Membrane bioreactors (MBR) with submerged flat sheet modules follow the airlift loop flow principle. The addition of particles is a new development in MBR fouling control. Experiments with plastic beads clearly show a mechanical cleaning effect. Biologically active beads (quorum quenching) can further reduce membrane fouling. The effectiveness of the particles depends on the flow conditions, though. To select or design such additives properly, as well as to optimize air flow rates, the complex fluid dynamics of the multiphase airlift loop flow and the rheology of the biomass need to be understood.