DYNAMICS OF BIOFILM PROCESSES
M. G. Trulear, Graduate Student
W. G. Characklis, Professor
Department of Environmental Science and Engineering
Rice University
Houston, Texas 77001
INTRODUCTION
The term fouling refers to the formation of inorganic and/or organic deposits on surfaces.
In cooling systems, these deposits form on condenser tube walls increasing fluid frictional
resistance, accelerating corrosion and impairing heat transfer. Biological fouUng, or biofoul-
ing, results from the attachment and growth of microbial organisms.
The Problem
The most common method for controlUng fouUng biofilm development is periodic
chlorination. Recently, concern over residual toxicity from hypochlorous acid and its reaction products has resulted in federal regulations which limit the allowable chlorine concentration in coohng water discharges. This investigation stems from the apparent need for a
better understanding of fouling biofilm development and fouUng biofilm destruction so that
the impact of these new regulations on power plant operations can be evaluated.
Processes Contributing to Biofdm Development
Based on experimental observation in this research, biofilm development on a surface
exposed to fluid flow is the net result of physical transport and biological growth rate
processes. The processes which contribute to the overall biofilm accumulation are:
• organic adsorption
• transport of microbial particles to the surface
• microorganism attachment to the surface
• growth of attached microorganisms
• reentrainment of biofilm by fluid shear
The adsorption of an organic monolayer occurs within minutes after exposure of an initially "clean" surface to a flowing fluid which contains dispersed microorganisms, nutrients
and organic molecules. Baier [1-4] has investigated the dynamics of this organic adsorption and suggests that it is a prerequisite for biological attachment because it conditions
the surface.
The transport of microbial particles from the bulk to the surface depends on the fluid
flow regime. In turbulent flow, molecular diffusion and turbulent eddy transport are
viable mechanisms. In a quiescent fluid, chemotaxis is possible.
Numerous mechanisms have been postulated for the process of microbial attachment
to the surface [5,6]. Most agree that the production of a polysaccharide binding material is necessary.
Biofilm growth is considered as the combined effect of cellular reproduction and
exopolysaccharide production. Numerous investigators [7-10] have postulated equations
which describe the rate of substrate removal as a function of Umiting nutrient concentration and biofilm mass. The biological rate equations presented by Atkinson [7] also
account for the fundamental process of substrate diffusion into the biofilm matrix. Few
studies have attempted to relate substrate removal rate to biofilm development rate.
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