Recycling the water of particle suspensions is often initiated with a partial flocculation. Partial, because the efficiency of FeCl3 is excellent in the beginning, but saturates. As a consequence, the coagulation is not completed. To overcome this imperfection, a cationic polymers is added to bring the flocculation to an end.
Finding the best switching moment, from FeCl3 to the polymer flocculant, requires a titration experiment with the selected polymer. The titration is repeated at various FeCl3 concentrations. In the example with Magenta pigments, 2/3rd of the FeCl3 could have been saved.