Investigation of biofilm structure, flow patterns and detachment with magnetic resonance imaging

Detachment of biomass from biofilms is still a nearly unknown process which has to be investigated in more detail. Magnetic resonance imaging (MRI) is a promising method which supplies information on the structural data of the biofilm, its surface and the hydrodynamic conditions at the bulk/biofilm interface. Both the structural data and the shear stress are key parameters for understanding biofilm detachment. In this paper a fast quantitative MRI technique was used to investigate the detachment from a heterotrophic biofilm which was grown in a tube reactor. The investigated biofilm was cultivated in a test segment (length 12 cm, diameter 7 mm) of a tube reactor at a constant Reynolds number of 3,000 and a substrate load from 6 g glucose/m² and day. For the MRI experiments, the test segment with a biofilm of 1,200 μm thickness was connected to the flow loop and placed inside the NMR magnet. During the experiment different hydrodynamic conditions were adjusted for two minutes (Reynolds number Re: 3,000, 4,000, 5,000, 6,000, 7,000, >9,000). Flow velocity and relaxation time were then measured at laminar flow conditions. The MR images show very impressively the increasing detachment of biomass from the biofilm surface with increasing Re. After the last step (Re > 9,000) only a thin biofilm of about 200 μm thickness with a very homogeneous surface remained in the test tube.