In the electronics industry, the preparation of silicon plates generates effluents that contain a great amount of colloidal silica. Two processes-decantation and ultrafiltration-are studied with in view the treatment of the effluents released by the firm Rockwood Electronic Materials. The feasibility of each of the two processes is studied separately and their operating parameters optimized. Both processes allow the recovery of a great proportion of the initial effluent (over 89%) as transparent and colorless water that can be reused at the start of a line. In view of the results and of the compared advantages and disadvantages of the two processes, ultrafiltration will be selected for the industrial unit. (C) 2004 Elsevier B.V. All rights reserved.