Relationship between concentration and hydrodynamic boundary layers over biofilms

The thicknesses of the concentration boundary layer and the hydrodynamic boundary layer above a biofilm are critical variables in the overall 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 thought to affect the thickness of these in a biofilm system are the velocity or Reynolds Number of the bulk fluid, and the roughness of the biofilm surface. This study shows experimentally the concentration boundary 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 minimal effect on CBL thickness. The study also shows, using particle image velocimetry, that the hydrodynamic boundary layer above a biofilm is much 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 used. It was possible to relate the HEL and CBL thicknesses over the biofilms through use of the Schmidt number relationship.