Abstract: In a filtration apparatus in which a flocculant is added to promote flocculation, efficient use of the flocculant is ensured by measuring an operational parameter of the apparatus dependent on the degree of flocculation, and adjusting the amount of flocculant being added to the minimum that causes maximum flocculation. As the operational parameter, measurement can be made of the depth of a pool of the material at the lower end of an upwardly inclined filter belt, or of the resistance to movement of a part of the apparatus e.g. the torque required to rotate a filter drum.