Rheology of biofilms formed at the surface of NF membranes in a drinking water production unit.

In this study, the mechanical properties of biofilms formed at the surface of nano-filtration (NF) membranes from a drinking water plant were analysed. Confocal laser scanning microscopy observations revealed that the NF biofilms formed a dense and heterogeneous structure at the membrane surface, with a mean thickness of 32.5 +/- 17.7 mum. The biofilms were scraped from the membrane surface and analysed in rotation and oscillation experiments with a RheoStress 150 rotating disk rheometer. During rotation analyses, a viscosity decrease with speed of shearing characteristic of rheofluidification was observed (eta = 300 Pa s for ý = 0.3 s(-1)). In the oscillation analyses with a sweeping of frequency (1-100 Hz), elasticity (G') ranged from 3000 to 3500 Pa and viscosity (G'') from 800 to 1200 Pa. Creep curves obtained with an application of a shear stress of 30 Pa were viscoelastic in nature. The G(0) and eta values were, respectively, 1.4 +/- 0.3 x 10(3) Pa and 3.3 +/- 0.65 x 10(6) Pa s. The relationship between the characteristics of NF biofilms and the flow conditions encountered during NF is discussed.