RELATIONSHIP BETWEEN CONCENTRATION AND HYDRODYNAMIC BOUNDARY-LAYERS OVER BIOFILMS

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.