Fouling biofilm development in a tubular flow system
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AbstractFouling biofilm development in a completely mixed tubular recycle reactor was studied. The geometry, materials and operation of the recycle circuit were designed to simulate conditions in industrial heat exchangers. The sampling system allowed direct (brightfield, epifluorescence, scanning electron and transmission electron photomicroscopy) and indirect (increased fluid frictional resistance, organic carbon, polysaccharide, adenosine triphosphate and deoxyribonucleic acid) observations of biofilms. Mixed culture biofilms were developed at two different fluid velocities. Biofilm accumulation on corrosion- resistant materials progressed in a sigmoidal fashion; biofilm accumulation on a corrosion susceptible metal was linear with time. Biofilms initially consisted of a monolayer of bacillary bacteria. Filamentous bacterial cells were invariably present in more fully developed biofilms. Fouling (increased fluid frictional resistance) always began after filamentous bacteria became a permanent part of the biofilm. Biofilms developed during low velocity runs were less dense (and had filamentous cells sooner) than biofilms developed during high velocity runs. Fouling biofilms were dynamic filamentous matrixes. Changes in the fluid frictional resistance were explained in terms of the appearance of the filamentous biofilm matrix. A technique is described that was effective in biofilm removal. The process is a practical approach to the problem of biofilm control in tubular heat exchanger systems. Rapid external cool-down of the metal tubular flow biofilm sampler (Robbins device) resulted in significant reductions in biofouling. Pure culture Pseudomonas sp. biofilm development was studied. Filamentous growth of sessile Pseudomonas sp. occurred. In liquid culture media, however, the organism grows as short bacillary rods. Accumulation of biomass by filamentous (but not bacillary) Pseudomonas sp. biofilms resulted in increased fluid frictional resistance. The biomass accumulation rate of filamentous biofilms was almost three times greater than the biomass accumulation rate of bacillary biofilms. The growth of Pseudomonas aeruginosa was characterized in nutrient broth batch cultures. Filamentous growth of P. aeruginosa strain MUCOID occurred because of a rapid shift-down in the log exponential growth rate. Pure culture Sphaerotilus natans biofilm development was studied. The filamentous growth of the organism caused fouling. The increase in fluid frictional resistance was directly proportional to the increase in adherent biomass. The growth of the biofilm was a linear function with time.
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