Biofilm Processes
The biofilm process is a process which uses a biofilm for the purposes of filtration, bioremediation or barrier formation. A biofilm is a complex and heterogeneous matrix of microorganisms attatched to and growing on a surface. Biofilms are most often found on solid substrates that are exposed to an aqueous solution. Biofilms are particularly effective due to the complex matrix structure which protects inner cells, facilitates communication and interactions amongst the cells, and allows for growth through cell division and adhesion.
The Formation of a Biofilm
A biofilm forms when free-floating microorganisms adhere to a surface through weak interactions, such as van der Waals forces, and then attach more sturdily through cellular adhesion molecules. Once this initial layer has attached to the surface, other cells are able to attach due to the diverse adhesion sites. This process continues with the addition of more and more cells, often of a variety of species of bacteria, fungi, algae and protozoa that excrete a significant amount of extracellular polymeric substances (EPS) that anchors them to the surface, creating a multilayer matrix. The biofilm is then able to disperse clumps of cells so that the bacteria can travel down stream and attatch to another surface, thus propagating the biofilm.
The formation of a biofilm can be broken down into the following 5 stages: initial attachment, irreversible attachment, maturation, continued maturation, and finally dispersion. This process of formation is illustrated in figure 1.
Figure 1. Biofilm Maturation in Five Stages
Because the cells on the surface of the biofilm matrix provide a certain amount of protection for the cells within, biofilms serve as an important survival mechanism for bacteria and other microorganisms. The cells within the biofilm behave differently from the cells on the surface. Depending on the thickness of the biofilm, cells on the interior can exist in a seemingly dormant state due to their lack of exposure to oxygen, and may remain in this state until cells on the outer layers of a biofilm are attacked. When the outer layers of cells are destroyed, the microorganisms within become active again and are able to regrow the biofilm. Thus biofilms are incredibly durable and can be difficult to destroy as they are able to develop a resistance to biocides.
Purpose
Biofilms can be used for treating wastewater, creating barriers to protect soil and groundwater from contamination or the bioremediation of hazardous waste sites. For wastewater treatment, biofilms can be grown on filters to be used in the treatment process. By running the wastewater over the filters, the biofilm will be able to break down and extract some undesirable organic compounds. This bioremediation can either occur in situ or in an engineered biofilm reactor. When engineering a biofilm to be used for filtration, it is important to focus on the thickness of the biofilm, because it will affect the rate of diffusion. Often it is more desirable to have a thin biofilm since the substrate will only be able to penetrate to a certain depth and excess depth is unnecessary.
History
Biofilms have been used for water treatment since the mid 1800’s. In 1860, British engineers treated water and wastewater with sand filter treatment processes. The sand acted as surfaces for the adhesion of the microorganisms, which in turn naturally developed into a biofilm capable of using the organic matter from that water for filtration purposes. Since this time, scientists in many fields have researched and developed a variety of biofilm processes to aid in water treatment, medical devices, bioremediation and other purposes.
Currently, there is a great deal of interest in the biofilm process for wastewater treatment. Many favor the biofilm process over the activated sludge process because the biofilm process requires less space and there is no need for sludge recirculation.
Biofilm Process Variables
There are many different biofilm systems currently in use and many more in the development stages. Some of the systems currently in use include the granular media biofilter, fluidized bed reactors, rotating biological contactors, trickling filters, fixed media submerged biofilters, pumped flow biofilm reactors, and annamox-based biofilm reactors.
Some other technology using the biofilm process includes microbial fuel cells, microbial mats, biofilm “traps” and microbial “canaries”.
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References
“What is Biofilm?” Automation in Microbiology and Biosciences . 2005. April 2, 2010.
