Pl. Bishop et al., RELATIONSHIP BETWEEN CONCENTRATION AND HYDRODYNAMIC BOUNDARY-LAYERS OVER BIOFILMS, Environmental technology, 18(4), 1997, pp. 375-385
The thicknesses of the concentration boundary layer and the hydrodynam
ic boundary layer above a biofilm are critical variables in the overal
l performance of any biofilm system. Their values determine the degree
of advective transport near the biofilm surface and the distance over
which molecules must move by diffusion alone. Two factors which are t
hought to affect the thickness of these in a biofilm system are the ve
locity or Reynolds Number of the bulk fluid, and the roughness of the
biofilm surface. This study shows experimentally the concentration bou
ndary thickness above biofilms of varying roughness under various flow
regimes. Using dissolved oxygen microelectrodes to determine the CBL
thickness, it was found that the thickness varied from about 200 mu m
to about 800 mu m depending on the bulk liquid flow velocity above the
biofilm. The roughness of the biofilm surface appeared to have minima
l effect on CBL thickness. The study also shows, using particle image
velocimetry, that the hydrodynamic boundary layer above a biofilm is m
uch larger than the concentration boundary layer. Here, though, there
is minimal relationship between HBL thickness and bulk flow velocity.
HBL thickness varied only slightly over the range of flow velocities u
sed. It was possible to relate the HEL and CBL thicknesses over the bi
ofilms through use of the Schmidt number relationship.