Abstract: A device for the determination of the rate of sedimentation of particles in a sample of a liquid and controlling the addition of a flocculant to the liquid in accordance with the rate of sedimentation so determined. The device comprises a sample container (18), a sample inlet duct (17) in fluid communication with the sample container (18) and extending downwardly from a lower end thereof and adapted to extend into a body of liquid to be sampled, a suction unit (34) in fluid communication with an upper end of the sample container (18) and adapted to create a reduced pressure in the sample container such that a sample of the liquid is drawn into the sample container. Units (not shown) are provided to measure the rate of sedimentation of particles in the liquid sample and flocculant addition units (not shown) are adapted to add flocculant to the body of liquid from which the sample was drawn.