CONDENSER BIOFOULING CONTROL WITH FERRATE(VI)
Joanne Fagan, Graduate Student
Department of Civil Engineering
Northwestern University
Evanston, Illinois 60201
Thomas D. Waite, Associate Professor
Department of Civil Engineering
University of Miami
Coral Gables, Florida 33124
The formation of a microbiological film on solid surfaces creates a costly problem for
many industries. Although the mechanisms by which microorganisms become attached
to solid surfaces are not well understood, the problems created by their attachment have
generated a great deal of research, especially in the area of condenser fouling. Traditionally,
the most common method of controlling slime growth in electric generating plant cooling
systems has been to treat cooling waters intermittently or continuously with chlorine. In
most cases, chlorination has proven to be effective in keeping condenser tubes clean, i.e.,
maintaining adequate heat transfer rates and low back-pressure.
The condenser unit in an electric generating plant consists of a matrix of slender (1- to
2-inch-diameter) tubes inside a heavy metal shell. The unit condenses steam that has insufficient energy to be useful in production of electricity, and is important in the conversion
of potential energy into electrical energy. Inefficiencies in condenser operation can have a
significant impact on the operating costs of a power plant. For example a $100 million
savings could be realized nationwide by a 0.1-in. Hg reduction in backpressure [ 1 ].
Biofouling is caused by bacterial growth on the walls of the condenser. The cells become
attached, increase in number, secrete extracellular material and create a slime. The organic
deposits attract additional deposits of organic and inorganic material. Also increases in heat
transfer resistance result in less efficient condensation and therefore less backpressure to
the turbine. With less vacuum in the system, the steam passes more slowly through the
turbine, generating less electricity, and resulting in greater fuel consumption per unit of
electrical energy produced. To reduce this extra energy generation cost, condensers must
be treated to reduce biofouling.
BIOFILM MICROBIOLOGY
The succession of a microbial ecosystem on solid surfaces in natural waters is comparable
to that described by Odum [2] for macroscopic environments. There is a progression from
nutritionally simple, unspecialized species to more diversified species with more complex
nutritional requirements. The pattern of succession observed for a microbial ecosystem is
that first gram-negative rods colonize the surface. Long filamentous rods, then stalked,
and budding bacteria develop followed by protozoa and rotifers. After a period of time
the film has become a dense heterogeneous assortment of particulate and biological material.
The relative abundance and diversity of the different organisms in the primary and developed film is a function of many environmental factors including temperature, pH, dissolved
oxygen, and light [ 3-5 ].
Marshall [6] describes three steps in the process of attachment. First, organisms come in
contact with the surface and are reversibly sorbed to it. Next the cells become bonded to
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