Naturally present in river water supplied cooling circuits, bacteria and free-living amoebae (FLA) can encounter conditions favourable to their multiplication in freshwater biofilms, especially since the water warms up. The presence of freshwater biofilms onto surface of industrial cooling circuits can reduce the thermal performance and represents a potential public health risk. Several factors play a role on the behavior of freshwater biofilm. Among those factors and to better manage those industrial problems, it is essential to understand how hydrodynamic conditions influence the dynamics of microorganisms into biofilms and identify the areas of the cooling system where it could occur.