place as the result of cell division thus increase in their biomass. This simple diagram is an example of how the biofilm layer forms on the hard surface of the heat exchanger over time.



Biofouling occurs as biological organisms start attaching to the heat exchanger surface. This attachment process usually starts through adhesion. Adhesion is the defined as the attachment of heterogeneous subjects. Adhesion of organisms in heat exchangers depend on the material wettability, its roughness, and the properties of the organism itself.



Electrostatic forces play a major factor in adhesion. Electrostatic forces act as the positive and negative charges of the different bacterial cells attract each other. It has been experimentally shown that bacterial adhesion does increase with the concentration of cations of NaCl, CaCl2, AlCl3, proving that electrostatic forces are playing a role.

Each biological organism attaches and grows differently on heat exchanger surfaces. After initial adhesion, each discharges different amounts of adhesive material to initiate attachment. Attachment may be temporary or permanent, based on the adhesive material released, its strength, and its amount.

The final stage of biofilm development is the growth of the organisms already attached to the heat exchanger surfaces. Reproduction of many organisms continuously occurs even after attaching. Growth rates are affected by the temperature of the heat exchangers, and the flow velocity which affects the movement of its nutrients.



Why is this a problem?
When using sea water as coolant in heat exchangers, biological fouling can occur as explained previously and evidently this is going to cause some problems. Even a thin layer of small organisms such as algae for example will add unwanted resistance effecting the rate of heat transfer and restricting the flow of fluids resulting in lost energy. The problem not only lies in the general observation of less efficient heat transfer but furthermore in numerically determining the heat transfer. The equation becomes more complex since the nor the thickness or thermal conductivity of the fouling is known. In the case of macroscopic organisms using mussels for example, their infestation is more severe because it can completely plug a system in only one growth season. Therefore biological fouling control needs to be carefully monitored and expensive mechanical or chemical techniques are required to do this. Fortunately, if the heat exchanger is designed well enough to expect this biological fouling, the problems associated with it become less significant. Previous: What is Biofouling? Next: What enables it?