The device (1) has a filter that is provided with a pair of chambers, and a pair of bags (11, 12) that is in fluid communication with a waste discharging line (8). A CPU (41) receives the weight signals from a gravimetric weighing unit associated with the bag (11), and controls fluid flow regulating units (31-33) for loading the liquid in one of the bags during which the other bag discharges the liquid.