This paper deals with the movement of flocs in suspension, as they appear in biological wastewater treatment (BWT) plants. Basic equations for solving the problem and analysis of wastewater composition of BWT plant Lendava are presented. Greater attention is given to the geometrical and sedimentation characteristics of solid flocs, key parameters for developing a fast numerical procedure for simulating flocs' movements. An extensive analysis regarding floc size distribution and settling velocity is presented. Based on the results of experimental investigations, the main physical parameters of the flocs are defined and calculated by considering floc porosity.