Membrane fouling is one of the most investigated topics in the field of membrane processes because it is the main weakness that prevents membrane bioreactors (MBRs) from being widely applied. With a view to reduce membrane fouling, all the phenomena involved in the operation are worth understanding through a fine-tuned and deep examination. In the present paper, the study focused on the hydrodynamics in an air-lift MBR with a flat sheet membrane, taking into account the application of a specific method for biofouling mitigation adding free-moving particles in addition of bubbles injection. The application of this technique creates a complex three-phase gas-liquid-solid contactor in the MBR in terms of hydrodynamics. Therefore, the main objective of this work was to properly characterize, using a Particle image velocimetry (PIV) technique, such a system in order to clarify the potential effect of the addition of free-moving solid particles on the hydrodynamics and the performances of the MBR. Three different shapes of particles have been tested: beads, hollow cylinders, and flat sheets. Local hydrodynamic parameters, such as the liquid velocity, the liquid shear stress, or the bubble sizes and velocities, were analyzed with and without the presence of solid particles. Specific conclusions are drawn to help future users of this antifouling technique to optimize the design of the solid particles.