PHYSICAL ASPECTS OF BIOFOULING
Margaret J. Bliss, Graduate Student
Charles R. O'Melia, Professor
Department of Environmental Sciences and Engineering
University of North Carolina
Chapel HiU, North Carolina 27514
INTRODUCTION
Biofouling refers to the biological formation of deposits or fdms on surfaces. These
fdms impede the transport of heat across such a surface and increase the resistance to
the flow of fluid passing the surface. Effects of biofouling include a reduction in the
heat transfer capabdity of condensers in power plants and a reduction in the hydraulic
capacity of water mains in distribution systems. The former reduces energy production
and the latter increases energy consumption.
The formation of biofdms in water mains and condenser tubes involves the transport
of nutrients and organic carbon to the surfaces of these systems, the growth of heterotrophic bacteria and fungi on these substrates, the formation of exoceUular polymeric
products during growth by these organisms and, assisted by these biopolymers, the
deposition of inorganic and organic particles from the flowing water on the walls of
the pipes. Excellent studies of the biological aspects of biofdm development have been
conducted by Characklis [ 1 ]. The research reported here has been undertaken to study
the physical factors involved in the transport of solid particles and soluble substrates
to pipe waUs under conditions of turbulent flow. The approach used is both conceptual
and experimental. Theoretical relationships for the transport of particles from water in
turbulent flow to pipe walls are developed by adapting concepts developed previously
by others for air systems. Experiments are conducted to test these theories by adapting
procedures used previously for studying the fdtration of particles from water by packed
beds under conditions of laminar flow.
THEORY
The concepts presented here follow directly from the approach presented by
Friedlander [2]  for the transport of aerosols. Two mechanisms of particle transport
are considered, e.g., diffusion and gravitational settling. Inertial effects, which can
dominate the transport of larger aerosols, are considered to be negligible because of
the lower fluid velocities used in aqueous systems. The following is a summary of
these theories; additional detads are provided by Bliss [3].
Diffusion
In studying the turbulent transport of particles suspended in a fluid, it is useful to consider
first the motion of the fluid itself. Turbulent flow in smooth pipes is conveniently classified
into three regions, as shown in Figure 1. A turbulent core occupies most of the pipe; time-
averaged velocities vary only slightly in this region. Momentum transport in this case is dominated by turbulent eddies. A thin, laminar sublayer with a linear velocity profile exists near
the wall. Momentum transport in this region is dominated by viscous forces and weak turbulent
fluctuations can be neglected. Connecting these two regions is a buffer layer within which
momentum transport is accomplished by both molecular and eddy diffusion.
In considering transport of particles from the bulk fluid to the pipe wall by diffusion, attention is focused on the laminar sublayer. In this region, the transport of mass
and momentum by diffusion are related but different. Because viscous momentum
transport (characterized for example by v, kinematic viscosity) is much greater than
molecular mass transport (represented by D, the molecular and Brownian diffusion
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