Critical particle size in membrane bioreactors - Case of a denitrifying bacterial suspension

The aim of this work was to present results from filtration of biological suspensions which had been subjected to defined hydrodynamic and biological conditions in a membrane bioreactor. In the context of our experiment, the granulometric distribution of particles and that of the soluble organic matter show that most of the suspension consists mainly of the particles with a radius of around 1 mu m. The filterability tests demonstrate the positive effect of the crossflow velocity and the possible presence of a particle layer close to the membrane surface. Reference models (Brownian diffusion model, Shear-induced diffusion model and Inertial lift model) are used to identify the size of those particles that were responsible for the major clogging phenomenon according to the experimental results. The theory according to which back-transport of particles is favoured by a sideways migration of particles due to inertial lift would appear to be better adapted, as the calculations lead us to particle radius values of around those measured. However, certain essential points and certain experimental findings demonstrate the necessity to take into account elements that have often been neglected or forgotten so far in the formalism of models, such as biological growth conditions. (C) 2000 Elsevier Science B.V. All rights reserved.