configuration C1 differs from the others in the separate thickening of the two sludges. in configuration C2, mixed thickening and elutriation of the resulting thickened sludge were carried out. in configuration C3, a stirred contact tank was used previous to the mixed thickening step instead of the elutriation system. in configuration C4, the EBPR sludge from the aerobic reactor was used as a "washing-stream" that elutriates the thickened sludge to a greater extent than in configuration C2.